Molybdenum bonding

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... 

T ike metals minerals also exhibit typical crystalline structures. As an example, the structure of molybdenite is shown in Figure 1.17. It is hexagonal with six-pole symmetry and contains two molecules per unit cell. Each sulfur atom is equidistant from three molybdenum atoms and each molybdenum atom is surrounded by six sulfur atoms located at the comers of a trigonal prism. There are two types of bonds that can be established between the atoms which constitute the molybdenite crystal stmcture. They are the covalent bonds between sulfur and molybdenum atoms and the Van der Waals bonds between sulfur-sulfur atoms. The Van der Waals bond is considerably weaker than the covalent sulfur-molybdenum bond. This causes the bonds of sulfur-sulfur to cleave easily, imparting to molybdenite the property of being a dry lubricant. Molybdenite adheres to metallic surfaces with the development of a molecular bond and the friction between metallic surfaces is replaced by easy friction between two layers of sulfur atoms. 

A number of excellent reviews have been written that address various aspects of metal-metal bonding. In particular, Cotton has contributed several eminent early reviews commensurate with his pioneering efforts in this area (54, 59, 60). More recently he reviewed quadruple bonds (56), molybdenum-molybdenum bonds (57), and multiple bond studies in his laboratory (55) in separate articles. Earlier reports discuss multiple metal-metal bonds in the context of general metal—metal bonding schemes and clusters at a time when the area was more limited in scope and therefore amenable to an encompassing survey (13, 128, 129, 164, 188, 189, 252). The redox behavior of metal-metal bonds has been reviewed by Meyer (193), and Walton has recently described ligand-induced redox reactions of strong multiple metal—metal bonds (253). 

One-electron oxidation of the Mo2+ unit in Mo2(S04)4" leads to an increase of about 0.05 A in the molybdenum-molybdenum bond length (78). 

To obtain the enthalpy contribution of the metal-metal bond in each case, it is suggested (25) that there is a logarithmic relation between bond length and bond enthalpy contribution both for the molybdenum-molybdenum bonds and for the molybdenum-oxygen bonds. The data base includes molybdenum metal and [Mo(acac)3] and M0O3. The enthalpy contribution E(Mo-Mo) of a... 

The shortest molybdenum-molybdenum bond (203.7 pm) known at the present time is found (20) in fM02(N(2-pyridyl)acetamide>4] for which E(Mo-Mo) is calculated to be -550 kJ mol l. 

Both silver nitrate and molybdenum hexacarbonyl react with the nine-membered ring system m,m,m-cyclonona-l,4,7-triene to form complexes of the types (C9Hi2)-3 AgN03 and C9Hi2Mo(CO)3, respectively (2S8). In the assigned structures, based on NMR and related data, the silver nitrate complex has the silver ions associated with the outertt-orbital lobes (XLIII), while the molybdenum tricarbonyl complex has the molybdenum bonded to the inner 7r-orbital lobes (XLIV). In both complexes, a crown conformation of the ligand is assumed. 

For the more general case, a coordinatively unsaturated intermediate of type A resulting from migration of the hydrocarbyl group may be stabilized in a number of ways . It has been pointed out that for CO insertion into the methyl-molybdenum bond of Me—Mo( -C5H5)(CO)3 [reaction (b)], which has been studied in tetrahydrofuran (THF) and in methyl-substituted THFs, the rate constant of the migratory step /ci significantly increases with the donicity of the solvent (the methyl-substituted THFs differ in donicity but have similar dielectric constants). 

Molybdenum dialkyldithiocarbamates Molybdenum dialkyldithiocarbamates are multifunctional lubricant additives as anti-wear, anti-friction and antioxidants. Molybdenum dialkyldithiocarbamates are also multifunctional antioxidants due to the hydroperoxide decomposing ability of dialkyldithiocarbamates, see Section 4.4.2, and the radical scavenging capacity of molybdenum. The best established structure for molybdenum dialkyldithiocarbamates is a six-coordinate complex of a dinuclear molybdenum centre with each molybdenum bonded to terminal oxygen or sulphur atoms, two bridging oxygen or sulphur atoms and one dialkyldithiocar-bamate ligand. Fig. 4.6 [44] ... 

---