SYNTHESIS AND CRYSTAL-STRUCTURES OF THE GOLD-CARBABORANE COMPLEXES [9-EXO-(AU(PPH3))-9-(MU-H)-10-ENDO-(AU(PPH3))-7,8-ME2-NIDO-7,8-C2B9H8] AND [10-EXO-(AU2(PPH3)2)-10-ENDO-(AU(PPH3))-7,8-ME2-NIDO-7,8-C2B9H8]

Treatment of the salt [NEt4][10-endo-{Au(PPh3)}-7,8-Me2-nido-7,8-C2B9H9] with [AuCl(PPh3)] in CH2Cl2 afforded the digold complex [9-exo-{Au(PPh3)}-9-(mu-H)-10-endo-{Au(PPh3)}-7,8-Me2-nido-7,8-C2B9H8]. Deprotonation of the latter with NaH in tetrahydrofuran followed by addition of [AuCl(PPh3)] gave the trigold compound [10-exo-{Au2(PPh3)2}-10-endo-{Au(PPh3)}-7,8-Me2-nido-7,8-C2B9H8]. The crystal structures of the new compounds have been determined by X-ray diffraction. In the digold species one Au(PPh3) group is endo-bound to the nido-C2B9 cage, primarily through a connectivity [Au-B 2.28(1) angstrom] with the boron atom which is in the beta-site with respect to the two carbons in the open CCBBB face. There is a weaker attachment [Au-B 2.37(1) angstrom] to one of the alpha-boron atoms, while the other alpha-BH unit forms a three-centre two-electron B-H half arrow pointing right Au bond with the second Au(PPh3) group [Au-B 2.28(1), Au-mu-H 1.9(1) angstrom]. The latter gold fragment lies exo with respect to the open face of the carbaborane cage, with an Au-Au separation of 2.875(1) angstrom. The structure of the trigold complex is formally related to that of its digold precursor by replacement of the mu-H atom in the latter by an Au(PPh3) moiety. This results in the molecule having an essentially isosceles triangle of gold atoms [Au-Au 2.691(1), 2.922(1), 3.010(1) angstrom], asymmetrically bridged by the beta-boron [Au-B 2.192(9), 2.227(7), 2.307(8) angstrom] of the open CCBBB face of the C2B9 cage. The NMR data (H-1, C-13-{H-1}, P-31-{H-1} and B-11-{H-1}) of the compounds are reported and discussed.