Download The Transition Metal-Nitrogen Multiple Bond

Volume 20
-
Number 5
May 1981
Pages 413-486
International Edition in English
The Transition Metal-Nitrogen Multiple Bond
By Kurt Dehnicke and Joachim Strahle'*'
Dedicated to Professor Josef Goubeau on the occasion of his 80th birthday
Numerous nitrido complexes of transition metals show very short metal-nitrogen bond
lengths, suggesting M=N-triple bonds. At present, compounds of this type are being intensively investigated. In particular the molybdenum complexes are considered as model substances for the study of at least an intermediate step of N,-assimilation. This article contains a review of the structure and bonding, as well as syntheses and reactions of these complexes.
1. Introduction
oxidation state. Hitherto, M N multiple bonds have been
found with following elements:
Apart from the carbyne ligand, the nitrido ligand is the
strongest known n-electron donor. As a terminal ligand
M=N: its formal bond order corresponds to a triple
bond. Here, the three bond lines represent one CT and two n
bonds, the latter resulting from overlap of occupied n orbitals of the nitrogen with two unoccupied, symmetry allowed d-orbitals of the metal. An important reason for the
high bond order is the relatively low electronegativity of
the nitrogen, which qualifies it for optimal overlap in the
series
MGN:
M-a
M--E:
V
Nb
Ta
Cr
Mo
W
Mn
(Tc)
Re
Fe
Ru
0s
Ir
The most favorable conditions are provided by the elements Mo, W, Re, Ru and 0s.
2. Bonding
2.1. Terminal Ligand M=N:
Although higher bond orders are possible in transition metal-oxygen and transition metal-fluorine bonds, the facts
are on the whole expressed by the above formulation. The
second condition for the occurrence of a high bond order
is a sufficient supply of unoccupied d-orbitals in the transition metal, which, as a result, exists in general in a high
[*] Prof. Dr. J. Strahle
Ti
The terminal nitrido ligand occurs, for example, in the
tetragonal pyramidal complex anions [MNCI,] - (M = Mo,
W, Re, Ru, OS)['-~]
(see Fig. 1). The M=N bond is ex-
['I
Institut fur Anorganische Chemie der Universitat
Auf der Morgenstelle 18, D-7400TiIbingen (Germany)
Prof. Dr. K. Dehnicke
Fachbereich Chemie der Universitst
Lahnberge, D-3550 Marburg/Lahn (Germany)
[ '1 Author to whom correspondence should be addressed.
Angew. Chem. Int. Ed. Engl. 20, 413-426 (1981)
Fig. 1. Structure of anions [MNX41- (M=Mo, Re, Ru, 0 s ; X = F, CI, Br, I)
with terminal nitrido ligands.
0 Verlag Chemie GmbH, 6940 Weinheim. 1981
057~0833/81/0505-0413
$ 02.50/0
413
tremely short; values range between 157 pm (Ru) and 166
pm (Mo). The triple bond can be understood as a superposition of one o bond ( I ) and two (degenerate) n bonds
(2).
The lone pair of electrons only bestows weak basic
properties to the nitrido ligand.
Q
@
2.2. Asymmetrical M=N-M
Bridges
Under certain conditions, the free electron pair at the nitrido ligand is able to form a second, significantly weaker
bond to the adjacent transition metal center. The continuance of the bond axis, according to M=N-M=N--,
leads to columnar structures, as in ReNC1,[51 (Fig. 2) and
n
Fig. 2. Structure of ReNCL with asymmetric nitrido bridges.
K,ReN(CN),- H,016]. The ReN bond lengths differ greatly:
158 and 248 pm, 153 and 244 pm respectively. Whereas the
short bond can be interpreted in the same way as the terminal one, the long bond is a a-type, in which the occupied
sp-hybrid orbital of the nitrogen overlaps with an unoccupied d2sp3-hybrid orbital of the metal, as indicated in (3).
Fig. 3. Structure of the tetramer [MoNCI,.OPCl& with asymmetric nitrido
bridges.
M=N
I
111
N
N
Ill
M
-M=o--M
--M
--N=u
I
I
In this arrangement, the dz2-and p,-orbitals involved in
the hybridization of the metal are strongly and unequally
involved in the o-part of the MEN triple bond, and hence
the o-interaction in the position trans to the n bond is only
very weak.
A much more favorable arrangement is that shown in
(4), in which the second o bond is achieved via an orbital
perpendicular to the z-axis.
Consequently, a rectangular bonding system is formed,
giving tetrameric units of type (5) (see Fig. 3). A zig-zag
chain, which is equally possible, has not yet been observed.
In fact, such tetramers occur far more frequently than
polymers of the type ReNCl,, as illustrated by the examples [MoNC~,],"~ and [MNCl,. POCl,], (see Fig. 3)
(M = Mo, W, Re[8-'01).The long Re-N bond in the tetrameric rhenium complex is even reduced to 217 pm. It is in-
414
0
0
II
M - O = M
(5)
M-F-M
I
I
1
I
I
F
M-F--M
(6)
I
I
F
(7)
teresting to compare these nitrido complexes with the tetrameric 0x0 (6) and fluoro complexes (7), in which the differences between the alternating bond lengths decrease in
the succession [MoNCI, .POC13]4[81,[NbOCl, .POCl&"]
and [NbFs]4[121:
Mo-N-Mo
166 216
Nb-N-Nb
174 209
Nb-F-Nb
207 207
[pm]
The solvating POCl, molecules are always arranged trans
to the n bonded ligand and are coordinated to the metal
center via the oxygen atom (see Fig. 3). Accordingly, the
bonding in the trinuclear nitridomolybdenum complex
[((Et,NCS2)3Mo~N]2Mo(SzCNEt2)3]3+r
which contains
the Mo-N-Mo-N=Mo
group as a structural element
can be understood[131.The structures of these types of compounds are discussed in Section 6.3.
Angew. Chem. Inr. Ed. Engl. 20. 413-426 (1981)
'A" Q 'A0
2.3. Symmetrical M=N=M Bridges
Symmetrical nitrido bridges are observed in some binuclear complexes, e.g. in the recently prepared ion
[Ta2NBrlo]3-['41(see Fig. 4). The TaN bonds (185 pm) can
n
been examined by crystallographic methods[20-221.
In these
complexes, real nitrido bridges are found. In the compounds [C14M=N=PPh3]2 (M= Nb, Ta) (see Fig. 5) the
MN- (178 pm and 180 pm respectively) and PN-bonds (164
r\
Fig. 4. Structure of the anion na2NBr,o]3- with symmetric nitrido bridges.
be interpreted as double bonds. In this case, apart from
two CJ bonds, the nitrido ligand forms, using the two perpendicular occupied n orbitals, two degenerate dn-pn-dn
three centered n molecular orbitals, each being occupied
by one electron pair (see Fig. 8)[lS1.
Fig. 5. Structure of the phosphaneiminato complex fCI4NbNPPhl],.
