THEORETICAL AND SPECTROSCOPIC STUDIES OF TETRAHEDRAL CLUSTERS OF THE TYPE [L3M(MU-H)X(AUL')3]2+ (M = RH, IR, X = 2, M = RU, X = 3)

The structural and bonding aspects of the hydride ligands in the cationic clusters [(triphos)MH2{Au(PPh3)}3]2+ (triphos = CH3C(CH2PPh2)3; M = Rh and Ir) and [(triphos)RuH3{Au(PPh3)}3]2+ were investigated by a variety of techniques. The P-31-CP-MAS spectra (162 MHz) of the clusters [(triphos)MH2{Au(PPh3)}3](CF3SO3)2 (M = Rh and Ir), [(triphos)RuH3{Au(PPh3)}3][PF6]2, and [(triphos)RuH2{Au(PPh3)}3][PF6] show that in the dihydrido complexes the phosphorus atoms of the triphos ligands, as well as those of the PPh3 molecules, are inequivalent while the corresponding donors in the trihydrido complex are equivalent. The Au-197 Mossbauer spectra of the rhodium and iridium complexes show that two of the gold atoms in the dihydrido clusters are different from the third. Furthermore, the values of the isomer shifts and quadrupole splittings indicate that the bridging hydride ligands are best described as being intermediate between mu2 and mu3 (quasi-mu3). Incoherent inelastic neutron spectroscopic (IINS) and IR studies of the rhodium and ruthenium clusters were carried out to unambiguously assign the M-H vibrational modes. These spectra show bands at ca. 1600 cm-1 (assigned to nu(as)(M-H-Au) that exclude a pure mu3 bridging mode for the hydride ligands but are consistent with a quasi-mu3 model. Finally MO model calculations of the extended Huckel type for [(PH3)3Rh(AuPH3)3](H)33+, [(PH3)3Rh(AuPH3)3](H)22+, and [(PH3)3Rh(AuPH3)3]-(H)+, which served as simplified models of real hydrido cluster cations, have been performed. A general bonding scheme is presented, analyzing electronic and molecular structures for this class of compounds, including the question of possible hydride locations, on the basis of a transparent fragment orbital (FMO) description and by means of various geometry optimization calculations. The theoretical results are in accord with spectroscopic data and allow their consistent interpretation. Thus, all these studies are in agreement with a quasi-mu3 structure for the hydride ligands in these clusters.