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Transcript
The 18-electron rule and its exceptions
(1) Octahedral complexes à can be split into 3 categories:
category
A
B
C
number of electrons
18
12-18
12-22
description
18-electron rule obeyed
18-electrons not exceeded
18-electron rule disobeyed
A. Those that obey the 18 electron rule
Complexes with strong π-acceptor ligands
(e.g. [V(CO)6]-, [Cr(CO)6], [W(CO)6], [Mn(CO)6]+)
- t2g strongly bonding ∴filled
- eg strongly antibonding (due to synergic bonding) ∴empty
t 1u*
4p (t1u )
a1g*
4s (a1g )
strongly anti-bonding
(empty)
t2g*
π* orbitals of CO (t2g)
e g*
large
∆O
3d (e g + t2g)
strongly bonding
(filled)
t2g
(a1g + t1u + e g)
eg
Cr
6 CO
t1u
a1g
- Complexes of strong π-acceptor ligands à tend to obey the 18-electron rule
irrespective of their coordination number (e.g. [Fe(CO)5], [Fe(CO)4]2-,
[Ni(PPh3)4]).
- Note: d8 and d10 configurations are exceptions to this rule (see later).
B. Octahedral complexes with 12-18 electrons
2nd and 3 rd row TM complexes (high in the spectrochemical series of metal ions)
with σ-donor or π-donor ligands (low to medium in the spectrochemical series)
- t2g non-bonding or weakly anti-bonding (because the ligands are either σ-donors
or π-donors) ∴ t2g can contain from 0 to 6 electrons
- eg fairly strongly antibonding (because 2nd/3rd row TMs bond more effectively to
the ligands)∴eg empty
e.g. [ZrF6]2- (Zr4+, d0, 12 e), [PtF 6]2- (Pt4+, d6, 18 e), [OsCl6]2- (Os4+, d4, 16 e),
[WMe6] (W6+, d0, 12 e), [Zr(OH2)6]3+ (Zr3+, d1, 13 e)
Octahedral, σ-donor ligands (e.g. [WMe6])
t1u*
4p (t1u)
a1g*
eg* is reasonably
antibonding for a 2nd or
3rd row transition metal
4s (a1g )
eg *
moderate ∆o for 2nd or 3rd
row transition metal
∆O
t 2g non-bonding
3d (e g + t2g)
t2g
(a 1g + t1u + eg)
eg
W
t 1u
a 1g
6 Me
C. Octahedral complexes with 12-22 electrons
1st row TM complexes (low in the spectrochemical series of metal ions) with
σ-donor or π-donor ligands (low to medium in the spectrochemical series)
= 1st row transition metal coordination complexes {e.g. [TiF 6]2- (Ti4+, d0, 12 e),
[Co(NH3)6]3+ (Co3+, d6, 18 e), [Cu(OH2)6]2+ (Cu2+, d9, 21 e)}
- t2g non-bonding or weakly anti-bonding (because the ligands are either σ-donors
or π-donors) ∴t2g can contain from 0 to 6 electrons
- eg only weakly antibonding (because 1st row TMs don’t bond as effectively to the
ligands) ∴eg can contain from 0 to 4 electrons
Octahedral, π-donor ligands (e.g. [Cu(OH2)6]2+)
t1u*
4p (t1u )
a1g *
eg* is only moderately
antibonding for a
1st row transition metal
4s (a1g)
eg*
small ∆o
t2g only weakly
anti-bonding
3d (eg + t 2g )
t2g *
π orbitals (t2g )
t2g
(a1g + t1u + eg)
eg
Cu
t1u
a1g
6 OH2
Tetrahedral Complexes
•
Tetrahedral complexes cannot exceed 18 electrons because there are no low
lying MOs that can be filled to obtain tetrahedral complexes with >18 electrons.
In addition, a transition metal complex with the maximum of 10 d-electrons,
will receive 8 electrons from the ligands à a total of 18 electrons.
•
∆t is small (~4/9 ∆o), so there is no particular preference for the e or t2 orbitals
to be filled (can have 8-18 electrons) – similar to class C octahedral complexes.
Tetrahedral - e.g. [Ni(PPh3)4] (Ni0, d10, 18-electron complex)
t2*
4p (t1u)
a1*
4s (a1g)
t2*
∆t
∆t ~ 4/9 ∆o
3d (eg + t2g)
e
(a1g + t 1u + eg)
a1
4 PPh3
Ni
t2
(2) Square Planar complexes (d8, 16 electrons)
d x2-y2
• d8-metals with 4 ligands, so 16-electron complexes
large ∆E
• common with metals and ligands high in the spectrochemical
series
d xy
d z2
I
I
II
II
III
- Rh , Ir , Pd , Pt , Au almost always square planar
All are very common oxidation states except AuIII which is quite
oxidising.
d xz, d yz
- NiII can be square planar, but only with strong π-acceptor ligands (because 1st
row TMs are lower in the spectrochemical series than 2nd or 3rd row TMs).