The factors responsible for the formation of symmetrical
or asymmetrical bridges have not yet been completely clarified (Section 6.4). An intermediate situation is found with
the nitrido bridge in the complex ion [W2NCllo]2-"61,
which contains a long (207 pm) and a short WN bond (166
pm), but where the long WN bond shows an even more
distinct share of multiple bonding. This could be a consequence of one of the tungsten atoms being pentavalent and
the other hexavalent, which would explain the asymmetry
of the bridge. A symmetrical bridge also occurs in p-nitrido-bis(porphyrinat0)iron. To date, it is the only known nitridoiron complex[171(see Section 6.4 for structures of these
complexes).
e
e
2.4. N-Bridges ("Nitrido Bridges") of the Type M=N=X
(X = 0, S, PRj)
N-bridges of this type with X = 0 or S, occur in nitrosyl
or thionitrosyl complexes. Nitrosyl complexes have been
adequately reviewed"81so there is no need to discuss them
here further. Formally, the nitrosyl ligand is considered to
be NO', which explains its extremely strong back-bonding behavior. In principle, it is suitably described by the linear arrangement M=N=O. Similar considerations seem
to apply to the thionitrosyl ligand. In the complex nC5H5Cr(CO)2NSit is also linear, the bond lengths CrN
(169 pm) as well as NS (155 pm) suggesting n bonding"91.
In both cases the formal oxidation number of the N atom
is +3, so that these compounds are actually not real nitrido complexes.
On the other hand, this does not apply to phosphaneiminato complexes of transition metals with a linear arrangement M=N=PPh3, of which three examples have already
Angew. Chem. Int. Ed. Engl. 20, 413-426 (1981)
and 159 pm respectively) can be understood as double
bonds on account of their lengths; the bond angle of the
bridge (171 and 177" respectively) practically corresponds
to an sp-hybridization of the N atom, so that the phosphaneiminato complexes are closely related to the type discussed in Section 2.3, and should be formulated as follows:
0
0
M=N=PPh,
-
20
0
M=N-fPh3
2.5. N-Bridges CNitridobridges") of the Type &&-X,
0
M = % l X and M=NR2
Compounds with these types of bridges have recently
been reviewedf231,
so they need not be discussed here in detail (for structures see Section 6.5). The ligands X, among
others, are hydrogen, halogens (see Fig. 6), alkyl, aryi, silyl,
NR, groups; MNX, X=NR2 is a hydrazido(2-) complex.
C
Fig. 6. Structure of the complexes CISVNX (X = CI or I).
With the exception of NR and NR2 respectively, all
these ligands, X, in complexes of the type
e e . .
M=N=NR
2a
0
e
or M=N=NR,
415
the ligands X show no tendency to A-bond with the central
nitrogen atom, so that the N-X group has o-character.
The bonds can be interpreted in a similar way as those of
MeN-M (Section 2.2); (9) shows only one of the doubly
degenerate n bonds of the MEN bond.
Ir3N is trigonal planar; the Ir-N bond length (192 pm) indicates appreciable n bonding1381.As shown in (10) the
bond system can be described by three IrN n bonds and
(pn-dz) overlap of the occupied N(p,) orbital with each
Ir(dxz)orbital.
3. Syntheses
The following contains a review of the most important
reactions for the syntheses of nitrido compounds of transition metals. A comprehensive review, up to 1971, has been
put forward by W. P. GrijjfithP91.As far as the organoimido
complexes (Section 6.5) are concerned we shall only deal
with those syntheses not covered in lz3'.
The N-X o bond thus arises via overlap of an sp-hybrid orbital of nitrogen with a hybrid orbital of the Xgroup. The bond is, contrary to the easily cleaved long
bonds in the MEN-M
bridges, very stable. It should be
noted that the M s N bond of nitrene complexes (Section
6.5) should only be regarded as a triple bond when the
MNX-axis is linear. This claim is supported by numerous
nitrene complexes, although smaller bond angles (up to
139") are known.
In some diazenido-complexes, which contain the
0
0
M=N-NR2
group, the NN bond is shorter than a N-N
2 8
0
0
single bond, so that the form M=N=NR2 is justified.
A selection of the numerous recent examples of these
types of bonds has been compiled in Table 1 (for further
examples see I2'I).
G
O
O
Table 1. Examples of structures with the characteristic groups M=N-X,
Complex
C1,VNI
3.1. Decomposition of Azides
The most effective preparative method is the reaction of
azides, mostly with transition metal haIides. The halogen
azides, chlorine azide and because of the higher polarity of
the I-N bond iodine azide, have proved to be of special
utility. Thus, for example, tungsten h e x a ~ h l o r i d eor
[ ~ the
~~
pentachlorides of niobium, tantal~m'~'],
molybdenum[401
and rhenium[421react with chlorine azide, to form metal
chloride azides:
b
R
e O
and M=NR2.
e
M=N&R,.MEN-RR~
Group
G
O
Q
O
V-N-I
[(sS-CSHsf2VN(SiMe,)1
V-N-Si
~MoC~(P~C~,)(NC~C~S)I
Mc-N-C
[Mo(NC6H,Me-p)CI2@-MeC6HAN2COPh)(
PMe2Ph)l
Mo-N-C
[MoC12(NH)O(EtPh2PO)zj
MhN-H
[MoCl(N2COPh)(NHNCOPh)(PMe2Ph)J
G
O
O
b
O
M-N-X
M-N
Ipml
Ref.
I"
I
165
163
[241
167
178
I251
169
172
L261
173
177
1271
170
178
157
174
1281
1291
[301
O
Q
O
0
0
G
O
[Mo(N2Ph)2(S2CNMe2121
Mo+ N-N
e
a
Mo--N-N
[MoI(N2C6H,,)(diphos),l [a]
Mo-N-N
[Mo(NMe2),1
Mo=NR2
AsPh t [CISW(NC&)]
W--N-C
168
168
(331
[WBr(diphos)2(NZCCIz)j* PF; [a1
W-N-N
175
169
f341
[(s5-C,H,)W(Co)z(NzCH,)I
[R~C~(POCI,)(NCZC~S)I
W --N-N
186
169
173
169
[361
182
173
[371
e
e
G
Q
170
177
1311
193
123
~321
m
m
O
O
Re--N-C
G
[IrCI(N2CsCh)(PPh,)2(C,Hs)l
174
183
(351
O
Ir--N-N
[a] diphos = ethylenebis(dipheny1phosphane).
2.6. Nitride Bridges of the Type M3N
This type of bridge has hitherto only been described in
the iridium complex (NH4)4[Ir3N(S04)6(H20)3].The group
+ CIN3
MoCIS + ClN3
WCl6
CCI,
-+
WCISN3
+ Clz
MoClSN3+ 1/2C1,
(1)
(2)
The explosive metal chloride azides of molybdenum,
tungsten and rhenium can be thermally decomposed in solution without any risk:
MC15N3 MNC13 f N,
M = M o , W, Re
-C
+ CI,
(3 )
This reaction is completed within a few hours, even at
room
On the other hand, the azides of
416
Angew. Chem. Inr. Ed. Engl. 20. 4M-426 (1981)
niobium and tantalum are more stable and have not yet
been thermally converted into nitrido complexes.
The tantalum azide [TaCI4N3],has been characterized by
single crystal structure analysis[411.However, decomposition of the azides [MN3CI4l2(M = Nb, Ta), occurs at room
temperature upon addition of a Lewis base such as triphenylphosphane. In an analogous Staudinger reaction[431,the
phosphaneiminato complexes [C14M=N=PPh3], are obtained[21.221
and PhN31461
undergo a reThe azides C1N3’44’,
markable reaction with vanadium tetrachloride; here, the
NX bond of the XN3 molecules is preserved and the nitrene complexes C13V=N-X
are formed (see Section 6.5).
The mechanism of this reaction has not yet been clarified.
In the reaction with IN3, the formation of ICI and the consumption of two mol IN3 per mol VC14 suggest the following reaction equation:
VC14 + 2 IN3
C13V=N--I
+
+ 2 1/2 N, + ICl
”(cl
Fig. 7. Structure of the triazido(nitrid0) complex MoN(N,),(bpy).
r\
AN..
