Hydrides, metal nonclassical structures

Polyhydrides often have coordination numbers in excess of 6, a consequence of the small size of the hydride ligand. Nine is the normal limit on the number of ligands imposed by the availability of nine orbitals, but if a polyhydride can adopt a nonclassical structure with an H2 molecule bound via a single metal orbital, this limit can be exceeded. A rare example of such a complex is [WH7(PPh(CH2CH2PPh2)2)] (Eq. 15.20), which is stable up to -20°C in solution. Since 15.3 is classical with terminal M—H bonds, and therefore d°, there are no metal lone pairs and so protonation must occur at the M—H bond to give an H2 complex directly. If it were classical, 15.4 would exceed the maximum allowed oxidation state and coordination number for a transition metal. 

In this case, there is no reason to suppose that hydrogen is coordinated in any way other than the classical dihydride manner (30) see Hydrides Solid State Transition Metal Complexes). However, similar experiments using Cr(CO)5 gave a product Cr(CO)5H2 for which strong circumstantial evidence pointed towards the nonclassical dihydrogen structure (31). Unfortunately, in low-temperature matrices... 

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