Molecular rearrangements in the formation of five-coordinate platinum(II) olefin complexes. Structural and mechanistic aspects

The olefin addition reaction to square-planar complexes of the type [PtXMe(N-N)] (X=Cl,CH(CO2Me)(2), N-N=bidentate nitrogen ligand), to form trigonal bipyramidal complexes has been shown to occur in two successive steps. First, a five-coordinate species (A) is formed in which the axial positions are occupied by the methyl group and by one nitrogen atom. In most cases this subsequently rearranges to the more stable isomer (B), in which both axial positions are occupied by the anionic ligands. The structure of the type A complex [PtMe{CH(CO2Me)(2)} (E-NCCH=CHCN) (dmphen)] has been determined by an X-ray diffraction study (triclinic, space group <P(1)over bar> (No. 2), a = 9.497(3), b = 10.884(4), c = 15.671(4) Angstrom,alpha = 75.62(3), beta = 78.47(2), gamma = 66.52(3)degrees, Z=2). For the first time the equatorial-axial coordination of the dmphen ligand has been unequivocally ascertained. The molecule reveals a significant tension due to the short contacts between the methyl groups. The electronic and steric factors affecting the relative stability of type A and B species are discussed, and a mechanism for the overall process is proposed. (C) 1997 Elsevier Science S.A.