(4)
In the case of ClN3, the intermediate azide VC14N3can be
isolated. It presumably decomposes [eq. (6)] in a manner
analogous to the Curtius reaction[471[eq. (511, in which an
isocyanate is formed by anionotropic migration of the
group R:
R-CO-Nj
+
+ N2
R-N=C=O
Cl4VN3 + CI~VEN-CI
+ N2
(6)
It has been known for some time, that the reaction of organoazides with metal compounds yields nitrene complexes[23J.
MoNCI3 can also be prepared from MoCI, and IN3; under
mild conditions it is even possible to isolate an intermediate compound with an N-N bond[481:
2 MoClS
+ 2 IN3
-+
CI~M-N-N=MOCI~
+ 2 ICI + 2 N2
(7)
which can be converted into MoNC13 by thermal dissociation of C1,.
Reactions with iodine azide are transferable to metal
bromides, as demonstrated by the examples for the preparation of M o N B ~ , [ ~
and
~ ’ WNBr3L721:
+ IN3
WBr, + IN, -+
MoBr,
- IBr
WNBr,
MoBr3N3
- NZ
MoNBr3
+ IBr + BrZ+ N2
(8)
(9)
According to Chatt et ~ 1 . [ ’ complex
~~,
chloro(nitrido) compounds can be obtained by reaction of trimethylsilyl azide
with molybdenum tetrachloride dissolved in acetonitrile,
in the presence of complex donor molecules[501:
+
LJ
(5)
MoC14(MeCN), + Me3SiN3 bpy + MoNCl,(bpy) +
N2 2MeCN + Me3SiC1
+
(10)
In a similar way, the azidonitrido complexes MoN(N3)3(bpy)[5’1
(see Fig. 7) or M O N ( N , ) , ( ~ ~ (see
) [ ~ ~Fig.
~ 8)
respectively result from the reaction of excess trimethylsilyl azide with the molybdenum(1v) complexes MoCl,(bpy)
and MoCl,(py),.
Equally, the only presently known nitrido iron complex
was prepared by thermal decomposition of an azide. Azido-(a,P,y,6 tetraphenylporphyrinato)iron(rrI) (FeTPP)N3
reacts to form (FeTPP)2N[531 with a symmetrical
Fe=N=Fe bridge[I7’:
Angew. Chem. Int. Ed. Engl. 20,413-426 (1981)
Fig. 8. Structure of the triazido(nitrid0) complex MoN(N,),(py).
2(FeTPP)N3 -* (FeTPP),N
+ 21/2N2
(11)
Here, the nitrido ligand is able to stabilize iron in the formal oxidation state 3.5.
Even dinitrogen complexes can occasionally be converted into nitrido complexes[541:
+
Lastly, tetrachloronitridomolybdate is obtained from a
straight-forward reaction of azide ions with molybdenum
penta~hloride[”~:
MoCl,
+ N;
-+
[MoNCIJ
+ N2 + 1/2C12
(13)
3.2. Ammonolysis Reactions
The preparation of the nitridoosmate [Os03N]- from
osmium tetraoxide and aqueous ammonia[561has been
known for some considerable time.
OSO4 + NH3 + KOH
+
K[Os03N] + 2 HZO
(14)
The same holds for nitridorhenates prepared from rhenium heptao~ide[’~I:
+
Re207 3 KNH2 -+ K2[Re03N) + K[Re04] + 2 NH3
(15)
Recently, nitrido complexes were also prepared by thermolysis of ammonium salts[’41:
2 NH,Br
+ 2 NH4TaBr6
+
(NH4)3[TaZNBrIO]
+ 4 HBr
(16)
417
3.3. Reactions with Nitrogen Trichloride
Nitrogen trichloride dissolved in carbon tetrachloride
reacts with ReCl, at slightly elevated temperatures[581:
ReCIS + NCl,
+
ReNC1,
+ 2.5 ClZ
(17)
The corresponding reaction, carried out at O"C, leads to
the thermally sensitive nitridochloride of the heptavalent
rheniumrs1:
ReCI,
+ NCl3
+
ReNCI,
+ 2CIZ
(18)
A very effective way is the synthesis of MoNCI3 and
WNC1, from the corresponding hexacarbonyl compound~~~~]:
W(CO),
+ NC13
+
WNC1,
+ 6CO
(19)
which are associated in the crystal lattice, can be transformed into the tetrahalogeno(nitrid0) complexes by treatment with the corresponding tetraphenylarsonium halides[68-711.
MNCl3 + AsPh4CI
MoNBr,
CH2ClZ
+ CI-
ReNCI,
3.4. Reactions with Hydrazine
Complexes of the type ReNCI,(PR,), and ReNCl,(PR,),
can be obtained from the 0x0 complexes Re0CI3(PR3),
with hydrazine in the presence of ethan01'~~-~~'.
3.5. Reaction of Vanadium Nitride with Chlorine
A method for the synthesis of halonitrene complexes,
which up till now has only been verified in one case, is the
reaction of vanadium nitride with gaseous chlorine; this
proceeds even at 120°C yielding C13VNCl, as well as a
small amount of VCI,'66J:
(24)
The complex anions form tetragonal pyramids having C4"
symmetry with the terminal nitrido ligand in an axial position.
The use of the smaller tetraethylammonium ion allowed
the preparation of the six-coordinated molybdenum complex (NEf4),[ M o N C ~ , ] ~ ~ ~ ] .
The
nitrene
complexes
CI,V=N-CI
and
C13PO(C14)M=N-R (M = Mo, W, Re; R = CCI,, C2C15)
are also capable of forming chloro c ~ m p l e x e s [ ~ ~ . ~ ~ - ' ~ ~ :
Simultaneous use of ammonia and sodium hypochlorite
recently led to the first nitridoItetra(pto1yl)porphyrinato]manganese(v)
(21)
AsPh,[MoNBr4J
CHLBr2
CI~PO(CI~)MZN-R
+ NH3 + N a 0 C l - t
MnN(TPP) + NaCl + H 2 0 + MeOH
(23)
-
+ AsPh,Br
As it is possible to obtain NC13 and NBr, from ammonium
salts and halogens, the syntheses of halogeno(nitrid0) complexes from ammonium halometalates and chlorine at
400- 500 0C160*611
are included in this section:
Mn(TPP)OMe
AsPh,[MNCIJ
C13V=N-CI
+ [Cl,V=N--Cl]-
+ CI-
+
[CI,M=N-R]-
(25)
+ POC13
(26)
In both cases, the linear group M=N-R
is preserved.
ReNCl,, which is sensitive to redox reactions, is smoothly
reduced to the rhenium(v1) complex by chloride or azide
i 0ns1~~1
:
+ C1ReNCI., + N;
+ 1/2C12
[ReNC14]- + 1 1/2Nz
+ [ReNC14]+
.
Phosphoryl chloride solvates the nitridohalides, forming
tetrameric units having alternating long and short MN
bonds. The 0 atom of the P0Cl3 molecule is always arranged trans to the N atom at the end of the short MN
(see Fig. 3):
4 MNC1, + 4 POCl,
M = Mo, W, Re
+
[MNC13 ' POCISL
In addition, the lattice of the complexes with M = W and
Re contains two POCI, molecules per tetrameric ~ n i t [ ~ . ' ~ ] .
