Metal carbonyl basicity
Key points: Most organometallic carbonyl compounds can be protonated at the metal centre; the acid ity of the protonated form depends on the other ligands on the metal. Many organometallic compounds can be protonated at the metal centre. Metal carbonylates provide many examples of this basicity:
The affinity of metal carbonylates for the proton varies widely (Table 22.6). It is ob- served that, the greater the electron density on the metal centre of the anion, the higher its Brønsted basicity and hence the lower the acidity of its conjugate acid (the metal carbonyl hydride). As we noted in Section 22.7, d-block MH complexes are commonly referred to as ‘hydrides’, which reflects the assignment of oxidation number 1 to an H atom attached to a metal atom. Nevertheless, most of the carbonyl hydrides of metals to the right of the d block are Brønsted acids. The Brønsted acidity of a metal carbonyl hydride is a reflection of the π-acceptor strength of a CO ligand, which stabilizes the conjugate base. Thus, [CoH (CO)4] is acidic whereas [CoH (PMe3)4] is strongly hydridic. In striking contrast to p-block hydrogen compounds, the Brønsted acidity of d-block MH compounds decreases on descending a group. Neutral metal carbonyls (such as pentacarbonyl iron, [Fe (CO)5]) can be protonated in air-free concentrated acid; the Brønsted basicity of a metal atom with oxidation number zero is associated with the presence of nonbonding d electrons. Compounds having metal-metal bonds, such as clusters (Section 22.20), are even more easily protonated; here the Brønsted basicity is associated with the ready protonation of MM bonds to produce a formal 3c,2e bond like that in diborane:
The MHM bridge is by far the most common bonding mode of hydrogen in clusters. Metal basicity is turned to good use in the synthesis of a wide variety of organometallic compounds. For example, alkyl and acyl groups can be attached to metal atoms by the reaction of an alkyl or acyl halide with an anionic metal carbonyl:
A similar reaction with organometallic halides may be used to form MM bonds:
