Classical Gold Carbonyl Complexes in Tetrahedral and Trigonal-Planar Settings

A unique four-coordinate, classical gold(I)-carbonyl complex with substantial backdonation from gold has been isolated by using a B-methylated and fluorinated tris(pyridyl)borate chelator. Its lighter silver(I) and copper(I) analogs enabled a study of trends in the coinage-metal family. The B-arylated ligand version also afforded a gold–carbon monoxide complex that displays a notably low C−O stretch value, but with trigonal planar geometry at the gold. A computational analysis shows that the Au^I−CO bonds of these tris(pyridyl)borate ligand-supported molecules consist of electrostatic attraction, OC→Au σ-donation, and very significant Au→CO π-back-bonding components. The latter is responsible for the observed C−O stretching frequencies, which are lower than in free CO.