Hydrides, metal bridging

Dinuclear elimination processes are possible only when at least one of the ligands to be eliminated is a hydride. This is supported by our observations (dinuclear elimination from Os(CO)4(H)CH3 ana Os(CO)4H2 but not from Os(CO)4(CH3)2) and is explained reasonably by the unique ability of a hydride to bridge a pair of transition metal atoms. Tne interaction of Os-H with a vacant coordination site on another Os to form a dinuclear species appears to be an essential part of the dinuclear elimination process. 

These carbonyl anions are strong nucleophiles (see Nucleophile) and can be used to form a diverse range of new compounds. Protonation gives the /t-hydride (see Bridging Ligand) [(/u.-H) Mo(CO)5 ]. Reactions with other metal carbonyls lead to CO substitution and the formation of metal metal-bonded heteronuclear anions, for example, [Mo(CO)s Fe(CO)4] and [Mo(CO)5-Co(CO)4] . Reaction with main group halides is shown in Scheme 1. The dianion reacts similarly. 

Similarly, the low-temperature - C and H NMR spectra of [HFe4(CO)i3] indicate the presence of two distinct metal frameworks - a butterfly (4) and a tetrahedral core (5). The first contains a dihapto CO ligand bridging the wingtips whereas the hydride ligand bridges the hinge. 

Fig. 5 Case studies of (a) bridging hydride, (b) bridging carbonyl with negligible -backbonding, (c) bridging thiolate, (d) bridging carbonyl with strong it-backbonding, (e) bridging carbonyl with its o-donation averaged over two metals, and (f) an alternative Lewis structural representation of a bridging CO with the addition of a metal-metal bond, an addition that is often invoked to satisfy the 18-e rule...

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