Pi Backbonding - Metal Carbonyls, Nitrosyls, and Isocyanides

Metal Carbonyls, Nitrosyls, and Isocyanides

The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogues). This electron-transfer (i) strengthens the metal-C bond and (ii) weakens the C-O bond. The strengthening of the M-CO bond is reflected in increases of the vibrational frequencies for the M-C bond (often outside of the range for the usual IR spectrophotometers). Furthermore, the M-CO bond length is shortened. The weakening of the C-O bond is indicated by a decrease in the frequency of the ν_CO band(s) from that for free CO (2143 cm−1) often by more than 200 cm−1. For this reason, IR spectroscopy is an important diagnostic technique in metal-carbonyl chemistry. The article infrared spectroscopy of metal carbonyls discusses this in detail.

Many ligands other than CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than is CO and ν_NO is a diagnostic tool in metal-nitrosyl chemistry. In the case of isocyanide complexes, the degree of π-bonding is again indicated by shortening of the M-CNR bond and by decrease in ν_CN. For the isocyanides however, an additional parameter is the MC=N-C angle, which deviates form 180° in highly electron-rich systems. Other ligands have weak π-backbonding abilities, which creates a labilization effect of CO, which is described by the cis effect.