1. Lecture 20 Dinuclear metal complexes with multiple M-M bonds 1) Transition metal-metal bonding: MO diagram
Compared to main group elements, transition metals (TM’s) have more options for bonding with one another.
In addition to one s- and up to two p-bonds common for organic compounds (acetylenes, cumulenes) and some other main group elements two more bonds of d-symmetry are possible for TM’s (five bonds in total).
If the metal dx2-y2 orbitals are involved in s-bonding with ligands, four metal atomic orbitals remain only available to form M-M bonds.
Species with TM-TM bond orders 1-4 are well known.
Multiple M-M bonds are most typical for such metals as V-Ta, Cr-W, Re, Ru, Os, Rh.

2. 2) Evidences for M-M double, triple and quadruple bonding
One of the arguments in favor of multiple bonds is M-M bond length (Å):
Look now at the solid state structures of salts derived from [Re2Me8]2- anion which shows the eclipsed conformation of the methyl groups.
This unfavorable conformation is possible if stabilized by dxy-dxy d-bonding between two rhenium atoms (l(ReRe)=2.18 Å).
The dxy-dxy orbital overlap is impossible in the staggered conformation.
The barrier of rotation about M-M quadruple bond was estimated in a dinuclear MoII porphyrin complex to be of 10 kcal/mol.

3. 3) More ways to form multiple M-M bonds
The M/M bond order is a function of transition metal electron configuration. The Table below is valid for the case when metal dx2-y2 orbital is used for s-bonding with ligands. d1 d2 d3 d4 d5 d6 d7 d8 1 2 3 4
Some examples are given on the previous page and on the right. A very week quadruple Cr/Cr bond is believed to exist in Cr2(OAc)4(OH2)2 paddlewheel complex above.
Another way to form M-M’ multiple bonds is through donor-acceptor mechanism.
Ti center is roughly tetrahedral (Rh-Ti-O = 109o; sp3 hybridization) while RhP3 fragment is almost planar (d2s hybridization). Rh atom has two filled dxz and dyz orbitals suitable for formation of two p-bonds with Ti atom. The latter uses the empty dxz and dyz orbitals.