Molybdenum complexes mononuclear

The reactions of dinuclear thioaldehyde and -ketone complexes with PR3 take a different course. When the dimeric complexes 112 are treated with PMe3 a Ti-S bond in 112 is cleaved and the monomeric complexes 113 are reversibly formed (see Scheme 26), 69 The reaction of 112 with benzene-thiol led to a Ti-C cleavage and formation of 1,3-propanedithiolato-(phenylthiolato) complex.69 The reaction of binuclear p-191 rf-thioketone molybdenum complexes related to 115 (M = Mo, E = S, R = Aryl) with P(OEt)3 also gave mononuclear complexes [Mo(CO)2(Et) i72-S = C(Aryl)2 (i75-C5H5)] by a rather complicated reaction sequence. When other phosphites were employed additionally binuclear complexes derived from addition of the phosphite to one molybdenum atom were isolated.225... 

Tables I and II summarize the structural studies of mononuclear and binuclear vinylidene complexes, and Table III those of propadienylidene complexes which had been reported to mid-1982. As can be seen, the C=C bond lengths range from 1.29 to 1.38 A, and the M-C bond (1.7-2.0 A) is considerably shorter than those found in alkyl or simple carbene complexes. Both observations are consistent with the theoretical picture outlined above, and in particular, the short M-C bonds confirm the efficient transfer of electron density to the n orbitals. In mononuclear complexes, the M—C=C system ranges from strictly linear to appreciably bent, e.g., 167° in MoCl[C=C(CN)2][P(OMe3)2]2(fj-C5H5) these variations have been attributed to electronic rather than steric factors. In the molybdenum complex cited, the vinylidene ligand bends towards the cyclopentadienyl ring (111).

Mononuclear vs. Dinuclear Sites. All molybdenum enzymes contain two molybdenum atoms. Dinuclear molybdenum complexes are well... 

A range of mononuclear molybdenum complexes containing chelate triazenide ligands has also been reported. The orange anion... 

There is very little information concerning one-electron reactions at oxo-molybdenum centers because of the tendency for oxo-molybdenum complexes to form dinuclear systems (37 and 38). To our knowledge, (L-N3)MoO,jX3, j complexes constitute the only family of mononuclear... 

Table 7-9. Formal electrode potentials (vs. SCE) for the redox processes exhibited by the ferro-cenyl-molybdenum complexes shown in Scheme 7-7 and related mononuclear species...

In addition to the compound classes mentioned so far, cyclopropenylidene complexes have also been reported. The first mononuclear carbene complex not stabilized by a heteroatom in the (X-position was the chromium complex 27, which proved to be unexpectedly stable. This complex was obtained in 20% yield by treatment of 3,3-dichloro-l, 2-diphenylcyclopropene with disodium pentacarbonylchromate at — 20 °C. However, isolation of the complex is easier when it is prepared from 3-ethoxy-l,2-diphenylcyclopropenium tetrafluoroborate. The corresponding molybdenum complex can also be obtained in this way. 

The lower nuclearity clusters [Mo5Cl13]2 were isolated from MoCls in AlCl3/KCl/BiCl3/Bi melts that led to the hexanuclear species (17). This is the only intermediate isolated from reaction mixture that produces the Mo6C18 4+ core. Solution chemistry has yielded Mo2, Mo3, and Mo4 halide species from mononuclear molybdenum complexes, suggesting that a nucleation process similar to that proposed for the tungsten systems may take place during the formation of hexanuclear molybdenum clusters (18-22). 

Fig. 8 Mononuclear molybdenum complex bearing a tridentate ditbiocarboxylato ligand...

Assuming that a mononuclear hydrazido complex is reduced to 2[NH4]", then intermolecular electron transfer from another molybdenum complex is necessary to account for all molybdenum appearing in the product as molybdenum(III). Intermolecular electron transfer can be ruled out. 

Puddephatt and co-workers have developed binuclear ruthenium complexes able to catalyze formic acid decomposition at room temperature in acetone and dichloromethane, which are more active than mononuclear complexes (38). Het-erobimetallic complexes containing Ru and either Mo or W, decomposed formic acid very slowly in THF (39). The molybdenum complex Cp Mo(PMe3)2(CO)H (where Cp = pentamethylcyclopentadienyl) has also been found to decompose formic acid into CO2 and H2 in benzene (40). 

Ni(CN)3(NO)] - + iOa + HaO—[Ni(CN)3(NO)]2- + 20H-The importance of the redox nature of molybdenum centres in biological systems has been recognized in several studies. Model systems for nitrogenase, involving molybdate, catalytic amounts of iron(ii), and cysteine, effect the reduction of aliphatic nitriles to alkanes and ammonia in the presence of sodium borohydride. The reaction is accelerated by substrate concentrations of ATP, the active species being considered to be mononuclear molybdenum-thiol complexes. The dinuclear molybdenum complex (7) formed by... 

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). 

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). 

Metal—Metal Bonding. The degree of nuclearity exhibited as a function of the oxidation state of molybdenum is shown in Table 1. In the highest oxidation state, Mo(VI), the tendency is to form mononuclear or a wide variety of polynuclear complexes in which there are no... 

Reactions of the metallocene derivatives of molybdenum with pyrazole lead to the mononuclear complexes of the type 22. Structure 22 shows that it cannot be used as a ligand for the preparation of dinuclear complexes owing to geometric constraints [80JOM( 197)291 83JOM(253)53]. In acetone, an unusual complex 23 is formed [83JOM(253)53]. The bidentate ligand is the product of the reaction of pyrazole and acetone. 

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

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