Synthesis and properties of gold alkene complexes. Crystal structure of [Au(bipyoXyl)(g2-CH2=CHPh)](PF6) and DFT calculations on the model cation [Au(bipy)(g2-CH2=CH2)]+

Unprecedented 16-electron gold(I) olefin complexes of general formula [Au(bipy(R,R'))(eta(2)-olefin)](PF6) and [Au-2(bipy(R,R'))(2)(mu-eta(2):eta(2)-diolefin)](PF6)(2) (bipy(R,R') = 6-substituted-2,2'-bipyridine) have been prepared by reaction of dinuclear gold(III) oxo complexes [Au-2(bipy(R,R'))(2)(mu-O)(2)](PF6)(2) with the appropriate olefin. The X-ray crystal structures of two mononuclear complexes (olefin = styrene) show in-plane coordination of the olefin and a C = C bond distance considerably lengthened with respect to the free olefin. The spectroscopic properties of the complexes are discussed and compared with those of analogous d(10) metal derivatives. Both structural and spectroscopic information indicate a substantial contribution of pi-back-donation to the Au-olefin bond in the three-coordinate species. Theoretical calculations carried out at the hybrid-DFT level on the model compound [Au(bipy)(eta(2)-CH2=CH2)](+) show excellent agreement with the experimental findings giving in addition an estimate of a p-back-bonding contribution higher than that of the sigma-bonding.