With chelate ligands, such as a,a'-bipyridyl, monomeric
six-coordinated complexes can be prepared1771:
MoNC1,
+ bpy + MoNCl,(bpy)
(30)
4. Chemical Properties
Relative to the strongly electrophilic character of the metal
atoms, as expressed in reactions (23)-(30), the nucleophilic character of the nitrido ligand is, in general, not very
distinct. Thus, neither MoNC1, nor the ion [MoNCI,]coordinate via the N atom with the strong Lewis acid
SbCl,. Only when the order of the MoN bond is reduced
to Mo=@ by competitive n-interaction of fluorine ligands,
is adduct formation at the nitrido ligand possible[781:
4.1. Addition Reactions
[MoNFJ
Many of the reactions dealt with in this section result in
formation of novel complexes with metal-nitrogen multiple bonds and are therefore useful synthetic routes to these
complexes. Thus, the ternary nitridohalides MNCI3
(M=Mo, W, Re, 0 s ) and the nitridobromide MoNBr,,
Furthermore, the oxidation state of the metal center seems
to influence the donor properties of the nitrido ligand,
since the rhenium(v) complexes ReNX,(PEt,Ph), (X = C1,
Br) form complexes of the type (PEt2Ph)3X2Re=N-BX3
with boron trihalide~[~'I.
VN
+ 2CI2 + C13VNCl
(22)
Under analogous conditions, all other metal nitrides investigated yielded only pure metal halides16".
41 8
+ BF3
+
[F~MoN-BF~I-
(31)
Angew. Chem. Int. Ed. Engl. 20. 413-426 (1981)
The reactions of Cl,V=N-Cl
with Lewis acids and Lewis
have been thoroughly investigated. Lewis
bases such as phosphane or nitrogen bases always attack at
the metal center. As illustrated by a crystal structure analysis of Cl,(bpy)VNCl, the V=N-CI
group remains in linear array. CI,VNCI does not react with Lewis acids as a
base. SbCfs was used to prepare the thermally unstable
trinuclear complex C13VNCI(SbC15)2,in which C1,VNCI is
linked to two molecules of SbCl, via CI-bridge~'~~'.
Sufficient nucleophilic character of the nitrido ligand is
presumably a prerequisite for the formation of thionitrosyl
complexes from nitrido complexes and elemental sulfur
or disulfur dichloridec"':
[MoN(SzCNR2)31+ S
[MO(NS)(SZCNR~)~I
(32)
[ReCI2N(PR3),] + 1/2SzC12 -* [ReCI2(NS)(PR3),] + 1/2C12
(33)
[OsC13(N)LZ] + 1/2S2C12 -+ [OsCI,(NS)L21+ 1/2C12
L=AsPh3, PMe2Ph, 1/2bpy
(34)
R2 = Me2, Et2, (CH&
The nitrido complexes are recovered from the thionitrosyl
compounds by means of tertiary phosphanes'"I.
Only in the cases of ruthenium, osmium and rhenium
has it been possible to obtain phosphaneiminato complexes of transition metals, containing the characteristic
group M=N=P, using nitrido complexes and triphenylphosphane as starting material^^^'^^^^^^^:
ReNCl,
+ 2 PPh,
-+
[(Ph3P)CI4Re=N=PPh3]
+ 2 Nr
(36)
The type of reaction (36) also applies to ionic azido complexe~[~~,*~':
+ PPh3
[BrsMN3]- + PPh,
[Cl,MN3]-
+
[ClSM=N=PPh3]-
+
[BrSM=N=PPh3]-
~ [ M o N C I ~ I+- 0
+ N2
+ N2
(37)
K(OSO~N]+ 6 H C l + KCI
+
[VOCI(NPPh,),]
+ 2Me3SiCI
(39)
(40)
4.2. Substitution Reactions
Reactions of this type occur at the MEN bond as well
as at the other metal-ligand bonds, resulting in the latter
case, in new nitrido compounds. In general, the M s N
bond is sensitive to hydrolysis, yet nitrido complexes are
known which are resistant to moisture. Thus, the red
AsPh,[MoNCI,] reacts in moiste air to form the green 0x0
complex AsP~,[MoOC~,]~'~.
The two compounds are isostructural, and the reaction could also be carried out in a
Angew. Chem. Int. Ed. Engl. 20. 413-426 (1981)
+ N2
(41)
K2[0sNCI,]
+ 3 H2O + C12
(42)
Recrystallization of this complex from water causes the
chlorine ligand trans to the nitrido ligand to exchange as
fOllOWS'8''
:
K2[OsNCIS]
+ H2O
+
K[OSNCI~(H,O)]+ KCI
(43)
Analogous reactions of the bromonitrido complexes of osmium are knowncg8'.
Treatment of tetraphenylarsonium tetrachloro(nitrido)rhenate(v1) with thiocyanate ions in boiling methanol gives
the corresponding pentakis(isothiocyanato)nitridorhenate(v1)[~~1:
+
+ AsPh4SCN
AsPh4[ReNC14] 4SCN-
CH30H
(ASP~,)~[R~N(NCS),]
+ 4CI -
(44)
In this complex the coordinatively unfavored position
trans to the nitrido ligand is also occupied by an NCS
group; however, the Re-N bond length is 23 1 pm, in contrast to a mean length of 202 pm for the four cis-NCS
groups. The position trans to the nitrido ligand remains
uncoordinated in exchange reactions of the [MoNCI4]ion"90.911.
AsPh4[MoNC1,]
+ 4AgF
CHiCN
AsPh,[MoNF,]
AsPh4[MoNCIJ
TiC12(S2CNEtZ)2
+ Me3SiNPPh3 +
[TiCl(NPPh3)(S2CNEt,)2]+ Me3SiCI
--*
(38)
UV-irradiation of [C15NbN3]- yielded the arsaneiminato
complex [ C ~ , N ~ = N = A S P ~ ~ ] -Several
[ ~ ~ ~ . phosphaneiminato complexes of titanium and vanadium have recently
been prepared by ligand exchange reactionsc85':
+ 2Me3SiNPPh3
~[MoOCIJ
Reaction (41) is remarkable inasmuch as molybdenum(v1)
is reduced to molybdenum(v) by Oz; however, the process
cannot be reversed. The Os=N bond of the [Os03N]- ion
is surprisingly inert towards water; the ion is transformed
into the chloro(nitrid0) complex by hydrochloric acid in
the presence of potassium chloride[86J:
M = N b , Ta
VOCl,
2 -+
(35)
More convenient for this purpose is the Staudinger reactionC2":
[NbC14N3]2 + 2 PPh3 --+ [C14N&-N=PPh3]2
single crystal. In boiling dichloromethane, the convertion
of the nitrido complex into the 0x0 complex can even be
achieved using dry oxygen[701:
+ 4AgCI
(45)
+ 4AgCl
(46)
+ 4AgN3
AsPh,(MoN(N3)4]
Chelate ligands such as a,a'-bipyridyl, slowly displace one
chloro ligand, resulting in nitrido complexes with six-coordinated metal centersr77':
[MNC14]- + bpy
M = Mo, W, Re
+
[MNCl3(bpy)]
+ C1-
(47)
Of note is the fact that tribromonitridomolybdenum undergoes a substitution reaction with t e t r a p h e n y l p ~ r p h y r i n ~ ~ ~ ~ :
MoNBr,
+ H2TPP
-.
[MoN(TPP)]+Br;
+ H2
(48)
The reaction of the N-iodine compound, Cl,VNI, with bromine or chlorine produces the corresponding N-halogen
derivatives by halogen exchange[451:
+ Br2
CI~VSSEN-I + Cl2
CI,V=N-I
-+
C13V=N-Br
+
Cl3VEN-CI
+ IBr
+ ICI
Furthermore, with iodine azide, exclusively the N-I ligand reacts, to form the highly explosive N - a ~ i d e ' ~ ~ ] :
CI3VsN-I
+ IN3 -,C13V=N-N3
+ 1,
( 5 1)
which is also obtained from the reaction of C13VNCl with
C1N3[921.
