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Principal mechanisms of ligand exchange in octahedral complexes
Dissociative
Associative
Dissociative pathway
(5-coordinated intermediate)
MOST COMMON
Associative pathway
(7-coordinated intermediate)
Experimental evidence for dissociative mechanisms
Rate is independent of the nature of L
Experimental evidence for dissociative mechanisms
Rate is dependent on the nature of L
Inert and labile complexes
Some common thermodynamic and kinetic profiles
Exothermic
(favored, large K)
Large Ea, slow reaction
Exothermic
(favored, large K)
Large Ea, slow reaction
Stable intermediate
Endothermic
(disfavored, small K)
Small Ea, fast reaction
Labile or inert?
L
L
L
M
L
L
Ea
L
L
L
L
M
L
L
M
L
L
L
X
L
X
G
LFAE = LFSE(sq pyr) - LFSE(oct)
Why are some configurations inert and some are labile?
Inert !
Substitution reactions in square-planar complexes
the trans effect
L
X
M
T
L
+X, -Y
L
Y
M
T
(the ability of T to labilize X)
L
Synthetic applications
of the trans effect
Cl- > NH3, py
Mechanisms of ligand exchange reactions in square planar complexes
L
L
X
L
S
+S
M
L
L
M
X
L
+Y
-X
Y
L
L
L
-d[ML3X]/dt = (ks + ky [Y]) [ML3X]
M
X
L
L
M
S
L
+Y
Y
L
-X
L
L
L
L
M
Y
-S
L
M
S
Electron transfer (redox) reactions
-1e (oxidation)
M1(x+)Ln + M2(y+)L’n
M1(x +1)+Ln + M2(y-1)+L’n
+1e (reduction)
Very fast reactions (much faster than ligand exchange)
May involve ligand exchange or not
Very important in biological processes (metalloenzymes)
Outer sphere mechanism
[Fe(CN)6]3- + [IrCl6]3-
[Fe(CN)6]4- + [IrCl6]2-
[Co(NH3)5Cl]+ + [Ru(NH3)6]3+
[Co(NH3)5Cl]2+ + [Ru(NH3)6]2+
Reactions ca. 100 times faster
than ligand exchange
(coordination spheres remain the same)
A
B
"solvent cage"
r = k [A][B]
Ea
Tunneling
mechanism
A
+
B
A'
G
+
B'
Inner sphere mechanism
[Co(NH3)5Cl)]2+ + [Cr(H2O)6]2+
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+
[CoIII(NH3)5(m-Cl)CrII(H2O)6]4+
[CoII(NH3)5(m-Cl)CrIII(H2O)6]4+
[CoII(NH3)5(H2O)]2+
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+
[CoIII(NH3)5(m-Cl)CrII(H2O)6]4+
[CoII(NH3)5(m-Cl)CrIII(H2O)6]4+
[CoII(NH3)5(H2O)]2+ + [CrIII(H2O)5Cl]2+
[Co(H2O)6]2+ + 5NH4+
Inner sphere mechanism
Ox-X + Red
k1
Ox-X-Red
k2
Reactions much faster
than outer sphere electron transfer
(bridging ligand often exchanged)
k3
k4
Ox(H2O)- + Red-X+
Ox-X-Red
Tunneling
through bridge
mechanism
r = k’ [Ox-X][Red] k’ = (k1k3/k2 + k3)
Ea
Ox-X
+
Red
Ox(H2O) - + Red-X +
G
Brooklyn College
Chem 76/76.1/710G Advanced Inorganic Chemistry
(Spring 2008)
Unit 6
Organometallic Chemistry
Part 1
General Principles
Suggested reading:
Miessler/Tarr Chapters 13 and 14
Elements of organometallic chemistry
Complexes containing M-C bonds
Complexes with p-acceptor ligands
Chemistry of lower oxidation states very important
Soft-soft interactions very common
Diamagnetic complexes dominant
Catalytic applications
The d-block transition metals
Group
4
5
6
7
8
9
10
11
3d row
4d row
5d row
Ti
Zr
Hf
V
Nb
Ta
Cr
Mo
W
Mn
Tc
Re
Fe
Ru
Os
Co
Rh
Ir
Ni
Pd
Pt
Cu
Ag
Au
8
7
6
5
4
3
2
1
9
8
7
6
5
4
3
2
10
9
8
7
6
5
6
3
10
9
8
7
6
5
4
dn
0
I
II
III
IV
V
VI
VII
4
3
2
1
0
5
4
3
2
1
0
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
Main types of common ligands
Ligand
F. C.
#e (A)
#e (B)
# CS
X
L
XL
XX
LL
XLL
LLL
-1
0
-1
-2
0
-1
0
2
2
4
4
4
6
6
1
2
3
2
4
5
6
1
1
2
2
2
3
3
A simple classification of the most important ligands
X
L
L2
L2X
L3
Counting electrons
Method A
Method B
Determine formal oxidation state of metal
Deduce number of d electrons
Ignore formal oxidation state of metal
Count number of d electrons for M(0)
Add d electrons + ligand electrons (A)
Add d electrons + ligand electrons (B)
The end result will be the same
Why is this relevant?
