The reactions of the nido-6-Me-5,6,9-C3B7H9- anion with MX2 (M = Ni, Pd, Pt) salts yield a series of new bis(tricarbadecaboranyl) (η4-MeC3B7H9)2M 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-MeC3B7H9)2 (4a). X-ray structural determinations and NMR studies of these complexes have revealed that the MC3B7 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 η6-fashion and function as a 5-electron donor ligand similar to an η5-cyclopentadienyl anion. However, the metal atoms in the (η4-MeC3B7H9)2M 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 η4-MeC3B7H9- ligands in these complexes appear to function as only bidentate, 3-electron donors to the metal, similar to an η3-C3H5- ligand. The (η4-MeC3B7H9)2M complexes can thus be considered analogs of M(η3-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 PMe3, 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-PMe3-7,8,10-C3B7H8) (5). An X-ray investigation demonstrated that the PMe3 is attached to a boron atom on the tricarbadecaboranyl cage that is coordinated to the nickel in an η4-fashion, while the other tricarbadecaboranyl cage is coordinated to the nickel in a distorted η6-manner. The distorted cage fragment exhibits a geometry that is intermediate between those found for 11-vertex nido (26 skeletal electrons) and closo (24 skeletal electrons) cage systems, suggesting that the odd electron in 5 may be localized in a cage-centered orbital.