Exopolyhedral Ligand Orientation Controls Diastereoisomer in Mixed-Metal Bis(carboranes).

Heterobimetallic derivatives of a bis(carborane), [μ-(1',3'-3'-Cl-3'-PPh--3',1',2'-RhCBH)-2-(-cymene)--2,1,8-RuCBH] () and [μ-(1',3'-3'-Cl-3'-PPh--3',1',2'-RhCBH)-2-Cp--2,1,8-CoCBH] () have been synthesised and characterised, including crystallographic studies. A minor co-product during the synthesis of compound is the new species [8-{8'-2'-H-2',2'-(PPh)--2',1',8'-RhCBH}-2-Cp--2,1,8-CoCBH] (), isolated as a mixture of diastereoisomers. Although, in principle, compounds and could also exist as two diastereoisomers, only one (the same in both cases) is formed. It is suggested that the preferred exopolyhedral ligand orientation in the rhodacarboranes in the non-observed diastereoisomers would lead to unacceptable steric crowding between the PPh ligand and either the -cymene (compound ) or Cp (compound ) ligand of the ruthenacarborane or cobaltacarborane, respectively.