The alkali metal-nickel carbonyl anions ENi(CO) 3 − with E=Li, Na, K, Rb, Cs have been produced and characterized by mass-selected infrared photodissociation spectroscopy in the gas phase. The molecules are the first examples of 18-electron transition metal complexes with alkali atoms as covalently bonded ligands. The calculated equilibrium structures possess C 3v geometry, where the alkali atom is located above a nearly planar Ni(CO) 3 − fragment. The analysis of the electronic structure reveals a peculiar bonding situation where the alkali atom is covalently bonded not only to Ni but also to the carbon atoms.