(5). The Y= O complexes represent the first example of cyclometalated ruthenium (III). The structure of 4b, ie, A with Y= O, R= Me, R'= H, and X= Br, RuC49H4<) N2OP2Br, has been determined by X-ray crystallography. The crystals of 4b are monoclinic in thespace group P2l/c withZ= 4 and unit cell dimensions a= 12.361 (2) Á, 6= 19.091 (3) Á, c= 18.560 (4) k, 0= 108.30 (1), and V= 4158.3 (13) Á1 23 4. The structure was refined to R= 0.046 and Rw= 0.059. The azo ligand and the bromide are bound in approximately planar C, N, O, Br tetracoordination, and the PPh3 ligands occupy trans positions. The coordination sphere is highly distorted octahedral. The EPR spectra of 4b and other Y= O complexes (low-spin d5) are uniformly rhombic (77 K, frozen solution). The magnitude of axial () and rhombic (V) distortionparameters and the optical transition energies (v, and v2) among Kramers doublets are computed to be~ 12000,~ 9000,~ 7000, and~ 16000 cm" 1, respectively. The v, transition is experimentally observed at~ 8000 cm" 1. Results are rationalized in terms of the orbital energy order dxy> dxz> d^. The redox potentials of the Y= O complexes in dichloromethane lie in thefollowing ranges (vs. SCE): ruthenium-(IV)-ruthenium (III),+ 1.0 to+ 0.6 V; ruthenium (III)-ruthenium (II),-0.3 to-0.7 V. In contrast, the Y= OMe, SMe complexes display only the latter couple but at much higher potentials:~ 0.7 V (OMe) and~ 1.0 V (SMe).