Molybdenum complexes phosphine ligands

Nitrido molybdenum complexes have been reported for MoVI, Mov, and MoVI. Among these, MoVI nitrides have been the most extensively studied. For the other two classes of nitrido molybdenum complexes, phosphine and thioalkoxide ligands are common, as they are suggested to stabilize the lower oxidation states through n-back bonding. 

Molybdenum(0) also forms a variety of dinitrogen complexes (41), especially when there are phosphine ligands in the molybdenum coordination sphere (see Fig. 7c). This type of complex has been extensively studied because the coordinated dinitrogen is reduced to ammonia upon acidification. 

The most extensive group of molybdenum hydride complexes are those which contain phosphine ligands, and mononuclear systems with one to six hydride ligands have been identified. 

Simple chiral phosphines have already been mentioned (Section 3.1.3) and the macrocycle enantiomers are discussed below (Section 4.6). Current research in this area is concentrated on bidentate chiral phosphines, such as the ligands (24)-(27). Although their transition metal complexes are normally used for stereospecific synthesis, Whitmire and coworkers used the molybdenum complexes to resolve their racemic bisphosphines via flash chromatography. The phosphines were decomplexed by reductive cleavage at low temperatures (-78 °C) using sodium naphthalenide (Scheme 1). 

Norbornadiene (NBD) in (NBD)M(CO)4 (353) is readily displaced by CO (274) or (2-allylphenyl)(diphenyl)phosphine (50, 312). Although the latter reaction gives the compound of expected composition, (C2iHi9P)M(CO)4, both the chemical and spectral data indicate that it has the structure (26) in which the C21H19P ligand is the isomeric (2-propenyl)(diphenyl)phosphine. For the molybdenum complex this structure has been confirmed by X-ray diffraction (379). 

The gradual substitution of the phenyl groups in the phosphine ligand for ferro-cenyl subunits affords the diferrocenyl- and triferrocenyl-phosphinepentacarbonyl-molybdenum complexes, respectively. With respect to the redox pathway shown in Fig. 7-11, each added ferrocenyl ligand involves the appearance of a further one-electron oxidation [50]. The relevant redox potentials are given in Table 7-7. 

Like the molybdenum complexes, a series of monoferrocenyl-, diferrocenyl- and triferrocenyl-phosphine pentacarbonyltungsten complexes have been prepared. Each ferrocenyl ligand undergoes reversible one-electron oxidation, whereas the tungsten moiety undergoes irreversible oxidation. The relevant potential values are reported in Table 7-12. 

Figure 2. Molybdenum complex depicting a short O-H-phosphine ligands were omitted for clarity).

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