FIGURE 1.23 The main metal 3d-dioxygen interactions: (a) bent structure; (b) side-on structure. (Adapted from .)
tion state and has no low-lying empty d-AO, showing a high electron donor potential, only the d -+ n* transfer of electron takes place, resulting in the formation of the molecular ion O2. By determining the structures of the iridium-oxygen complexes, Ibers  discovered that the 0-0 distance and the tendency to bind oxygen increases as the ancillary ligands become more electron donating. For example, in CI-Ir(CO)L202 the 0-0 distance is 1.30 A, in the iodide analogue it is 1.51 A, and in the phosphine chelate Ir(Ar2P)402 it is 1.66 A. The valence frequency of the 0-0 bond is considerably lowered (800-900 cm-1) in comparison with the free molecule (1555 cm-1).