Stable mononuclear diamagnetic complexes
generally contain 18 or 16 electrons
The reactions of such complexes
generally proceed through 18- or 16-electron intermediates
Although many exceptions can be found, these are very useful practical rules
for predicting structural and reactivity properties
C. A. Tollman, Chem. Soc. Rev. 1972, 1, 337.
Why 18 electrons?
antibonding
Organometallic complexes
18-e most stable
16-e stable (preferred for Rh(I), Ir(I), Pt(II), Pd(II))
<16-e OK but usually very reactive
> 18-e possible but rare
generally unstable
A closer look at some important ligands
Typical -donor ligands
Hydride:
M-H (terminal) is a -1, 2e (1e) ligand
Halide:
M-Cl (terminal) is a -1, 2e (1e) ligand
Alkyl:
M-CH3 (terminal) is a -1, 2e (1e) ligand
Alkoxide: M-OR (terminal) is a -1, 2e (1e) ligand
Thiolate: M-SR (terminal) is a -1, 2e (1e) ligand
Amide:
M-NR (terminal) is a -1, 2e (1e) ligand
Phosphide: M-PR2 (terminal) is a -1, 2e (1e) ligand
H
M
M
Cl
M
M
H3
C
M
M
R
O
M
M
R
S
M
M
R
N
M
M
R2
P
M
M
(m-bridging) is a -1, 2e (1e) ligand
(m-bridging) is a -1, 4e (3e) ligand
(m-bridging) is a -1, 2e (1e) ligand
(m-bridging) is a -1, 4e (3e) ligand
(m-bridging) is a -1, 4e (3e) ligand
(m-bridging) is a -1, 4e (3e) ligand
(m-bridging) is a -1, 4e (3e) ligand
Other important C-donor ligands
M
M
M
terminal, 1-aryl, alkenyl, alkynyl, -1, 2e (1e)
M
M
M'
M'
bridging, m2-alkenyl, alkynyl, -1, 4e (3e)
M
or 1-allyl -1, 2e (1e)
M
por 3-allyl -1, 4e (3e)
Other important ligands
M
M
M
M
2- (2e)
4-diene, 4e
4- (4e)
6- (6e) arene
M
M
1-Cp -1, 2e (1e)
5- Cp -1, 6e (5e)
O
M
M
M
M
C
2-alkene or alkyne, 2e
N
O
C
M
M
N
C
2- / side-bonded and 1- / end-bonded
aldehyde/ketone, 2e
imine, 2e
C
Other important ligands
M
CR2
M
Fischer carbene, 2e (2e)
Schrock carbene, -2, 4e(2e)
M
O
M
C
O
M
Fischer carbyne, 4e (3e)
Schrock carbyne, -3, 6e(3e)
M
Oxo, -2, 4e (2e)
NR
M
imido, -2, 4e (2e)
N
N
CR
M
N
nitrido, -3, 6e (3e)
N
O
M
N
O
carbonyl, 2e
M
NR3
amine, 2e
dinitrogen, 2e
M
PR3
phosphine, 2e
linear nitrosyl
+1, 2e (3e)
M
AsR3
arsine, 2e
bent nitrosyl
-1, 2e (1e)
M
SbR3
stibine, 2e
The M-L-X game
Group
4
5
6
7
8
9
10
3d row
Ti
V
Cr
Mn
Fe
Co
Ni
4d row
5d row
Zr
Hf
Nb
Ta
Mo
W
Tc
Re
Ru
Os
Rh
Ir
Pd
Pt
Neutral stable compounds
0
I
II
III
IV
V
ML7
ML6
MXL 6
MX2L6
MXL 5
MX2L5
MX3L4 (16e)
MX4L4 (16e)
ML5
ML4
MXL 3 (16e)
MX2L4
MX3L4
MX4L3 (16e)
MX2L2 (16e)
MX3L3
MX4L3
MX5L2 (16e)
Each X will increase the oxidation number of metal by +1.
Each L and X will supply 2 electrons to the electron count.
MX4L2
Group
4
5
6
7
8
9
10
3d row
Ti
V
Cr
Mn
Fe
Co
Ni
4d row
5d row
Zr
Hf
Nb
Ta
Mo
W
Tc
Re
Ru
Os
Rh
Ir
Pd
Pt
Stable monocationic compounds
0
I
II
III
IV
V
Now looking at compounds having a charge of +1 to obey 18 e rule.
