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), (Mes3P)­Au­(CN)
(4), [(Mes3P)2Au]­[Au­(CN)2] (5), [(SIDipp)­Au­(CNtBu)]­[SbF6] ([6]­[SbF6]), [(SIDipp)­Au­(cyclooctyne)]­[SbF6] ([8]­[SbF6]), and [(Mes3P)­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]),
[(Mes3P)­Au­(CO)]­[SbF6] ([2]­[SbF6]), and [(Mes3P)­Au­(CNtBu)]+ ([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 π-acid ligand on (SIDipp)­Au+ from CO, CN–, CNtBu,
cyclooctyne as in [1]+, 3, [6]+, and [8]+ did not
lead to large changes in the Au–Ccarbene 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 Mes3P supported
[Au–L]+ (L = CO, CN–, CNtBu to cyclooctyne) have higher electrostatic character
than covalent character. The Au←L σ-donation and Au→L
π-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 σ and π 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 Mes3P supported gold­(I) complexes of cyanide
and isocyanide also exhibit a significant blue shift in υ̅CN 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 ν̅CO of these compounds
may at least partly be caused by intermolecular forces.