Polyhydride-Dihydrogen Complexes

Up until now, we have concentrated mainly on complexes with only two hydrogens, but complexes with up to seven are known, and often it is these that show stretched H--H bonds. The factors governing the stabilities of classical versus nonclassical isomers in polyhydiide complexes have been extensively studied theoretically by Hall and coworkers, who point out that electron correlation is important in computations but that has been recognized only since 1991. For example, calculations by Hay in 1992 on ReH7(PH3)2, showed the presence of one Hj ligand with a short dgn of 0.80 A, but Cl techitiques, which include election correlation, showed that only the classical heptahydride form. The actual PPI13 complex 

For simple polyhydrides, [MHJ (x = 3-9), orbitally ranked symmetry analysis correctly predicts and rationalizes the geometries of over ICD complexes in comparison to their ab initio structures.Such use of symmetry and group theory can rationalize unusual experimental geometries. 

The story was not completely over, however, because the 1.49 A distance between H,and Hi, in 18 is short for a hydride (other dim re longer than 1.75 A), and H H, and H, are coplanar, both of which details were not reproduced by the elaborate calculations. Further calculations by Eisenstein rationalized the coplanarity as due to ds interactions (see Section 4.11), where the absence of an H-H bond does not affect the attraction between the hydrides. The reason that the H.-H bond is essentially broken (and not an H2 ligand) is the presence of sufficient (but unquantifiable) BD into a H,-H from a nonbonding orbital different in energy than other backdonafing orbitals that apparently spread the other hydrides further apart. 

One method for stabilizing an H2 complex in a polyhydride system is protonation to a cationic species to lower the overall electron density at the metal. Although the above protonafion of OsH4(PPh3)j failed to do this, the strategy is viable in many other systems and can even be effected by attachment of Lewis adds such as 

BF3 or BH3 to a hydride in, e.g., Cp2NbH3. Computations show that an Hj complex results, and again there is experimental support Addition of HBR2 lowers the temperature for H2 evolution from Cp2NbH3 on heating, consistent with increased / -H2 character. 

BF3 or BH3 to a hydride in, eg., Cp2NbH3. Computations show that an H2 complex 

Many polyhydride complexes contain Cp ligands and have piano-stool geometries. Calculations by Lin and Hall on CpMHaL4 a (M = Ru, Rh, Os, Ir n = 1-4 L = PH3) show that hydrides with four-legged piano-stool structures such as 19 and 

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The use of polydentate phosphines has received a renewed interest, with the search for new families of polyhydrides or molecular dihydrogen complexes see Dihydrogen Complexes Related Sigma Complexes). For example, PP3RUCI2... 

After the advent of the nonclassical if-Vi.2 complexes, polyhydrides had to be reinterpreted [14]. Thus, RuH4(PPh3)3 is not a classical tetrahydride of coordination number seven at the ruthenium atom but a dihydride-dihydrogen complex Ru(H2)(H)2(PPh3)3 of octahedral geometry [25]. 

Polyhydrides are complexes with a H M ratio exceeding 3 [79]. Complexes of the type IrH5(PR3)2 are polyhydrides of Ir(V). but their protonation products are not hexahy-drides of Tr(VII) but Ir(III) complexes having two dihydride and two neutral dihydrogen ligands (eq (68)). 

Table 1 A selection of H... H distances in dihydrogen rj -XrXz) and polyhydride (H) complexes determined by neutron diffraction ...

The intramolecular exchange of polyhydride complexes often goes through dihydrogen complexes. It is beyond the scope of this work to discuss the many examples of exchange within polyhydrides [13, 14, 15]. 

Up to now we have dealt with quantum phenomena in polyhydride and dihydrogen complexes. In both cases the tunnelling effect is used to explain... 

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