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

Heterobimetallic derivatives of a bis(carborane), [mu(7,8)-(1 ',3 '-3 '-Cl-3 '-PPh3-closo-3 ',1 ',2 '-RhC2B9H10)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (1) and [mu(7,8)-(1 ',3 '-3 '-Cl-3 '-PPh3-closo-3 ',1 ',2 '-RhC2B9H10)-2-Cp-closo-2,1,8-CoC2B9H10] (2) have been synthesised and characterised, including crystallographic studies. A minor co-product during the synthesis of compound 2 is the new species [8-{8 '-2 '-H-2 ',2 '-(PPh3)(2)-closo-2 ',1 ',8 '-RhC2B9H10}-2-Cp-closo-2,1,8-CoC2B9H10] (3), isolated as a mixture of diastereoisomers. Although, in principle, compounds 1 and 2 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 PPh3 ligand and either the p-cymene (compound 1) or Cp (compound 2) ligand of the ruthenacarborane or cobaltacarborane, respectively.