Syntheses and structural characterizations of metallabis(tricarbadecaboranyl) sandwiches, (eta(4)-MeC(3)B(7)H(9))(2)M (M=Ni, Pd, Pt): Tricarbadecaboranyl analogs of (eta(3)-C3H5)(2)M complexes

The reactions of the nido-6-Me-5,6,9-C3B7H9- anion with MX(2) (M = Ni, Pd, Pt) salts yield a series of new bis(tricarbadecaboranyl) (eta(4)-MeC(3)B(7)H(9))(2)M sandwich complexes including the following: commo-Pd-(8-Pd-9-Me-7,9,10-C3B7H9)(2) (1a), commo-Pd-(9-Pd-8-Me-7,8,10-C3B7H9)(8'-Pd-9'-Me-7',9',10'-C3B7H9) (1b), commo-Pd-(9-Pd-8-Me-7,8,10-C3B7H9)(8'-Pd-10'-Me-7',9',10'-C3B7H9) (2), commo-Pt-(8-Pt-9-Me-7,9,10-C3B7H9)(2) (3a), commo-Pt-(9-Pt-8-Me-7,8,10-C3B7H9)(8'-Pt-9'-Me-7',9',10'-C3B7H9) (3b), and commo-Ni-(8-Ni-9-Me-7,9,10-MeC(3)B(7)H(9))(2) (4a). X-ray structural determinations and NMR studies of these complexes have revealed that the MC(3)B(7) cage fragments in each complex adopt open cage geometries. Previous studies of the coordination properties of the nido-6-Me-5,6,9-C3B7H91- anion demonstrated that it can bind to a transition metal in an eta(6)-fashion and function as a 5-electron donor ligand similar to an eta 5-cyclopentadienyl anion. However, the metal atoms in the (eta 4-MeC(3)B(7)H(9))(2)M complexes 1-4 are situated in only four-coordinate cage positions, and the structural and spectroscopic data combined with electron-counting arguments suggest that the metals are providing only two orbitals for cluster bonding. Therefore, the eta(4)-MeC(3)B(7)H(9)(-) ligands in these complexes appear to function as only bidentate, 3-electron donors to the metal, similar to an eta(3)-C3H5- ligand. The (eta(4)-MeC(3)B(7)H(9))(2)M complexes can thus be considered analogs of M(eta(3)-C3H5)(2) complexes (M = Ni, Pd, Pt) in which the metals are 16 valence electron systems. The nickel complex 4a was found to react with PMe(3), but instead of forming an 18 valence electron adduct, oxidation, accompanied by loss of one hydrogen atom, occurred to form the 17 valence electron compound, commo-Ni-(1-Ni-5-Me-2,3,5-C3B7H9)(9'-Ni-7'-Me-11'-Pme(3)-7',8',10'-C3B7H8) (5). An X-ray investigation demonstrated that the PMe(3) is attached to a boron atom on the tricarbadecaboranyl cage that is coordinated to the nickel in an eta(4)-fashion, while the other tricarbadecaboranyl cage is coordinated to the nickel in a distorted eta(6)-manner. The distorted cage fragment exhibits a geometry that is intermediate between those found for Ii-vertex nido (26 skeletal electrons) and close (24 skeletal electrons) cage systems, suggesting that the odd electron in 5 may be localized in a cage-centered orbital.