Molybdenum complexes dithiolenes

Transition metal complexes of unsaturated 1,2-dithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties.169 Molybdenum dithiolene complexes have featured prominently170 in these studies and have special significance following the suggestion171,172 that the molybdenum-containing cofactor of the oxomolybdoen-zymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-1,2-dithiolate. 

A combination of descent in symmetry (Ctv —> C3v —> Cs) and orbital overlap considerations have been utilized in order to assign the six lowest energy bands in the (L-/V )MoO(dithiolene) series. The inherently low oscillator strengths of the Mo ligand-field bands in oxo-molybdenum complexes (s < 100 M em ), coupled with the presence of low-energy LMCT excitations, result in ligand-field transitions that are difficult to observe in (L-/V3)MoO(dithiolene) compounds. As a result, LMCT bands are anticipated to dominate the absorption spectra of these complexes. 

CT transitions occur in Mo(IV) oxo-molybdenum-bis(dithiolene) complexes since the dxy acceptor orbital is completely filled. Therefore, it has been proposed that a thermally induced intramolecular electron/charge transfer (valence tautomeric) process is operable in [MovO(qdt)2J1 and responsible for the observed thermochromism. Such a S —> Mo dXi CT process formally results in the conversion of [Mo 0(qdt)2] to the Mo(IV) species LMoIV0(qdt Kqdt) 1 (Fig. 46), which is consistent with the attenuation of both the S —> Mo dxy CT and the dianionic qdt n —> n bands. 

A nitrogenase model reaction has been accomplished in which significant amounts of NH3 (0.013-0.044 mol NH3/M0) are produced when trans-[Mo(N2)2(Ph2PCH2CH2PPh2)2] is added to[FeS(SCHPhCHPhS)]4 reduced to the 4 — or lower level. The molybdenum complex is also reducible by an excess of Na (naphthalene) to afford NH3 (0.100—0.300 mol NH3/M0). No N2H4 was observed in either reduction. The iron-dithiolen system alone is capable of fixing N2 at the 4— level under an N2 atmosphere NH3 up to... 

Scheme 2.22 Synthetic strategy for preparing molybdenum pterin dithiolene complex.

Scheme 2.24 Garner and Joule s method of preparing unprotected dithiolene, and its molybdenum complex.

Owing to their involvement in bioinorganic processes and catalysis, much interest has been attracted by the chemistry of Mo and W 1,2-dithiolene complexes.39,40 Molybdenum oxo-1,2-dithiolene complexes can be typically... 

Lowe and Garner (1993a,b) have synthesized three new dithiolene ligands and formed complexes with a variety of transition metals (Fig. 23) including molybdenum [40]-[42], tungsten [43] and [44] and nickel [45]—[48]. The electrochemical properties of the complexes and free ligands were studied by... 

Molybdenum(IV) complexes with dithiolenes, of general formula [Mo(S2C2R2)3]2, adopt trigonal prismatic coordination. These compounds are discussed in Section 36.6.6. 

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