Carbon monoxide as a σ-donor ligand in coordination chemistry

In addition to the numerous and varied examples of organometallic and coordination compounds where carbon monoxide functions as a π-acceptor ligand, there is a small but select group of derivatives wher the CO-metal bonding relies predominantly on σ-bonding only. The metal derivatives, which include Pt(CO)2Cl2, known since 1868, are formed by electron-rich metals (Pd, Pt, Ag and Au), in oxidation states +1 and +2, are cationic in nature, and are for the most part thermally stable. Their generation in strong protonic and Lewis acids (HSO3F, HSO3FSbF5 and liquid SbF5) allows convenient synthesis. The predominance of σ-bonding results in unusual spectroscopic properties, such as vCO stretching vibrations as high as 2280 cm−1, relative to 2143 cm−1 in free CO, and 13C chemical shifts which are 180−130 ppm rather than 190–220 ppm for classical transition-metal carboyls. Consequently two basic principles, the synergic bonding model and the rule of 18, fail for these complexes. Recent structural studies point to the importance of secondary OC…OS or OC…FSb contacts, which in place of π-back-donation provide for a measure of charge compensation both in solution or in the solid state. This review concentrates on very recent (since 1990) developments in this field, which is expanding rapidly beyond the transition metals.