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 AuI-CO bonds of these tris(pyridyl)borate ligand-supported molecules consist of electrostatic attraction, OC -> Au sigma-donation, and very significant Au -> CO pi-back-bonding components. The latter is responsible for the observed C-O stretching frequencies, which are lower than in free CO. Pyridyl-borate scorpionates decorated with anionic and fluoro-alkyl groups serve as ideal ligands to capture gold(I)-carbon monoxide fragments in trigonal planar and tetrahedral environments that display remarkably low C-O stretching frequencies, and have significant Au-CO pi-bond character.image