THE SYNTHESIS AND CHARACTERIZATION OF SIGMA-BONDED GOLD CARBABORANE COMPOUNDS - CLOSO-CARBABORANES AS GOOD SIGMA-DONOR LIGANDS

A series of sigma-bonded phosphine and arsine gold carbaborane compounds, 1-R'-2-ER3Au-1,2-closo-C2B10H10 (R'- CH2OCH3 or Ph, ER3 = PPh3, P(o-tol)3, PCy3, PEt3 or AsPh3) have been prepared by reaction between ER3AuCl and LI[1-R'-1,2-closo-C2B10H10] in Et2O. Consideration of their B-10 NMR and (where appropriate) P-31 NMR chemical shifts implies that the closo-carbaborane ligands function as efficient electron donating groups. Comparison of the Au-C distance in 1-CH2OCH3-2-ASPh3Au-1,2-closo-C2B10H10 (1e) and AsPh3AuCH3 (3) suggests that the gold-carbon bond in the former is stronger, and this is supported by analysis of the results of molecular orbital calculations at the extended Huckel level on model compounds. closo-Carbaboranes bonded to metal atoms via a M-C bond are thereby shown to be good sigma-donor ligands.