Heterobimetallic lanthanum-nickel and cerium-nickel carbonyls, LnNi(CO)(n)(-) (Ln = La, Ce; n = 2-5), were generated using a pulsed laser vaporization/supersonic expansion ion source. These compounds were characterized by photoelectron velocity-map imaging spectroscopy and quantum chemical calculations. The binding motif in the most stable isomers of the n = 2 and 3 clusters consists of one side-on-bonded carbonyl. A new building block of two side-on-bonded carbonyls is favored at n = 4, which is retained at n = 5, evidencing the increase of the number of extremely activated CO molecule in the larger clusters. The experimental and theoretical results demonstrate the ligand-enhanced CO activation by the early lanthanide-nickel heterodimers, which would have important implications for the design of alloy catalysts for activation of a molecular ligand.