Reactions of metal carbonyl cations (Mn(CO)6+, Re(CO)6+, Mn(CO)4PPh3+, Mn(CO)4(PPh3)2+, Mn(CO)5PEt3+, Mn(CO)5PPh2Me+, Re(CO)5PPh3+, and CpFe(CO)3+) with metal carbonyl anions (Co(CO)3PPh3, Co(CO)4, Mn(CO)5−, Mn(CO)4PPh3−, Mn(CO)4PEt3−, Mn(CO)4PPh2Me−, Mn(CO)3(PPh3)2−, CpFe(CO)2−, Re(CO)5−, and Re(CO)4PPh3−) are reported. Peak potentials are reported for all ions, and nucleophilicities (as measured by reaction with Mel) are reported for the anions. Reaction of any metal carbonyl cation with any metal carbonyl anion leads ultimately to binuclear products, which are the thermodynamic products. The binuclear products are formed by a single-electron transfer. In over half of the reactions between metal carbonyl cations and anions, a two-electron change results in a new metal carbonyl cation and anion. The two-electron change may be considered mechanistically as a CO2+ transfer with the more nucleophilic of the two anions retaining the CO2+. The kinetic and thermodynamic driving forces and the suggested mechanism are examined.