Metal carbonyls dissociative character

Some metal carbonyl hydrides dissolved in polar solvents such as water, alcohols, nitriles, ketones, etc., are acids. The acidic character of these complexes is due not only to the polarity of the metal-hydrogen bond, but to solvation of the proton and the carbonylate anion. Acidic metal carbonyl hydrides dissolved in nonpolar solvents do not dissociate, and their chemical properties are like those of nonacidic metal hydrides. [HCo(CO)4] and [HV(CO)6] are strong acids (Table 2.28). 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

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