Trigonal-Bipyramidal Five-Coordinate Trisolefin Complexes of Platinum(II)

Article abstract of DOI:10.1021/ja00006a028

Five-coordinate trigonal-bipyramidal platinum(II) becomes the ground-state geometry in the presence of the chelating diolefin 3,7-dimethylenebicyclo[3.3.1]nonane [DMBN]. Anionic σ donors invariably take up the axial positions, and the third trigonal position is generally held by a π donor-acceptor. These trigonal-bipyramidal complexes are inert to addition at the platinum center, but will react by dissociation of the third trigonal ligand. As long as the dissociative pathway is prohibited (e.g. by addition of excess olefin) the complexes are quite stable, and in particular do not react via. σ group migration to coordinated olefin. The prototypic complex [(C₂H₄)(DMBN)Pt^IICl₂] crystallizes in the space group P2₁/c, a = 7.629 (1) A?, b = 12.431 (1) A?, c = 14.714 (2) A?, and β = 100.88 (1)°. The trigonal-bipyramidal structure has C_2v geometry with the 2-fold axis passing through the Pt atom and ethylene midpoint. The mirrors are respectively parallel and perpendicular to the trigonal plane. A final R factor of 2.6% was obtained based on 3126 3σ reflections observed of 3990 unique reflections collected at -79?C. Replacement of one axial chloride by a methyl group greatly enriches the NMR spectra due to removal of the 2-fold axis and one mirror and allows for detailed analysis of the solution structure as well as for the observation of dynamic process(es) related to ethylene dissociation.