A comparative study of analogous nitrogen and carbon monoxide complexes of iridium(I)

Abstract

Complexes of the type trans-[lrXL(PR₃)₂](X = Cl, Br, I, or N₃; L = N₂ or CO; PR₃= tertiary phosphine) have been prepared. The carbonyls are stable, crystalline substances, but the nitrogen complexes were so unstable that few were obtained pure. In the complexes, the nitrogen and carbonyl stretching frequencies undergo almost identical fractional lowerings, in comparison with molecular nitrogen and carbon monoxide respectively. Both frequencies are almost independent of X and have little dependence on R. The more aliphatic phosphine complexes tend to have slightly lower frequencies. Nitrogen and carbon monoxide are not significantly different in their trans-influence, as shown by almost identical metal–halogen stretching frequencies in the two series of complexes. Similarity between the carbonyl and nitrogen complexes was expected, but the very close similarity between their i.r. spectroscopic properties is remarkable in view of the difference between the electronic structure of the two molecules. This we interpret to indicate that nitrogen is weaker in both its σ-donor and its π-acceptor functions in bonding to the metal, than is carbon monoxide. These weaknesses would explain the instability of the nitrogen, as compared with the corresponding carbonyl complexes. We also consider that it is more critical for the metal site to satisfy the donor function of the nitrogen molecule than the π-acceptor property, although both are important.

Attempts to prepare a series of nitrogen complexes of cobalt containing alkarylphosphines gave products too unstable for characterisation.