Download 1 5.03, Inorganic Chemistry Prof. Daniel G. Nocera Lecture 9 May 11

5.03, Inorganic Chemistry
Prof. Daniel G. Nocera
Lecture 9 May 11: Bimetallic and Cluster Complexes
Metal-metal bonding is common for metals in low oxidation states, and generally
increases in strength along the series 3d << 4d < 5d. There are limiting forms of
metal-metal bonding depending on d-orbital occupation. Usually d1 and d2 metals do
not form complexes with unsupported (i.e. no bridging ligands) metal-metal bonds.
Triple and quadruple metal-metal bonds are formed between d3 and d4 metals,
respectively. Single metal-metal bonds are formed between d7 and d9 metals.
Polynuclear clusters can support a wide variety of metal d-electron counts.
Single Metal-Metal Bonded Complexes
The paradigmatic single single metal-metal bonded complex is M2(CO)10 (M = Mn(0),
Re(0)). The general features of this compound are:
The dimers are formed from d7 metals. The d electron count is better described as
(d6)d1–d1(d6). From this formalism, we see that the metal-metal bond is formed
from one electron that is donated from each metal and that the coordination
geometry about the metal should be octahedral, as is typically the case for d6
metals. Here, one of the octahedral coordination sites is occupied by a metal-metal
bond. The (d6)d1–d1(d6) formalism also suggests that the compound is diamagnetic,
which is also the case.
A more quantitative description is given by the molecular orbital diagram, which is
formed from the dimerization of two M(CO)5fragments.
We can determine the electronic structure of the M(CO)5 fragment by beginning
with an octahedral M(CO)6 complex and removing a CO ligand. We know that the Oh
complex has a t2g-eg electronic structure format. What happens to the d orbitals
when one CO ligand is removed from the z axis?
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Note that the dz2 orbital is singly occupied and coordinatively unsaturated. Thus the
Mn(CO)5 fragmant is the “methyl” radical of inorganic chemistry. With regard to the
isolobal analogy:
CH3
Mn(CO)5
Dimerizing •Mn(CO)5 fragments,
A diamagnetic complex formed from the
dimerization of two metallic radicals
This orbital is cylindrically
symmetric, thus no electronic
preference for staggered vs
eclipsed—so steric factors
dominate and system is
staggered
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The energetic stabilization of the e– in dz2 by dimerization is the driving force for
metal-metal bond formation. As shown in the table below, the MM σ-σ* splitting
increases along the series Mn < Tc < Re owing to the increase in SML as a result of
the increased radial extension of the dz2 orbitals. This increase in the SMM of the dz2
orbitals is reflected in an increased metal-metal bond strength.
E(σ-σ*) / cm–1
Complex
BDE / kcal mol–1
Mn2(CO)10
29740
37
Tc2(CO)10
32400
45
Re2(CO)10
32800
53
MnRe(CO)10
31950
–
This MO diagram rationalizes the photochemistry of M2(CO)10 complexes. Excitation
of the σ-σ* transition will result in the cleavage of the metal-metal bond (since the
excited state born order is 0). Thus, the photochemistry is a radical based one:
•Mn(CO)5
2 •Mn(CO)5
therefore
Mn2(CO)10
Mn2(CO)10 + Re2(CO)10
Fe(CO)5 + Re2(CO)10
h
CCl4
2 Mn(CO)5Cl
2 MnRe(CO)10
(CO)5ReFe(CO)4Re(CO)5
Cluster Formation
As mentioned above, odd electron occupancy of the eg orbital (Oh) of a M(CO)6
complex prompts ligand loss in order to stabilize the dz2 orbital. Further stabilization
occurs by metal-metal single bond formation. We can take this argument to a limit
to explain cluster formation from metals across the periodic table. The carbonyl
clusters of the first row transition metals have the following compositions and
structures:
Group 6:
M(CO)6
Group 7:
M2(CO)10
Group 8:
M3(CO)12
Group 9:
M4(CO)12
Group 10:
M(CO)4
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Group
6
7
8
9
10
–
Ni(CO)4
Cr(CO)6
Mn2(CO)10
Fe3(CO)12
Mo(CO)6
Tc2(CO)10
Ru3(CO)12
Rh4(CO)12
–
W(CO)6
Re2(CO)10
Os3(CO)12
Ir4(CO)12
–
eg
t2g
bury d6
no e– in σ*
stable
bury d6
1e– in σ*
lose 1 CO
1 M–M
bury d6
2e– in σ*
lose 2 CO
2 M–M
bury d6
3e– in σ*
lose 3 CO
3 M–M
filled d-orbitals
no electronic
stabilization
maximum stabilization afforded
by metal-metal σ bond formation
In each case, the clusters assume an octahedral coordination as a result of burying 6 d
electrons in what is formally t2g orbitals. The system loses the number of CO’s that is
equivalent to the number of e–s in M-Lσ*. This permits maximum M-M bond formation
and thus maximum stabilization.
Metal-Metal Bonding: Quadruple Bonds
This class of compounds was discovered on the 4th floor of Building 6 with the
determination of the X-ray crystal structure of Re2Cl82– in 1963.
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The dimers are formed from d4 metals (usually Cr(II), Mo(II), W(II), Re(III)). The
compounds have the shortest metal-metal bonds in inorganic chemistry; this is not
surprising as there is a formal quadruple bond. The complexes are diamagnetic and
they are always eclipsed. As depicted above, the MO strategy is to be to correlate
to ReCl63– (Oh), remove two axial Cl– ligands to give square planar ReCl4– (D4h)
fragment and then dimerize.
Accounting for the π-donating properties of Cl–, the correlation diagram for ReCl63– (Oh)
and ReCl4– (D4h) is
The ReCl4– fragments can now be dimerized to furnish the Re2Cl82– quadruple bond.
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The MO diagram is essentially that of M2L10 in form. The differences between M2L10
and M2L8 MO diagrams are due to M–M separation—the larger splittings of σ,π and δ
bonds arises from the shorter M–M distance. This electronic structure accounts for:
•
the quadruple bond results from σ2π4δ2 configuration
•
the eclipsed structure a result of the alignment of the dxy orbitals to form the
quadruple bond
•
the ground state is diamagnetic
•
lowest energy transition is δ → δ*
The δ → δ* transition is spectroscopically anomalous. Though a fully allowed
transition, its intensity is weak (ε ~ 103 M–1 cm–1 when it is expected to be
104 – 105 M–1 cm–1). Also there is a large shift in the transition energy upon
oxidation:
λmax (δ→δ*) for Mo2Cl4 (PMe3)4 = 17094 cm–1 [d(M-M) = 2.13 Å]
λmax (δ→δ*) for Mo2Cl4 (PMe3)4+ = 6350 cm–1 [d(M-M) = 2.17 Å]
How do we account for these observations ?
small 1e– (MO splitting) energy
large 2e- (e–-e– repulsion) energy
This problem is treated in a 5.37 lab module.
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