Reactivity of [Co4(CO)12].Synthesis and characterization of tetracobalt carbonyl anions of general formula [Co4(CO)11X]

Salts of the green-brown anions [Co4(CO)11X]- (X = Br, I, or SCN) have been obtained at -70 °C starting from [Co4(CO)12] and tetrasubstituted ammonium ([NEt4] Br or [NEt4]I) or phosphonium {[PMePh3]Br, [N(PPh3)2]I, or [N(PPh3)2][SCN]} salts. Using sodium alkoxides or propylamine, formation of purple-red solutions containing, on the basis of the i.r. spectra, similar monosubstituted anions (X = COOMe, COOPri, or CONHPrn) has been observed. The transformation of [Co4(CO)12] into [Co2(CO)8] has been studied in PriOH at room temperature under cabron monoxide. The considerable acceleration of the rate of this reaction by halide ions is in agreement with the ready decomposition observed for all the [Co4(CO)11X]- anions in Lewis-base solvents.