- CoI is almost never square planar because it is a first row transition metal in a
low oxidation state (very low in the spectrochemical series for metals).
- d8 CuIII and AgIII complexes are extremely rare and highly oxidising.
- d8 Ru0 and Os 0 are not square planar (2nd or 3rd row TMs but low in the
spectrochemical series of metal ions because their oxidation state is zero).
- d9 complexes are sometimes square planar (e.g. [CuII(py)4]2+) – this may be
considered an extreme form of Jahn-Teller distortion.
EXAMPLES:
Ph3P
Ph3P
Ph3P
OC
RhI
Ir I
PPh3
Cl
CO
Cl
Ph 2
P
P
Ph 2
Cl
Cl
PtII
PtII
NC
NC
Cl
Cl
Cl
Cl
2-
CN
CN
NiII
Cl
Cl
2-
2-
NiII
Cl
Cl
CO
-
CO
CN
OC
Fe
CO
CO
CO
OC
Mn
CO
CO
NC
CO
OC
Co I
CN
2-
(3) Linear complexes (d10, 14 electrons)
p
small ∆E
• d -metals with 2 ligands, so 14-electron complexes
10
s
• Common for AgI, AuI and HgII
small ∆E
• Less common for Cu , Zn and Cd
I
II
II
d
Explanation:
• For d 10 complexes, there is a relatively small energy difference between the d, s
and p orbitals (e.g. 5d, 6s and 6p for AuI).
• This permits extensive hybridization between the dz2, s and p z orbitals as shown
below:
Step 1
Step 2
+
filled d z2
+
+
empty s
filled
sd-hybrid
empty
sd-hybrid
+
Overall:
filled dz2
empty
sd-hybrid
+
empty p
filled
sd-hybrid
empty
+
+
+
empty s
empty
empty p
empty
empty
orbitals suitable for
forming a pair of
linear covalent bonds
• More common for group 11 (Cu, Ag, Au) than group 12 (Zn, Cd, Hg) because
the energy difference between the d, s and p-orbitals is smaller for group 11.
• More common for the heavier elements (AgI, AuI, HgII).
• Examples: [Ag(CN)2]-, [Ag(NH3)2]+, [Cu(NH3 )2]+, [(R3P)AuCl], [HgMe2 ],
[CdMe2], [ZnMe2 ]
• However, there are also lots of tetrahedral complexes of AgI, AuI, CuI, ZnII, Cd II and
HgII (e.g. 14 e- linear [(R3P)AuCl] + 2 PR3 ßà 18 e- tetrahedral [(R3P)3AuCl]).
Distribution of coordination numbers for crystallographically characterized CuI,
AgI, and AuI compounds as found in the Cambridge Structural Database (2003).
S. Alvarez et al., JACS, 2004, 1465-1477
(4) Steric Effects and Early Transition Metal Compounds
• Steric effects can produce low-coordinate (not many ligands) complexes which
often have <18 electrons.
Me3Si
SiMe 3
Si
Me3Si
ScIII
N
N
Me3Si
SiMe3
N
Cl
SiMe 3
MII
SiMe3
Si
SiMe3
d0
M = Cr2+
M = Mn 2+
M = Fe2+
6 electrons
SiMe3
SiMe 3
Me3Si
M = Sc3+
SiMe3
d4
d5
d6
10 electrons
11 electrons
12 electrons
• For early transition metals (e.g. with d0 metals) it is often not possible to fit the
number of ligands necessary to reach 18 electrons around the metal.
CO
Me
Me
Me
WVI
Me
OC
Me
OC
V0
CO
CO
Me
CO
W6+ d0 12 electrons
V0 d5 17 electrons
Sc III
Cl
PMe3
Sc3+ d 0 16 electrons
(5) Strong oxidants or reductants
• Many 18 electron complexes can be reduced or oxidised to give 17 or 19
electron complexes. Such compounds are often good oxidising or reducing
agents (i.e. they want to get back to being 18-electron compounds).
- eFeII
FeII,
FeIII
+ e-
Ferrocene
d6, 18 electrons
Ferrocenium cation
FeIII, d5, 17 electrons
Orange, diamagnetic
Perfectly Happy !
Dark blue, paramagnetic
Oxidant
+ eCoIII
CoII
- e-
Cobaltocenium cation
CoIII , d6, 18 electrons
Co , d7, 19 electrons
Yellow, diamagnetic
Dark purple, paramagnetic
Perfectly Happy !
Strong Reductant
Cobaltocene
II