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A few dioxomolybdenum complexes of the type MoO2L and MoO2L·B of a new diprotic tridentate ONS chelating ligand, H2L, have been synthesized with the aim to examine their potential to behave as models for the active site of an oxidoreductase molybdoenzyme like xanthineoxidase. The MoO2L complex produces MoO2L·B on treatment with neutral monodentate Lewis bases such as γ-picoline, 2-methylimidazole or 1-allylimidazole, utilizing the vacant sixth coordination site. They have been characterized by spectroscopic and electrochemical techniques. The complexes MoO2L (1), MoO2L(γ-pic) (2) and MoO2L(1-allyl imz) (4) were structurally characterized by single crystal X-ray diffraction. The complex MoVIO2L exhibits oxotransfer to PPh3 in acetonitrile medium leading to the formation of MoIVOL, which is reoxidized to MoVIO2L on treatment with DMSO or pyridine N-oxide. DFT calculations on the ligand and complexes 1, 2 and 4 were also carried out.

The new rhenium(I) carbonyl complexes containing {Re(CO) 3 } + fragment with redox non-innocent ligands L 1 /L 2 (L 1 , 1-methyl-2-{( o -thiomethyl)phenylazo}imidazole and L 2 , 1-ethyl-2-{( o -thioethyl)phenylazo}imidazole) having general formula fac -[ReX(CO) 3 (L 1 /L 2 )] ( 1 / 2 ) (X = Cl ( 1 ) and Br ( 2 )) have been synthesized and characterized by both experimental and theoretical studies. The structural confirmation has been carried out for 1a . The complexes show one irreversible Re I /Re II oxidation and one reversible along with an irreversible ligand reduction in the cyclic voltammetric studies. The electronic structure and the nature of Re–CO bonding has been explained by means of DFT and NBO calculations. The spin allowed singlet–singlet electronic transitions of 1a and 2a have been calculated with TDDFT method, and the experimental spectra of the complexes have been discussed on this basis.