Molybdenum dinuclear chemistry

Divalent molybdenum compounds occur in mononuclear, dinuclear, and hexanuclear forms. Selected examples are shown in Figure 6. The mononuclear compounds are mostiy in the realm of organometaUic chemistry (30—32). Seven-coordinate complexes are common and include MoX2(CO)2(PR3)2, where X = Cl, Br, and I, and R = alkyl MoCl2(P(CH3)3)4, heptakis(isonitrile) complexes of the form Mo(CNR) 2 (Fig. 6d), and their chloro-substituted derivatives, eg, Mo(CNR)3CR. The latter undergo reductive coupling to form C—C bonds in the molybdenum coordination sphere (33). 

In the preceding paper, Malcolm Chisholm (1) has presented a cogent case for the modeling by metal alkoxides of certain aspects of the structural chemistry and reactivities of metal oxides. The focus of this work has been the dinuclear and polynuclear alkoxides of molybdenum and tungsten, an area of research which has also attracted our interest 02-4) and upon which I would now like to take this opportunity to comment. 

One of the broadest and historically most important areas of dimetal chemistry is that of the simple carbonyl complexes (see Carbonyl Complexes of the Transition Metals). As mentioned above, Co2(CO)s, Fe2(CO)9, and Mn2(CO)io were among the first metal-metal bonded complexes characterized. To this day, these complexes continue to be involved in new chemistry, for example, Co2(CO)g found recent use in a one-pot synthesis of tricyclic 5-lactones .In the case of molybdenum, the zerovalent carbonyl is monomeric however, reduction gives a dinuclear metal carbonyl dianion in which the metal is in the [-1] oxidation state (equation 6). 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

Reviews have appeared of the photophysics of molybdenum complexes, primary and secondary processes in organometallic chemistry, flash photolysis of Pe(CO)5 and Cr(CO)g, dinuclear manganese carbonyl compounds, the photochemistry of metal complexes isolated in low temperature matrices, cluster complexes, diene complexes, photoproduction of coordinativeiy unsaturated species containing rhodium or iridium, and redox chemiluminescence of organometallic compounds.Synthetic and metal organic photochemistry in industry has also been reviewed. 

Reaction Schemes for Dinuclear Compounds Containing Metal-Metal Triple Bonds Illustrated by Recent Findings in the Chemistry of Molybdenum and Tungsten... 

Molybdenum and copper form many sulfido-bridged complexes, ranging in size and complexity from simple dinuclear species through incomplete and complete cubanes to large, complex clusters. The initial driver of Mo/Cu/S chemistry was its relevance to Mo-induced Cu-deficiencies in ruminants (Mo-Cu antagonism). The importance of Mo/Cu/S chemistry in medicine " and the synthesis of electro-optical devices and metal-organic frameworks " now stimulate Mo/Cu/S chemistry. Efforts to model... 

However, by development of the chemistry of the kinetically and thermodynamically stable dinuclear tris(3,5-dimethylpyrazolyl)borato molybdenum nitrosyl and oxo complexes, it has been possible to perform combined studies of electronic and magnetic interactions. The purpose of this case study is to illustrate how a wide variety of physical methods has been used in this area, and to show how, in these complexes, the electronic and magnetic interactions—despite being quite different phenomena—share the same relationship to the structure of the bridging ligand. 

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