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Second order rate constants are reported for the reactions of metal carbonyl anions ([M(CO)nL]−) with several vinyl halides: PhCClC(CN)2, Z‐ and E‐Ph(CN)CCHHal (Hal = Cl, Br) which follow the addition–elimination (AdNE) substitution mechanism. The obtained data show that the nucleophilic reactivity of [M(CO)nL]− anions towards vinyl halides increases in the same order as in aliphatic SN2 reactions, but much more steeply, by 14 orders of magnitude in the row log{$k_{[{\bf M}({\bf CO})_n {\bf L}]{\bf M'}} {\rm /}k_{[{\bf CpMo}({\bf CO})_3 ]{\bf K}}$}: [CpFe(CO)2]− (∼14), [Re(CO)5]− (7.8), [Mn(CO)5]− 2.1, [CpW(CO)3]− (0.7) > [CpMo(CO)3]− (0). A good correlation exists between nucleophilicities of [M(CO)nL]− anions towards vinyl (sp2‐carbon) and alkyl halides (sp3‐carbon) with slope 2.7. The reactivity of [M(CO)nL]− in a halogen–metal exchange process (with Z‐PhC(CN)CHI) follows a similar ‘large’ scale as in the AdNE process. The nucleophilicity of [M(CO)nL]− anions correlates better with their one‐electron oxidation potentials (Eox) than with their basicity (pKa of [M(CO)nL]H). Copyright © 2008 John Wiley & Sons, Ltd.

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