CONFORMATIONAL POLYMORPHISM AND FLUXIONAL BEHAVIOR OF M(PR(3))(2) UNITS IN CLOSO-12-ATOM METALLAHETEROBORANES WITH MX(2)B(9) (X=C OR AS) AND MZB(10) CAGES (Z=S, SE OR TE)

The compounds [3,3-(PEt(3))(2)-closo-3,1,2-PtAs2B9H9] 1 and [2,2-(PMe( 2)Ph)(2)-closo-2,1-PtSB10H10] 2 were synthesised and characterised by NMR spectroscopy and X-ray crystallography. Crystals of 1 exhibit conf ormational polymorphism with five conformers of the Pt(PEt(3))(2) unit above the As2B3 face; those of 2 contain single conformers. The free energy of the barrier to rotation of the Pt(PMe(2)Ph)(2) unit above th e SB10 cage ligand in 2 has been determined in chloroform solution and is compared with data from other platinum and palladium metallahetero boranes containing C2B9H11, As2B9H9, SeB10H10 and TeB10H10 ligands. A mechanism for the rotation of M(PR(3))(2) units above heteroborane lig and faces is suggested. It involves shifting the M(PR(3))(2) unit via eta(n)-bonded species (n < 5) with a concomitant twisting of the M(PR( 3))(2) unit about an axis passing approximately through the metal atom and the antipodal B atom.