Molybdenum mixed ligand complexes

The molecular complexity of molybdenum (IV) alkoxides is determined by the size and ramification of the alkyl group — the polymeric (R = Me), dimeric (CN = 5, two bridging OR-groups, R = Pr ) with a double metal-metal bond, and even monomers (R = Bu ). The first representatives of W(OR)4 homologous series are tetramers with the [Ti4(OMe)16]-type structure. The derivatives of ramified or bulky alcohols are known only as mixed-ligand complexes (such as dimeric solvates of alkoxide halids with alcohols, various alkoxide hidrides and monomeric complexes with phenantroline, see Table 12.19). 

J. Selbin, J. Sherrill and C. H. Bigger, Model compounds of biological molybdenum. Flavin and mixed-ligand complexes of molybdenum(IV) and [y], Inorg. Chem., 1974,13, 2544-2549. 

Infrared and electronic spectra confirm the nature of the hexa-N-thiocyanatometallate anions of chromium(III) (293, 639, 647, 656, 663, 665) and molybdenum(III) (293, 489, 647, 663, 664), and molyb-denum(II) (534) a variety of salts have been reported containing these anions (see the foregoing and also Refs. 30 and 528). Infrared spectra and magnetic measurements have been used to characterize [M(NCS)e] (M = Mo, W) and [W(NCS)g] (386). A large number of mixed-ligand thiocyanate complexes of chromium(III) have been reported, and many of these are of the type M[Cr(CNS)4L2], similar to the original Reinecke s salt where L = NH3. 

These techniques are applicable only to paramagnetic Mo(V) centers, but the EPR parameters are extremely sensitive to coordination changes at the molybdenum center 17, 64). The molybdenum and ligand hyperfine splittings can provide additional information about the coordination environment of the molybdenum(V) species and the chemical reactions at the molybdenum center. EPR spectra from xanthine oxidase were first reported in 1959 by Bray et al. (65), and Bray and co-workers have continued to develop the application of EPR spectroscopy to molybdenum enzymes 17, 64). In 1966 it was shown (66) that mixing [Mo04] with dithiols produced EPR signals with (g) and (A( Mo)) values similar to those of xanthine oxidase. Only recently, however, have the structures of such thiolate complexes been determined (see Section IV.B.2.b). 39) and P (67) ENDOR spec-... 

Hexacoordinated mixed-ligand dinitrosyl complexes of molybdenum(O) of the composition [Mo(NO)2(L)(OH)] with a variety of Schiff base ligands have been reported [3]. Such complexes are readily obtained by a single-step reaction between molybdate(VI) and the ligand in presence of hydroxylaminehydrochloride in a dimethylformamide solvent. A ds-octahedral structure has been observed in these complexes, as shown in Figure 12. 

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