Electronic structure of M(η4 -P2C2But2) where M = Ge, Sn, and Pb:: Photoelectron spectroscopy and density functional studies

Density functional calculations are used to optimize the geometry of M(eta(4)-P2C2R2) (M = Ge, Sn, Ph; R = H, Bu-t) and examine the orbital structure. He I and He II spectra are reported for M(eta(4)-P2C2Bu2t) (M = Ge and Sn), and the spectra are assigned. Ionization energy calculations are in good agreement with the experimental values. The binding of the diphosphacyclobutadienyl. ring to the metal involves donation from the ring T orbital into the metal s, p(x), and p(y) orbitals. A possible intermediate in the formation of a M(eta-P2C2R2) complex is presented; this has a MP2C2 square-pyramidal structure with P at the apex and M in the square base. The transition states for rearrangement to the product, where M is at the apex of a square pyramid, are identified. The reactivity of M(eta(4)-P2C2R2) as a base and toward oxidative addition is explored.