C13VNCI can transfer the nitrogen to 4d and 5d elements158.701.
ClsVsN-CI
M = Mo, Re
+ MCI,
+
MNC13 + VCli + C1z
(52)
The primary product is presumably Cl,M-N=VCI,,
in
which two different metals compete for 71-interaction with
the nitrogen. As the d-orbitals of the 4d and 5d metals are
more extensive than those of vanadium, the MEN bond
(M = Mo, Re) is favored over the VN bond, and hence the
latter is cleaved.
5. Vibrational Spectra
In general, it is possible to determine nitrido groups or
nitrene groups quickly and reliably by infrared spectroscopy, since the stretching vibrations of MN multiple bonds
give rise to very strong absorptions at characteristic frequencies (for examples see Table 2). Because of the intense
color of most of the substances, the vibrational data are,
on the whole, confined to IR spectra; Raman spectra have
only been recorded in a very few cases.
Compounds with terminal bonds of the type MEN:
normally exhibit a very intense MN stretching vibration in
cm-I; this is, however, a result of the competitive n bonding of the fluorine ligands. When the metal is reduced to
the oxidation state ( + v) or (+ IV), the wave number of the
MN vibration is lowered to about 950 cm-'. In structurally comparable species, depending on the position of the
metal within the periodic table, the wave numbers increase
from left to right along a period and generally down a
group, as shown by the following examples:
[WNC14]- 1036 cm-'[681, [ReNCI,I- 1085 cm-'[69],
[OsNCI,]- 1123 cm-'[IOO],
[RuNCI3(AsPh3),l 1023 cm- ' [82],[ O S N C I ~ ( P E ~1070
~ ) ~ ]cm-' [82]
The reason for this appears to be the increased ability of
transition metals to effect n-overlap, which proceeds in the
same direction.
A similar spectroscopic situation is found in compounds
with M=N-M
bridges. A strong absorption in the region
of 1000- 1100 cm-', which is occasionally split, corresponds to the stretching vibration of the short MN bond.
No data is available on the vibrational frequency of the
long MN bond, however this is expected to occur at very
low wave numbers (<200 cm-I). This is also the reason
for the absence of significant coupling: v(M=N) for this
type of bridge occurs in the same region as v ( M 5 N ) for
the terminal type. The few spectra of complexes with symmetrical M=N=M bridges hitherto reliably studied, form
a similar picture. Here, the two stretching vibrations
should be regarded as v,(M=N=M) and va,(M=N=M);
the latter appear again between 1000 and 1100 cm-'. Since
Table 2. Selected frequencies of MN stretching vibrations in complexes with M N multiple bonds. O = P or As.
Terminal Type MEN:
v(MEN:)
[cm - '1
Ref.
969
1054
1060
1019
948
1036
1085
1022
1023
I123
1073
(I5N: 1041)
1070
I107
1032
963
I200
1286- 1332
1265
1240
1190-1 196
1184-1215
G
v(MrN)
[cm-'1
Ref.
I045
951
1068, 1084
944, 1015
1080
944,995, 1011
[401
IlW
1401
[I61
[581
151
I821
510
435
390
928
851
the region 1000-1100 cm-I, when the metals have the oxidation number (+VI). The MN-vibration thus occurs at
least 100 cm-' higher than the corresponding MO stretching modes, so that they can generally be easily distinguished. An exception is v(MoN) in [MoNF,I- at 969
420
O
Bridge Type M=N-M
the symmetrical vibration leaves the N atom unmoved, it
has a very low wave number.
An interesting aspect of the vibrational spectra of anions
of the type [X,M=N=MX,]"and of the corresponding
p-0x0 complexes is a strong, sharp band which occurs at
Angew. Chem. Int. Ed. Engl. 20. 413-426 (1981)
520-540 cm-’ in the p-nitrido compounds and at 400470 cm-’ in the p o x 0 compounds, first mentioned by
Mattes et ~ 1 . ’ ~ ~ ’ .
Its assignment is problematic, since except for the asymmetric vibration of the bridge v,,(M,N), no absorption is
expected above 500 cm-’, this being the upper limit of the
characteristic region for metal-halogen stretching vibrations. Mattes et al. assigned this vibration to a p(XM0)
or p(XMN) mode of the species E,. The IR spectrum of
the anion [Ta2NBr10]3-,apart from v,,(Ta,N) at 985 cm-’,
also shows a band of medium intensity at 735 cm-’. This
has been tentatively assigned to the bridge deformation vibration 6(M,N)1’41, which due to the high mass of the
TaBr, groups lies at a higher wave number than the symmetrical vibration of the bridge v,(M,N).
Apparently, the situation in the anion [W2NC110]2-cannot be compared herewith; this ion contains an asymmetric nitrido bridge. Here, two absorptions occur at 1015
cm-’ and 944 cm-’, which are assigned to WN stretching
The vibrations of bridges of the types M=N=X and
M=N-X
have to be regarded in a somewhat different
way. Depending on the frequency of the NX stretching vibration, strong couplings are observed in some cases,
which permit the MN stretching mode to be considered as
a characteristic vibration, to a limited extent. In the phosphaneiminato complexes, significant n bonding of the
M=N=P type can be attributed to the PN bond on account of its short length. Here, a strong band, generally
above 1100 cm-’ is usually found (occasionally split into a
doublet as a consequence of Fermi resonance), and a second band appears between 500 cm-’ and 600 cm-’. The
former is normally described as v(PN), the latter as
v(MN); more correctly these should be v,,(MNP) and
v,(MNP). Undoubtedly the frequencies of both vibrations
are influenced by strong coupling, which accounts for the
low wave number of the MN stretching vibration. Similar
conditions are encountered in nitrosyl metal complexes
where the MNO group is linear:
0 0 . .
M=N=Q
20
I3
or M-N=8:
lengths by more than 100 cm-’ relative to uncoupled
v(NC) stretching vibrations. Similar behavior is found in
the carbyne complexes M=C-R[961, which are related to
the bridges of the type MEN-X.
6. Crystal Structures
A remarkable feature of all crystal structures of nitrido
and nitrene complexes (Section 6.5) is the extremely short
metal-nitrogen bond length and the distinct trans-effect of
the n bonded nitrogen ligand.
After a brief consideration of the trans-effect, the compounds will be discussed in detail. Characteristic data of
selected compounds are compiled in Tables 3 to 7 (for the
problem bond length/bond order in MoN bonds see
[l051).
6.1. The trans-Effect
The trans-effect is the weakening of the bond to a ligand, coordinated in a position trans to a ligand which is
particularly strongly bonded, so that substitution reactions
preferentially take place at this site (see Section 4.2). Not
only is the weakening evident in reactions but it is also expressed by the differences observed in the bond lengths between the central atom and cis or trans-positioned ligands:
this difference can amount to 30 pm in mononuclear complexes.
In the previously mentioned
complex,
[ReN(NCS),]2-1891,the Re(NCS) bond length is 202 pm for
the cis-ligand (relative to the nitrido nitrogen) and 231 pm
for the trans-iigand. A similar situation is found in
[OsNCl5l2- (236 pm and 261 pm respectively)[’06’(Fig. 9).
+
In these cases v(N0) is assigned to frequencies between
1600 and 1700 em-’, and v(MN) to that at ca. 500 cm-’.
