Terminal M—H Bonds

Of the monomeric metal carbonyl hydrides [HMn(CO)s,H2Fe(CO)4,HCo(CO)4], only HMn(CO)s has been analyzed crystallographically 10 4S). The lack of results for H2Fe(CO)4 and HCo(CO)4 is probably related to the fact that the compounds are thermally unstable. The neutron diffraction analysis of HMn(CO)s by La Placa and co-workers45) produced a Mn—H distance of 1.60(2) A. This result, together with the earlier measurement of the Re—H distance of 1.68(1)A in K2ReH9, provided conclusive proof that M—H distances in metal hydride complexes are normal (i.e., are consistent with the known covalent radii of the elements), and not anomalously short as suggested by some researchers (this controversy is reviewed in Refs. 1—3). 

2 NEt4]2lRe4(CO)16]2-, and its companion (Re4(CO)16l2 anion is shown on the right (Ref. 50) 

Other monomeric carbonyl hydrides whose structures have been investigated are the cis and trans isomers of [H2Re(CO)4]- 49,5°). In neither case have the hydride ligand s been directly located. Trans-[H2 Re(C0)4 ] was prepared in low yield from the pyrolysis of c/s-[H2 Re(CO)4 ], and was isolated as crystals of an unusual mixed salt [NEt4]2[Re4(CO)I6]2 [NEt4]+[H2Re(CO)4] 5°) (Fig. 8). 

In virtually all mononuclear hydride complexes, the H ligand occupies a stereo-chemically active position in other words, it is usually possible to infer H positions by searching for holes in the coordination sphere around the metal atom. Exceptions, however, are found in the structures of HCo(PF3)4 6I), HRh(PPh3)4 62), and HRh(PPh3)3(AsPh3)63). In these complexes, the P and As atoms define almost tetrahedral coordination spheres around the central atom (Fig. 12). Under these circumstances, it is not obvious from X-ray results alone where the H atoms might be located. 

Finally, the unusual ruthenium hydrido complex HRu(PPh3)3[HC=C(Me)C(0)0C4H9] is formed in the reaction between H2Ru(PPh3)4 and n-butylmethacrylate77). The Ru atom has oxidatively added to a vinylic C-H bond, and this compound represents the first example of such a structure. 

---

Terminal M-H bonds in metal cluster complexes are occasionally found (see Table 3). Some of them are noteworthy in that they contain both terminal and bridging H ligands on the same metal atom. Examples include H2Os3(CO)n 78>... 

These complexes are sometimes called nonclassical hydrides because prior classical hydrides always had terminal M-H bonds or M-H-M bridges. Dihydrogen complexes are... 

The temperature variation of the Tj of the hydride resonance of a polyhydride complex in the NMR can be used to determine whether the structure is classical, with terminal M-H bonds only, or non-classical, containing one or more dihydrogen ligands and to estimate the H-H distance. 

After completing our work on C-H activation in 1985, we participated in the development of the chemistry of dihydrogen complexes by showing their generality and that they can be synthesised by protonation of compounds with a terminal M-H bond (Equation 1). ... 

The sum of the total bond strengths for a bridging hydride can be greater than the strength of a terminal M-H bond. Vites and Fehlner have demonstrated that the interaction energy of the three atoms in the Fe(fx-H)Fe unit is about 83 kcal/mol. 

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. 

---