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Rhodium and iridium complexes of N-heterocyclic carbenes ( 3a − c and 4a − c) were obtained by transmetalation from the corresponding Ag(I) complexes. The structure of 3b was verified by X-ray diffraction. The compounds display restricted rotation about the metal−carbene bond, the rate of which can be controlled by altering the steric bulk of the auxiliary ligands. Infrared spectroscopy provides an estimate of the electron-donor power of the carbene ligands from ν(CO) of the carbonyl derivatives.
Chianese et al. (2003) studied this question.