On the other hand, in compounds with M=N-X
bridges in which the ligand X does not display n bonding
to the N atom, v(MN), by analogy with nitrosyl complexes, appears shifted to shorter and v(NX) to longer
wave lengths as a result of vibrational coupling. The decreasing influence of coupling is especially evident in the
series of compounds C13V=N-X,
in which the linear arrangement of the VNX group has been confirmed by crys-
Fig. 9. Structure of the anion [OsNC15]’- with a terminal nitrido ligand.
Frequently, the trans-effect is so strong, that the trans-position is not occupied at all. As a result, in a solution of pentachloro(nitrido)molybdate in CH2CI2, an equilibrium is
observed:
tal structure analyses (X =C1[94’;X = 1 [ ~ >:
~1
[MoNCl,]’CI,V=N-CI
v(VN) 1107
v(NX) 510
[44] Cl,V=N-Br
1032
43 5
[4S] CI,V=N--I
963
390
When X is an organic group, v(MN) appears at even
shorter wavelengths, generally between 1200 cm
and
1300 cm-’; in this case v(NC) is shifted to longer wave-
-’
Angew. Chem. Inr. Ed. Engl. 20. 413-426 (1981)
+ [MoNCIJ + CI-
[45]
In the azido(nitrid0) complex M ~ N ( N ~ ) ~ ( b p y )(Fig.
[ ~ ’ l 7)
the bipyridyl ligand is bonded asymmetrically, since the nitrogen atom in the trans-position to the nitrido ligand can
only bond very weakly to the molybdenum atom. In attempts to prepare the analogous pyridine complex, the
trans-position remains unoccupied and the resulting com42 1
pound, M o N ( N ~ ) ~ ( ~ ~with
) [ ” )coordination number five is
formed (Fig. 8).
In general, the trans-effect has been attributed to electronic influences. According to Grinberg[’*’], an easily polarizable ligand causes polarization at the central atom,
giving rise to an accumulation of negative charge at the
trans-position, and, in consequence, to repulsion of the
trans-ligand’s electron sphere.
Chatt et a1.[1081and O r ~ e l [ ’ ~
have
~ 1 interpreted the transeffect as deriving from a strong 7t bond. As two ligands located in trans-position to each other compete for the same
atom orbitals, increased overlap on one side of the central
atom results in a decrease of electron density and thus a
weakening of the bond on the other side. This interpretation explains the distinct trans-effect of the nitrido ligand,
which is exceedingly strongly bonded by two n bonds in
planes perpendicular to one another. It is however possible to account for the trans-effect with purely steric arguments. This opinion was first expressed by Bright and
Ibers[’06’ and later by the a~thors[~.’~.
Accordingly, the
trans-effect is a consequence of the mutual repulsion of the
ligands. The nitrido atom, which is extremely close to the
central atom, displaces the cis-positioned ligands to the
other side of the central atom. The latter atom is therefore
situated above the plane of the four cis-positioned ligands
in an octahedral coordination. Thus, the sixth coordination site can be occupied only by a weakly bonded ligand
at a greater distance.
In this model it is assumed that the six ligands bonded
to the same central atom already have the minimum distance to each other. If the central atom bonds especially
strongly to one ligand, it is displaced from the center of the
octahedron towards this ligand, thus weakening the bond
to the trans-position.
This concept is supported by the fact that the trans-effect occurs particularly frequently in the sterically unfavorable square planar coordination.
Both ReNCI2(PPh3)2[’lo1,
coordinated in the form of a tetragonal
pyramid,
and
the
octahedral
ReNCl,(PEt,Ph),” ”I, have been investigated crystallographically by Ibers et al., and clearly show the steric influence. In the latter six-coordinate complex the inter-ligand repulsion is so strong, that even the nitrido ligand is
forced away to a bond length of 179 pm. In the five-coordinate complex only two bulky phosphane ligands are
bonded to rhenium. The ReN bond length is 160 pm,
which is the value expected for a triple bond. It is interesting to note, that the analogous tris(tripheny1phosphane)
complex ReNC12(PPh3)3 does not exist, presumably for
similar steric reasons.
6.2. Structures of Mononuclear Nitrido Complexes with
Square Pyramidal and Octahedral Coordination
Nitrido complexes having the stoichiometry [MNX4](M = Mo, Re, Ru, 0 s ; X = F, CI, Br, I, N3) form tetragonal
pyramids with the nitrido ligand at the apex (Fig. 1). In all
cases the metal atom M is situated about 55 pm above the
pyramid’s base. All complexes [MNX4]-, which were investigated crystallographically, have tetraphenylarsonium
ions as cations since this combination was found to produce readily crystallizable products. An interesting crystallographic publication concerning AB types with EPhf; ions
(E = P, As, Sb) should be mentioned here[l”l. The most important geometrical details of the complex ions are specified
in
Table 3, including the
compounds
Table 3. Characteristic bond lengths and angles in selected mononuclear nitrido complexes with tetragonal pyramidal coordination.
Compound
Bond length [prn]
MEN
M-X
Angle [“I
NMX
AsPh[MoNFa]
AsPh,[MoNCLI
AsPhalMoNBr,]
AsPhdMoN(N,),]
MoN(N3)d~y)
A.sPhafReNCl+]
ReN(S>CNEt&
ReNCI2(PPh&
183
I66
I63
163.0
163.5
162
165.6
160.3
AsPhJRuNC14]
AsPh,[OsNCl.,]
AsPhJOsNL]
157.0
160
163
99
101.5
103
99.5
102.2
103.5
107.7
109.7 [a]
98.4 [b]
104.6
104.6
103.7
173
234.5
248.8
206.8
204.3
232.2
239.1
237.7 [a]
244.8 b]
231.0
231.0
266.2
Ref.
[a] X = CI. [b] X = P.
M o N ( N ~ ) ~ ( ~ ~ReN(SZCNEtZ)2[1131
)[”~,
and ReNClZ(PPh3)’,
in which the coordination of the central atom is also approximately tetragonal pyramidal.
It is interesting to compare the structures of
M o N ( N ~ ) ~ ( ~ ~and
) [ ”[~M O N ( N ~ ) ~ ] - [ In
~ ] ] the
.
complex
MoN(N,),(py) (Fig. 8), steric effects cause the azide
groups to arrange so that the free electron pair of the sp’hybridized N, atom points radially away from the nitrido
ligand. The same configuration exists in three of the four
azide groups of [MoN(N3W-. The fourth group, which occupies the site of the pyridine ligand, is oriented in the op-
Table 4. Characteristic bond lengths and angles in selected mononuclear nitrido complexes with octahedral coordination.
Bond length [pm]
M-X,,,
M-X,,,,
Average
Angle I“]
NMX
Ref.
M=N
K2[OsNC15]
AsPhdReN( NCS)S]
ReNCI2(PEt,Ph),
161
166
179
236.2
202. I
245.4 [a]
260.5
230.1
256.3
11061
MoN(NMbpy)
164.2
224.0 [c]
241.9
96.2
96.0
99.2 [a]
92.2 p]
92.0 [c]
IW.6 [d]
Compound
~~
[a] X = CI. p ] X
422
-
I891
11111
PI1
P. [c] X =bpy. [dl X = N,.
Angew. Chem. lnt. Ed. Engl. 20. 413-426 (1981)
posite direction, so that its free electron pair is located in
the vicinity of the nitrido ligand.
Complexes with the coordination number five generally
have the sterically favorable trigonal pyramidal geometry.
