Unprecedented 16-electron gold(I) olefin complexes of general formula [Au(bipy^R,R′)(η²-olefin)](PF₆) and [Au₂(bipy^R,R′)₂(µ-η²:η²-diolefin)](PF₆)₂ (bipy^R,R′ = 6-substituted-2,2′-bipyridine) have been prepared by reaction of dinuclear gold(III) oxo complexes [Au₂(bipy^R,R′)₂(µ-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 CC bond distance considerably lengthened with respect to the free olefin. The spectroscopic properties of the complexes are discussed and compared with those of analogous d¹⁰ metal derivatives. Both structural and spectroscopic information indicate a substantial contribution of π-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)(η²-CH₂ CH₂)]⁺ show excellent agreement with the experimental findings giving in addition an estimate of a π-back-bonding contribution higher than that of the σ-bonding.