Widely Varying Olefin Rotation Geometry in Platinum(II)-Olefin Complexes Containing the η3-Allyl Ligand. Molecular Structure, Solution Stability, and Molecular Orbital Studies

Article abstract of DOI:10.1021/ja00218a031

The molecular structures of six olefin complexes <3-CH2CMeCH2)(olefin)(PPh3)>PF6 (2) (olefin = CH2=CH2 (2a), CH2=CHPh (2b), CH2=CHC6H4Cl-o (2c); (E)-MeCH=CHPh (2d), (E)-MeCH=CHMe (2e), (Z)-MeCH=CHMe (2f))> were determined by X-ray diffraction.Crystal data: 2a monoclinic, space group P2₁/n, a = 9.610(2) Angstroem, b = 10.786(4) Angstroem, c = 24.082(8) Angstroem, β = 91.77(3)^o, Z = 4; 2b monoclinic, space group P2₁/n, a = 15.560(3) Angstroem, b = 11.342(3) Angstroem, c = 17.594(3) Angstroem, β = 110.91(3)^o, Z = 4; 2c*1/2CH2Cl2 orthorhombic, space group Pccn, a = 11.421(2) Angstroem, b = 27.127(6) Angstroem, c = 20.414(4) Angstroem, Z = 8; 2d*C6H6 monoclinic, space group P2₁/c, a = 11.408(2) Angstroem, b = 14.910(10) Angstroem, c = 21.397(3) Angstroem, β = 103.17(2)^o, Z = 4; 2e monoclinic, space group P2₁/c, a = 13.886(2) Angstroem, b = 11.872(4) Angstroem, c = 16.740(2) Angstroem, β = 91.17(1)^o, Z = 4; 2f monoclinic, space group P2₁/c, a = 11.149(2) Angstroem, b = 15.460(3) Angstroem, c = 15.701(2) Angstroem, β = 90.21(2)^o, Z = 4.The ethylene ligand in 2a lies almost in the coordination plane, defined by Pt, P, and the center of gravity of the allyl triangle, with the C=C axis making an angle of 6.0^o with this plane (in-plane geometry).Depending on the steric requirement of the olefin substituents, the angle between the C=C axis and the coordination plane varies quite widely (2b 11.1^o, 2c 24.5^o, 2d 46.1^o, 2e 67.0^o, 2f 7.4^o).The o-chloro- and o-methyl-substituted styrene complexes of 2 showed considerably smaller solution stabilities than the corresponding meta- and para-substituted counterparts owing to the steric repulsion nearly on the coordination plane between the ortho substituent and the allyl hydrogen, while the reversed stability order was observed in those complexes that contain the C=C axis perpendicular to the coordination plane (upright geometry).The complexes 2 containing (E)-olefin ligands showed unprecedented higher stabilities than the corresponding (Z)-olefin complexes by a factor of ca. 2.Extended Hueckel MO calculations were carried out on 3-CH2CHCH2)(CH2=CH2)(PH3)>⁺ in both in-plane and upright geometries to give comparable total energies for the two conformers.The nature of the Pt-olefin bond in 2 is discussed in terms of these structural and MO results.