In the nitrido complexes, however, the trans-effect of the
nitrido ligand forces the formation of a tetragonal pyramid. In some cases the tetragonal pyramid can be replenished to form an octahedron by an additional, less tightly
bonded ligand (Fig. 9). Some selected complexes of this
kind are given in Table 4. Even in the octahedral complexes, the central atom is situated above the plane of the
cis-positioned ligands. Yet the distance here (about 20 pm)
is smaller than in the tetragonal pyramids.
Except for two examples, the MEN bond lengths range
from 157 to 166 pm. A comparison of the values in Tables
3 and 4 shows that oxidation state and coordination number have practically no effect on the bond lengths, which
correspond to MEN triple bonds. In [ M O N F , ] - ~ ~
the
~ ] ndonor ability of the fluorine ligands accounts for the very
long MoN bond length which is equivalent to a low bond
order, as determined from the low frequency of the MoN
stretching vibration (Section 5).
Steric reasons account for the great length of the ReN
bond in ReNC12(PEt2Ph)$1"1(Section 6.1).
6.3 Structures of Polynuclear
Asymmetrical Nitrido Bridges
As mentioned in Section 2.1, the free electron pair of the
nitrido ligand renders it weakly basic and it can form a
[WNC13.POC13],-2POCI, (386 pm)I9l, but exactly the same
as in (HNMe3)2[W408C14(0H2)4]
. 2 H20, which contains
linear WOW bridges with alternating Wv and W"'
atom~["~l.
Further evidence should be provided by the planned
structural analyses o f complexes of the type (PMe,Ph),C1,ReNMoC14(NCEt)r''61,which exhibit a nitrido bridge
between different metal atoms.
In the trinuclear complex [(Et2NCS2)3Mo=N]2Mo(S~CNE~,),(PF,),"~', the central Mo atom accepts the
free electron pairs of two neighboring Mo=N: groups
forming a slightly bent chain M-N-Mo-N=Mo.
The
bond lengths in the Mo=N-Mo
bridges are, within the
limits of error, the same as in [MoNCl,],"].
The numerous tetrameric cyclic structures are completely planar and approximately square, the edges being
formed by the MEN-M
bridges (Fig. 3). The small deviation of the nitrido bridge from linearity is caused by steric
effects. The van der Waals distances of neighboring N
atoms within the M N eight-membered ring are comparatively short; hence, these atoms are pushed outwards
slightly. Again, the strong trans-effect (see Section 6.1.) directs the most weakly bonded ligand to occupy a position
trans to the nitrido ligand. In [MoNCI,],, a C1 atom of a
neighboring tetramer occupies this position and forms a
very weak C1 bridge. In the POCl, adducts,
[MNCI,. OPC13]4(Fig. 3), the trans-position is occupied by
the oxygen atom of the Lewis base POCI,. The Mo-N
eight-membered ring has the same geometry in the adduct
as in pure [MoNC13],.
e
Q
Table 5. Characteristic bond lengths and angles in selected polynuclear complexes with asymmetric nitrido bridges MzN-M.
Compound
mononuclear
(AsPh&[W,NCIioI
dinuclear
[Et2NCS2)3Mo=N]2
[Mo(S2CNEthI(PF&
tetrameric
[MoNCI],
[MoNCI~.OPCI&
[WNCI3.OPCI,J4.2 P0Cl3
polymeric
[ReNC141,
1K2IReN(NCM. H z O L
Angle ["I
M=N
Bond length [pm]
M-N
M-L
M-Xrra,,,
MsN-M
166
207
231.1
242.6
177
166
213
-
166
166
167
217
216
215
227 [a]
229 [a]
232 [a]
158
248
244
227
213 [c]
153
Ref.
N-M-X,,,
92
[I61
99
98
99
171
181
I91
180
288 [a]
237 PI
231 PI
173
172
171
174
180
[a] X=CI. [b] X=O. [c] X = N .
weak dative bond with another metal atom, resulting in a
linear, highly asymmetrical nitrido bridge. This type of
bridge gives rise to binuclear, trinuclear and tetranuclear
complexes, as well as polymeric structures. The anion
[W2NCllo]2-1161
has hitherto been the only example of the
species [M2YXl0]"- with M=transition metal, Y = O or N
and X = C l or Br, in which an asymmetric bridge is observed. Here, it is assumed that the different oxidation
states of the tungsten atoms are the decisive factor. Participation of the d ' system of the tungsten(v) atom, leading to
Jahn-Teller stabilization, in the [W,NC110]2- ion also
seems to be evident from the W.. .W distance (373 pm).
This is significantly shorter than the W.. .W distance in
Angew. Chem. Int. Ed. Engl. 20. 413-426 (1981)
ReNC14r51and K,[ReN(CN),]- H2Oi6]form polymeric linear chains. Here, the ReN single bonds are very long (248
and 244 pm) in contrast to the other complexes with asymmetrical bridges, since they are located in trans-positions
and are sterically influenced by the ligands in the cis-positions.
It is interesting to note that the MN bond length is evidently not significantly affected by the donating function
of the free electron pair. The M=N bond lengths of the tetrameric complexes in Table 5 are therefore, within the
limits of error, the same as in the compounds with terminal
nitrido ligands (Tables 3 and 4). Extremely short ReN
bond lengths are observed in the polymers [ReNC14],f'51
423
and (K,[ReN(CN)4]. H20)_[61,however, these values have
relatively high standard deviations.
positions
to
each
other.
The
complexes
[ R u ~ N C ~ ~ ( H-,
~ [OO) ~S ]~~N C ~and
, ~ [Ta2NBrIol3]~~
have
D4,,symmetry. However, it is surprising to find a staggered
configuration in the iron complex [Fe(TPP)],N with the
porphyrinato ligands twisted at a torsion angle of 58”.
6.4. Structures of Binuclear Complexes with Symmetrical
‘40
0
Nitrido Bridges M=N=M
’40
6.5. Structures of Complexes with N-Bridges (“Nitrido
0
@
Symmetrical nitrido bridges, M=N=M, are exclusively
Bridges”) M=N-X
and M=%,
X
observed in binuclear complexes. When the bridges are linearly continued to form oligomeric and polymeric chains,
Complexes with the groups M=N-X
or M=N-X
are
e. g. in [(Et2NCSZ)3Mo=N]2Mo(S2CNEtz)3(PF6)3[131
and in
formally derived from amines H,N-X, by substitution of
[ReNC14]_151,the trans-effect prevents formation of symthe two hydrogen atoms by a metal atom. In the case of
metrical nitrido bridges. It is however surprising, that
transition metals in high oxidation states, the free electron
asymmetrical bridges also occur in the tetrameric ring strucpair of the nitrogen atom participates in the MN bond, retures of the type [MoNC~,],[~’,since the trans-effect has no
sulting in a linear MNX-arrangement with a MEN triple
direct influence there. It might be possible-as in the
bond. A bent MNX structure with an sp2-hybridized nitrans-effect-to invoke steric effects to account for the fortrogen atom is only observed when the metal atom attains
mation of an asymmetrical bridge. The MN eight-meman inert gas configuration without the free electron pair of
bered ring is strained, because neighboring N atoms in the
the nitrogen. In contrast to this, corresponding compounds
ring are situated very close to each other. Consequently,
of the main group elements always show a bent arrangethe ring is extended slightly and the nitrido bridge bonds
ment[791.
consequently lengthened. In a symmetrical bridge this
In order to distinguish both types, we have proposed the
lengthening would unfavorably affect z overlap, whereas
terms “imido” and “nitrene” complexes[791
:
in an asymmetrical arrangement it would only influence
e
m
the o bond which is known to tolerate longer bond
M=R,
M=N-X
MsC-X
lengths.
sp2
SP
SP
imido complex
nitrene complex
carbyne complex
In accord with this supposition, an increase in the metalmetal bond lengths of the nitrido bridges from dimeric
In the electrophilic nitrene complexes a nitrene 8-X is
(332-370 pm) to tetrameric (381-384 pm) and polymeric
formally
stabilized by bonding to the transition element.
