Isotope exchange reactions direct metalation

Substitution reactions are well documented for the three Group 6 metal hexacarbonyls. Since they are low-spin d systems, reactivity is normally lower than that of other metal carbonyls. To promote substitution reactions, it is necessary to use elevated temperatures or uv irradiation. The hexacarbonyls usually undergo substitution via a dissociative mechanism, unless direct attack of a nucleophile at coordinated carbonyl groups occurs. Usually MoCCO) is more reactive than its two congeners. This is shown, inter alia, by kinetic studies on the isotopic exchange reaction between M( CO)6 and The higher kinetic lability of Mo(CO)6 may effect some of the reductions and oxidations, when the electron transfer process is accompanied by loss of carbonyl groups, e.g., equations (a), (e)-(g), and (1). 

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