Bimetallic and Cluster Complexes Metal-Metal Bonding Is

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. 3 4 Triple and quadruple metal-metal bonds are formed between d and d 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?

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:

CH Mn(CO)5 3

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

– 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.

Complex E(σ-σ*) / cm–1 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:

h •Mn(CO)5 2 •Mn(CO)5 therefore Mn2(CO)10 2 Mn(CO)5Cl CCl4

Mn2(CO)10 + Re2(CO)10 2 MnRe(CO)10

Fe(CO)5 + Re2(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: Group 7: Group 8:

M(CO)6 M2(CO)10 M3(CO)12

Group 9: Group 10: M4(CO)12 M(CO)4

Group 6 7 8 9 10

Cr(CO)6 Mn2(CO)10 Fe3(CO)12 – Ni(CO)4

Mo(CO)6 Tc2(CO)10 Ru3(CO)12 Rh4(CO)12 –

W(CO)6 Re2(CO)10 Os3(CO)12 Ir4(CO)12 –

t2g

bury d6 bury d6 bury d6 bury d6 filled d-orbitals no e– in σ* 1e– in σ* 2e– in σ* 3e– in σ* stable lose 1 CO lose 2 CO lose 3 CO no electronic 1 M–M 2 M–M 3 M–M 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 2– determination of the X-ray crystal structure of Re2Cl8 in 1963.

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 3– – – to ReCl6 (Oh), remove two axial Cl ligands to give square planar ReCl4 (D4h) fragment and then dimerize.

– 3– Accounting for the π-donating properties of Cl , the correlation diagram for ReCl6 (Oh) – and ReCl4 (D4h) is

– 2– The ReCl4 fragments can now be dimerized to furnish the Re2Cl8 quadruple bond.

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:

–1 λmax (δ→δ*) for Mo2Cl4 (PMe3)4 = 17094 cm [d(M-M) = 2.13 Å] + –1 λmax (δ→δ*) for Mo2Cl4 (PMe3)4 = 6350 cm [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.