An unexpected cobalt-containing zwitterionic complex, [{(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4)}{mu,eta-(R1)PC CCH(2)(R2)}] (8; R1 = Ph(2)Bz; R2 = 3-CoBr(3)-1-benzimidazole), has been obtained during an attempt to modify the imidazole moiety in the alkyne-bridging dicobalt complex [{(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4)}{mu,eta-Ph(2)PC CCH(2)(IM)}] (5; IM = benzimidazole) to a carbene-type substituent by reaction with benzyl bromide. As revealed in the crystal structure of 8, it is clear that the benzyl bromide was nucleophilically attacked by the phosphane moiety of 5 with the result the addition of the benzyl moiety. This phosphane site in 8 is in a tetrahedral environment and presumably cationic in nature. In addition, an unusual anionic species CoBr(3)(-) was found to have attached itself to the free nitrogen site of the imidazole moiety. Thus 8 can be regarded as an unusual metal-containing zwitterionic complex. When the phosphane site of 5 was replaced by a non-reactive substituent, for example, [{(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4)}{mu,eta-Ph(2)PC CCH(2)(IM)}] (5': R = H; 5_O: R = Ph(2)(O)P), nucleophilic attack took place exclusively on the imidazole moiety and led to the formation of [{(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4)}{mu,eta-Ph(2)PC CCH(2)(IM')}(+)]Br(-) [9': R = H; 9_O: R = Ph(2)(O)P; IM' = 3-benzyl-1H-benzimidazole]. Complex 9' could act as a carbene precursor, which was demonstrated by the formation of the bis-9'-coordinated palladium complex [trans-[{(mu-PPh(2)CH(2)PPh(2))Co(2)(CO)(4)}{mu,eta-HC CCH(2)(IM')})(2)PdBr(2)] (11) on treatment with [Pd(cod)Cl(2)]. Fair-to-good efficiencies were observed in Heck reactions employing either isolated 11 or in situ 9'/PdCl(2) (2:1) as the catalytic precursor