(397-406 pm) complexes is observed.
This
makes
them formally similar to the carbyne comMetal-metal distances as short as 332 pm (in the iron
plexes.
The
nucleophilic
imido complexes, on the other
complex [Fe(TPP)[2N[’71)are possible in the symmetrical
hand,
are
characterized
by
a free electron pair at the ninitrido bridges of binuclear complexes, since the cis-positrogen
atom.
tioned ligands can move away from both sides of the nitrido ligands. Thus, the metal atoms are located about 2030 pm above the plane of the cis-ligands. The trans-position either remains unoccupied, as in the porphyrinatoiron
complex, or is coordinated by a relatively weakly bonded
ligand. In the ruthenium complex [ R u ~ N C ~ ~ ( H , O ) , ] ~ - [ ” ~ ~
(Table 6), this ligand is a water molecule, and in
[Ta,NBr,o]3-[141a bromine atom, with a TaBr bond 23 pm
longer than the bonds of the cis-ligands.
The metal-nitrogen bond lengths (Table 6) in the symmetrical nitrido bridges are, in view of their double bond
character, longer than those in terminal nitrido ligands
expected
(Table 3). They correspond well to the
Fig. 10. Detail from the structure of the cis-bis(diethy1dithiocarbamafor double bonds. In agreement with the conception of the
to)bis(phenylimido)molybdenum complex Mo(NPh)z(SzCNEt2)2with bent
(d,-p,-d,)-x overlap (Section 2.3.) in the nitrido bridge, the
imido and linear nitrene geometry. For the sake of clarity, only the S atoms
of the diethyldithiocarbamato ligands are shown.
cis-ligands of the two metal atoms are arranged in eclipsed
%-
rn
%-
Table 6. Characteristic bond lengths and angles in binuclear complexes with symmetrical nitrido bridges M=N=M.
Compound
K,RuzNCls(OHz)z
(NH~),RU~NCMOH~)Z
CsrOszNC1,o
(NH413Ta2NBrtO
OszN(SZCNMe2)s
ffTpP)Fel~N
M=N
M-X,,
172.0
172.5
177.8
184.9
166.1
176
236 [a]
238 [a]
237.1
25 1.4
240
199.1 [c]
Bond length [pm]
M-X,,,,,
218 PI
220 To1
243.4
273.8
242
-
Angle [“I
Ref.
M-N=M
N=M-X,,,
180
180
180
180
165
180
94.7
95
93.7
94.1
98
99.2
[a] X = CI. b]X = 0. Ic] X = N.
424
Angew. Chem. lnf. Ed. Engl. 20, 413-426 (1981)
Complexes with a bent “imido” geometry have hitherto
been rarely observed; in addition, the expected bond angle
of 120” for spz-hybridization is significantly exceeded,
whereas in compounds of main group elements, the corresponding angle is 120” or usually even smaller.
An example of an imido transition metal complex is the
molybdenum compound MoOC1z(NH)(OPEtPh,),’z81 in
which the angle is 157”. The complexes cisM O ( N P ~ ) ~ ( S ~ C N E ~ (Figure
~ ) ~ [ ~ ’ ~ 10) and
OsOz[NC(CH3)3]2[1211
each contain a bent and a linear MNR
group (see Table 7).
Table 7. Characteristic bond lengths and angles of selected imido and nitrene
complexes M-N-X.
Compounds
Bond length [pm]
MN
N-X
CI,VNCI (solid)
CI,VNCi (gas)
C13VNCI(SbCIS)2
Cl,(bpy)VNCI
CI,VNI
CI3PO(CI4)MoNC2CIS
AsPh,[CISWNC2C15f
164.2
165.1
165.5
168.8
165
169.2
168.4
175.4
c~s-Mo(NP~)~(S~CNE~~)~
178.9
MoOC12(NH)(OPEtPh2)2
170
Os03(N-Adamantyl)
169.7
{
Os02INC(CH,h12
RuCI~(NPEtzPh)(PEtzPh),
[NbCIXN PPh3)h
ITaCL(N PPh,)l2
{ ::::;
184.1
177.6
180.1
158.8
159.7
160.3
158.5
193
145
147
138.6
139.2
92
144.8
143
145
158.6
163.7
159.3
Angle I”]
MNX
Ref.
175.2
170
179.5
175.0
163
171.8
163
169.4
139.4)
157
171.4
155.1
174.9
171.1
176.8
The metal-nitrogen distance in nitrene complexes is consistent with the value expected for a triple bond. A lengthening of the MN bond in the linear form MEN-X
is only
observed in the latter cases, in which there is a large supply
of electrons and one of the two MNX groups is bent.
It is interesting to consider the NX bond lengths in the
nitrene complexes more closely. On account of sp-hybridization of the nitrogen atom, the bonds are always significantly
shorter
than
in
sp3-hybridization.
In
C13V=N-C1[941 and C13V=N-IL9” (Fig. 6), for example,
the shortest hitherto measured N-Cl
and N-I
bond
lengths have been observed (Table 7): the values being respectively ca. 17 and 22 pm smaller than in halogen amines. Approximately the same difference is found in C-CI
bonds in halogen alkanes and alkynes.
Apart from halogens, organic groups and amino groups,
the substituent, X, in nitrene complexes can also be an organophosphorus group. The resulting phosphaneiminato
complexes also exhibit an almost linear configuration of
the M=N=PR3 group (see Fig. 5), in which the following
mesomeric structures can be assumed on account of the
observed bond lengths:
20
0
M=N-PR,
e
0
c*
o
M=N=PR,
In all cases the MNP unit is slightly bent, the bond angle
being about 174”. This can be explained by pn-d, overlap
in the NP bond, since in the tetrahedral coordination of
the phosphorus only the less favorable orbitals d,. or dX2-,,>
can be utilized for the n bondizo1.
Angew. Chem. Inr. Ed. Engl. 20. 413-426 (1981)
7. Future Prospects
Since the fundamental investigations of G o ~ b e a d ’ ~ ~ ~ ,
who suggested that multiple bonds between two elements
outside the first row of eight elements can occur under certain conditions, many examples have been found experimentally. Transition metals can form multiple bonds with
each other, as well as with carbon, nitrogen and oxygen.
The still new field of metal-nitrogen multiple bonds is rapidly expanding. Apart from the attractive structural chemistry and the remarkable bonding of these compounds,
considerable stimulus has arisen from attempts to develop
models of N,-assimilation. Starting with the fixation of a
Nz molecule at a molybdenum center, the formation of the
nitridomolybdenum group is an important step towards
the production of ammonia in which the nitrogen is transformed into a form utilizable by plants. However, much
still has to be done to clarify the complex paths of such sequences of reactions and to simulate them in the laboratory.
We thank the Verband der Chemischen Zndustrie and the
Deutsche Forschungsgemeinschaftfor their generous financial support of our investigations. We also wish to extend our
thanks to the many co-workers who have contributed signzficantly to thisfield.
Received: December 1, 1980 [A 365 IE]
German version: Angew. Chem. 93, 451 (1981)
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