End-On and Side-On π-Acid Ligand Adducts of Gold(I): Carbonyl, Cyanide, Isocyanide, and Cyclooctyne Gold(I) Complexes Supported by N-Heterocyclic Carbenes and Phosphines

N-Heterocyclic carbene ligand SIDipp (SIDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and trimesitylphosphine ligand have been used in the synthesis of gold(I) cyanide, t-butylisocyanide, and cyclooctyne complexes (SIDipp)Au(CN) (3), (Mes(3)P)Au(CN) (4), [(Mes(3)P)(2)Au][Au(CN)(2)] (5), [(SIDipp)Au((CNBu)-Bu-t)][SbF6] ([6][SbF6]), [(SIDipp)Au(cyclooctyne)][SbF6] ([8][sbF(6)]), and [(Mes(3)P)Au(cyclooctyne)][SbF6] ([9][SbF6]). A detailed computational study has been carried out on these and the related gold(I) carbonyl adducts [(SIDipp)Au(CO)][SbF6] ([1][SbF6]), [(Mes(3)P)Au(CO)][SbF6] ([2][SbF6]), and [(Mes(3)P)Au((CNBu)-Bu-t)(+) ([7](+). X-ray crystal structures of 3, 5, [6][SbF6], [8][SbF6], and [9][SbF6] revealed that they feature linear gold sites. Experimental and computational data show that the changes in pi-acid ligand on (SIDipp)Au+ from CO, CN-, (CNBu)-Bu-t, cyclooctyne as in [1](+), 3, [6](+), and [8](+) did not lead to large changes in the Au-C-carbene bond distances. A similar phenomenon was also observed in Au-P distance in complexes [2](+), 4, [7](+), and [9](+) bearing trimesitylphosphine. Computational data show that the Au-L bonds of naked [Au-L](+) or SIDipp and Mes(3)P supported [Au-L](+) (L = CO, CN-, (CINBu)-Bu-t to cyclooctyne) have higher electrostatic character than covalent character. The Au <- L sigma-donation and Au -> L pi-back-donation contribute to the orbital term with the former being the dominant component, but the latter is not negligible. In the Au-CO adducts [1](+) and [2](+), the cationic gold center causes the polarization of the C-O sigma and pi orbitals toward the carbon end making the coefficients at the two atoms more equal which is mainly responsible for the large blue shift in the CO stretching frequency. The SIDipp and Mes(3)P supported gold(I) complexes of cyanide and isocyanide also exhibit a significant blue shift in (v) over bar (CN) compared to that of the free ligands. Calculated results for Au(CO)Cl and Au(CF3)CO suggest that the experimentally observed blue shift in (v) over bar (CO) of these compounds may at least partly be caused by intermolecular forces.