Synthesis and structure of Pd4(mu-P(t)Bu2)2(mu-CS2)(PPh3)2(I)2, a neutral palladium(I) derivative with a hexacoordinate carbon and a CS2 molecule bridging four palladium centers

The dinuclear CS2 complex [Pd-2(mu-(PBu2)-Bu-t)(mu,eta(2):eta(2)-CS2)(PPh3)(2)]BF4, (2)BF4, reacts with Me4NI to yield the neutral tetranuclear Pd(I) derivative Pd-4(mu-(PBu2)-Bu-t)(2)(mu(4)-CS2)(PPh3)(2)I-2 (4), which has been characterized by single-crystal X-ray diffraction. Its unusual molecular structure with a formally hexacoordinate carbon atom, as well as that of (2)(+) with a planar tetracoordinate carbon can be explained on the basis of fragment MO considerations and EHMO model calculations. The reaction proceeds stepwise, with the relatively fast substitution of CS2 by the iodide ion giving the intermediate complex Pd-2(mu-(PBu2)-Bu-t)(mu-I)(PPh3)(2) (3). The latter further reacts slowly with free CS2 to give complex 4 and free PPh3. Complex 3 was also prepared and isolated pure by reacting [Pd-2(mu-(PBu2)-Bu-t)(PPh3)(3)]BF4 with Me4NI.