Molybdenum compounds, dinuclear

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). 

M1M) + Complexes (2). In scores of dinuclear molybdenum compounds, and to a lesser extent ditungsten compounds, a central unit has a M-M quadruple bond of configuration a2 1 2. 

Analysis of the Bonding in Dinuclear Molybdenum Compounds with Bridging Methyl and Hydride Ligands as a Representative Study... 

Table 4 Comparison of calculated bond orders for a series of dinuclear molybdenum compounds with those predicted by two alternative electron-counting methods. In each case, the bond order predicted by the [p.-LX] half-arrow method corresponds exactly to that derived by consideration of the orbital occupations, whereas the half-electron method fails for compounds with bridging hydride and methyl ligands. Data ttiken from reference 6...

Molybdenum(V) compounds generally occur as mononuclear or dinuclear species. Molybdenum pentachloride [10241-05-1] MoCl, formed by combination of the elements, serves as a usebil and reactive starting material (Fig. 1). MoCl has a dinuclear stmcture (Fig. 3) in the soHd state but is mononuclear in the gas phase. In solution or in the soHd state the compound, actually Mo2C1 q (Fig- 3a), is readily hydroly2ed in air to form MoOCl ... 

Molecular examples of trivalent molybdenum are known in mononuclear, dinuclear, and tetranuclear complexes, as illustrated in Figure 5. The hexachloride ion, MoCk (Fig- 5a) is generated by the electrolysis of Mo(VI) in concentrated HCl. Hydrolysis of MoCP in acid gives the hexaaquamolybdenum(III) ion, Mo(H20) g, which is obtainable in solution of poorly coordinating acids, such as triflic acid (17). Several molybdenum(III) organometaUic compounds are known. These contain a single cyclopentadienyl ligand (Cp) attached to Mo (Fig. 5d) (27). 

Dinuclear stmctures are known for molybdenum(III) in a series of air and moisture sensitive compound containing multiple Mo—Mo bonds. Examples include Mo2(N(CH2)2)g (Fig- 5b) and Mo2(CH2Si(CH2)3)g in which there is a strong Mo—Mo bond, presumably of triple-bond character (28). 

The dinuclear compounds of molybdenum(II) have strong, quadmple, molybdenum—molybdenum bonds and ecHpsed stmctures, such as those of Mo2CF g (Fig- 6 ) Mo2(02CCH3)4 (Fig. 6b). These quadmply bonded complexes have been intensely studied from the point of view of their... 

Mononuclear molybdenum(V) compounds, 77 23. See also MolybdenumCV) dinuclear complexes... 

A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. Two of the older methods, namely, the reaction of the hexacarbonyls with halogens in the presence of an isocyanide (775,116) or reactions of the salt Ag4Mo(CN)8 with isocyanides (74), have given six- and seven-coordinate products. Recently, however, the discovery of the reductive or nonreductive cleavage of multiple metal - metal bonds in dinuclear group VIA compounds by isocyanides has provided a facile route to the synthesis of a variety of homoleptic and related isocyanide complexes of these metals in reasonable yields. 

Dithiocarbamates, in Ru and Os half-sandwiches, 6, 493 Dithiocarbenes, Pt complexes, 8, 439 Dithiocarboxy ligands, in molybdenum carbonyls, 5, 447 Dithiolate-bridged compounds in dinuclear iron compounds with Fe-Fe bonds, 6, 238 as iron-only hydrogenase biomimetic models, 6, 239 Dithiolate diamides, with Zr(IV), 4, 784 Dithiolene—uranium complexes, synthesis and characterization, 4, 212 Ditopic receptors, characteristics, 12, 489 Ditungsten complexes, associated reactions, 5, 748 Divinyllead diacetates... 

The compound loses a chlorine to give M0NCI3 when heated in CC14 at 100°C, and water produces dinitrogen, ammonia, and molybdenum blue. It is superficially analogous to the tantalum dinuclear species discussed above, but its paramagnetism is still a puzzle. 

I should like to propose that all the types of reactions which have been established for mononuclear transition metal complexes will also occur for dinuclear transition metal complexes, and furthermore, that the latter will show additional modes of reactivity which are uniquely associated with the metal-metal bond. In this article, I shall support this proposal by illustrations taken from the reactions of dinuclear compounds of molybdenum and tungsten, two elements which enter into extensive dinuclear relationships (J ). 

For the sake of brevity, I have restricted my attention here to M-M bonds of integral order and have considered only the cases where all the available dn electrons are used to form M-M bonds. There are, however, dinuclear compounds having M-M bonds of fractional order (14) and dinuclear compounds in which not all the available dn electrons contribute to M-M bonding. Well known examples of the latter are Cp2Mo2(C0) (MsM) 0 5) and Cp2Mo2(C0)g (M-M) ( 6) compounds which both contain molybdenum atoms in oxidation state number +1 (formally they are d -d dimers), but by considerations of the EAN rule and the observed M-M distances (2.448(1) and 3.235(1) 8) are commonly considered to have M-M triple and single bonds, respectively. 

Molybdenum and tungsten in their lower oxidation states have a profound tendency to form M—M bonds, and therefore cluster compounds, as well as the dinuclear species just discussed in Section 18-C-10. One class of these clusters has already been presented in Sections 18-C-7 and 18-C-8, namely, the trinuclear species such as the M3Of+ and M3SJ+ cores and the cuboidal M4S3+ ones. This section will now deal with a number of other clusters of these elements. 

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. 

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