Structure and bonding in octafluoro-trans-but-2-enebis(triphenylphosphine)platinum, Pt[CF₃CFCFCF₃][P(C₆H₅)₃]₂, a compound with unusually long carbon-fluorine bonds

The crystal and molecular structure of a substituted olefin complex of platinum, Pt[CF₃CFCFCF₃] [P(C₆H₅)₃]₂, has been determined from three-dimensional X-ray diffraction data. The complex crystallizes in space group P2₁/c of the monoclinic system, with four molecules in a unit cell of dimensions a = 11.635 (2) Å, b = 19.213 (4) Å, c = 18.107 (3) Å, β = 114.39 (2)°. The structural parameters were refined by least-squares techniques, the R factor on F converging to 5.4% for the 3841 independent reflections (measured using a four-circle diffractometer) for which F₂ > σ(F²) and 2θ ≤ 50°. No symmetry is crystallographically imposed upon the molecules but they have C₂ symmetry to a good approximation. Coordination around platinum is not exactly planar, the largest distortion being the displacement of one olefinic carbon atom by 0.29 (1) Á from the plane defined by the other four atoms; the dihedral angle between the PtP₂ and PtC₂ planes is 10.8 (7)°. The olefinic carbon-carbon separation is 1.429 (14) Å; this value does not differ significantly from the corresponding parameter in the analogous 4,4′-dinitro-trans-stilbene complex, suggesting that the separation of the olefinic carbon atoms in complexes of this type formed by acyclic olefins is not a function of the olefinic substituents. The olefinic carbon atoms are essentially equidistant from platinum, the Pt-C distances being 2.028 (12) and 2.048 (11) Å. The Pt-P bond lengths are 2.302 (3) and 2.322 (3) Å. The bonds from each olefinic carbon atom to its fluoro substituent are unusually long at 1.426 (13) and 1.437 (12) Å. The chemical shift in the 19F NMR spectrum of these two fluorine nuclei is much further upfield in the complex than in the free olefin; the chemical shift of the CF3 resonances does not change greatly upon formation of the complex, and these groups are of normal geometry in the complex. The binding energy of the Pt Ah 12 electrons in Pt[CF₃CFCFCF₃][P(C₆H₅)3]2 is 1.0 eV higher than in the analogous 4,4′-dinitro-trans-stilbene complex and is close to that in PtCl₂[P(C₆H₅)₃]₂· These results were interpreted to mean that considerable electron density is transferred from platinum to the olefinic fluoro substituents upon formation of the complex. Since the general characteristics of fluoro substituents are such that this electron transfer would be through molecular orbitals of A symmetry, then the Dewar-Chatt-Duncanson model for the bonding in olefin-metal complexes is inappropriate.