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), -8-Me-7,8,10-C3B7H9)(8'-Pd-9'-Me -7',9',10'-C3B7H9) (1b), 8-Me-7,8,10-C3B7H9)(8'-Pd-10'-Me-7',9',10'-C3 B7H9) (2), commo-Pt-(8-Pt-9-Me-7,9,10-C3B7H9)(2) (3a), -8-Me-7,8,10-C3 B7H9)(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 st udies of these complexes have revealed that the MC(3)B(7) cage fragmen ts in each complex adopt open cage geometries. Previous studies of the coordination properties of the nido-6-Me-5,6,9-C3B7H91- anion demonst rated 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-cyclopentadi enyl 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, an d the structural and spectroscopic data combined with electron-countin g arguments suggest that the metals are providing only two orbitals fo r cluster bonding. Therefore, the eta(4)-MeC(3)B(7)H(9)(-) ligands in these complexes appear to function as only bidentate, 3-electron donor s 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 ele ctron systems. The nickel complex 4a was found to react with PMe(3), b ut instead of forming an 18 valence electron adduct, oxidation, accomp anied by loss of one hydrogen atom, occurred to form the 17 valence el ectron compound, 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 th e nickel in an eta(4)-fashion, while the other tricarbadecaboranyl cag e is coordinated to the nickel in a distorted eta(6)-manner. The disto rted cage fragment exhibits a geometry that is intermediate between th ose found for Ii-vertex nido (26 skeletal electrons) and close (24 ske letal electrons) cage systems, suggesting that the odd electron in 5 m ay be localized in a cage-centered orbital.