Elec count: 4 (M) +2 (NO) +12 (L6) = 18
Group 4
5
6
7
8
9
10
3d row Ti
V
Cr
Mn
Fe
Co
Ni
4d row Zr
5d row Hf
Nb
Ta
Mo
W
Tc
Re
Ru
Os
Rh
Ir
Pd
Pt
Stable monocationic compounds
[M(NO)L6]+
0
[M(NO)L5]+
[ML6]+ (16e)
I
[MXL7]+
II
IV
[ML6]+
[MXL6]+
[MX2L5]+ (16e)
III
[M(NO)L4]+
[MX3L5,6]+
[ML4]+ (16e)
[MXL5]+
[MX2L5]+
[MX3L4]+ (16e)
MX2L2 (16e)
[MX2L4]+
[MX3L4]+
[MX4L3]+ (16e)
V
NO+ is isoelectronic to CO
X increases O N by 1
ML4
Elec Count: 4 (M) + 4 (L2) + 10 (L5)
MX4L2
Actors and spectators
Actor ligands are those that dissociate or undergo a chemical
transformation
(where the chemistry takes place!)
Spectator ligands remain unchanged during chemical
transformations
They provide solubility, stability, electronic and steric influence
(where ligand design is !)
Organometallic Chemistry
1.2 Fundamental Reactions
Fundamental reaction of organo-transition metal complexes
Reaction
(FOS) (CN) (NVE)
Association-Dissociation of Lewis acids
0
±1
0
Association-Dissociation of Lewis bases
0
±1
±2
Oxidative addition-Reductive elimination
±2
±2
±2
0
0
0
Insertion-deinsertion
FOS: Formal Oxidation State;
CN: Coordination Number
NVE: Number of valence electrons
Association-Dissociation of Lewis acids
(FOS) = 0; (CN) = ± 1; (NVE) = 0
Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2
H
H
+ BF3
W:
H
BF3
W
H
This shows that a metal complex may act as a Lewis base
The resulting bonds are weak and these complexes are called adducts
Association-Dissociation of Lewis bases
(FOS) = 0; (CN) = ± 1; (NVE) = ±2
A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…)
in this case the metal is the Lewis acid
HCo(CO) 4
HCo(CO) 3 + CO
Crucial step in many ligand exchange reactions
For 18-e complexes, only dissociation is possible
For <18-e complexes both dissociation and association are possible
but the more unsaturated a complex is, the less it will tend to dissociate a ligand
Oxidative addition-reductive elimination
(FOS) = ±2; (CN) = ± 2; (NVE) = ±2
Mn+ + X-Y
M(n+2)+
X
Y
H
Ph3P
Cl
I
Ir
CO
PPh3
Vaska’s compound
+ H2
Ph3P
IrIII
Cl
H
PPh3
CO
Very important in activation of hydrogen
Oxidative addition-reductive elimination
H becomes H-
Concerted reaction
H
Ph3P
CO
IrI
Cl
+ H2
Cl
IrI
PPh3
H
H
M
PPh3
via
H
CO
Ir: Group 9
SN2 displacement
CO
IrIII
Cl
PPh3
Vaska’s
compound
Ph3P
Ph3P
+ CH3I
cis addition
CH3+ has become CH3+
CH3
Ph3P
IrIII
Cl
CO
PPh3
I-
CH3
Ph3P
IrIII
Cl
CO
PPh3
I
trans addition
Also radical mechanisms possible
Oxidative addition-reductive elimination
Mn+ + X-Y
M(n+2)+
X
Y
Not always reversible
Mn+ + R-X
Mn+ + R-H
M(n+2)+
X
R
M(n+2)+
H
R
Insertion-deinsertion
(FOS) = 0; (CN) = 0; (NVE) = 0
M-X + L
(CO)5Mn-CH3 + CO
M-L-X
O
(CO)5Mn-C-CH 3
Mn: Group 7
Very important in catalytic C-C bond forming reactions
(polymerization, hydroformylation)
Also known as migratory insertion for mechanistic reasons
Migratory Insertion
CH3
OC
CO
+ CO
CO
Mn
OC
OC
O
C
CH3
Mn
CO
OC
CO
CO
CO
k1
k2
O
OC
+ CO
C
CH3
Mn
OC
CO
CO
Also promoted by including bulky ligands in initial complex
Insertion-deinsertion
The special case of 1,2-addition/-H elimination
R2C
LnM
CR'2
H
LnM
R2
C  H
 C
R'2
A key step in catalytic isomerization & hydrogenation of alkenes
or in decomposition of metal-alkyls
Also an initiation step in polymerization
Attack on coordinated ligands
Nu- Favored for electron-poor complexes
(cationic, e-withdrawing ligands)
M
L
E+
Favored for electron-rich complexes
(anionic, low O.S., good donor
ligands)
Very important in catalytic applications and organic synthesis
Some examples of attack on coordinated ligands
Electrophilic addition
Nucleophilic addition
Cl
Pt
py
py
Cl
Et
Pt
Cl
py
O
O
+
N
Cl
Et3O+
+
Fe(CO)3
Fe(CO)3
Electrophilic abstraction
Nucleophilic abstraction
Cp
Cp
+
Ta
CH3
CH3
Cp
Me3PCH2
+ Me4P+
Ta
Cp
Cp
CH2
CH3
Fe
OC
OC
OH-
OH
Cp
Fe
OC
OC
+
OH2
-H2O
Cp
Fe
OC
OC