Reduction without formation of M-H bonds

On the other hand,24 the replacement of chloride ligands by hydride ligands should result in higher electron density at the metal center, a common criterion for a reduction.54 Such reactions have been termed nucleophilic ionic hydrogenation .55 Others24 have used such terms as reduction and substitution interchangeably. In this section reactions are discussed which can be unambiguously identified as redox processes. 

Chatt and Rowe showed56 that the platinum(O) complex, [Pt(dppe)2l, can be prepared by reduction of (Pt(dppe)2]Cl2 with sodium borohydride in aqueous solution, Eq. 10.8  

Vanadium(O), chromium(O), molybdenum(O), and tungsten(O) complexes of the type [M dmpe)3] have been prepared by treatment of the appropriate metal chlorides with three equivalents of ligand and Na[BH4] or Li[AlH4l in THF. Reductive cleavage of the dimeric complexes, [Mn(CO)5l2/ [Co(CO)4]2, [Co(CO)3(PPh3)]2, [Mo(CO)3Cp]2, and [Fe(CO)2Cp]2, can be effected with potassium hydride in tetrahydrofuran or in a mixture of THF and HMP, Eq. 10.9  

The reduction products are isolated in quantitative yields. It has been shown that alkali metal trialkylborohydrides can also be used in reductive cleavage reactions, but the resultant trialkylborane by-products are difficult to separate and lead to problems in product isolation. 

At least some of the reduction reactions discussed above proceed by 

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