A Ce atom reaction with ethylene was carried out in a laser-vaporization metal cluster beam source. Ce(C2H2) formed by hydrogen elimination from ethylene was investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The theoretical calculations include a scalar relativist correction, dynamic electron correlation, and spin orbit coupling. The MATT spectrum exhibits two nearly identical band systems separated by 128 cm(-1). The separation,is not affected by,deuteration. The two-band. systems are attributed to spin orbit splitting and the vibrational bands to the symmetric metal ligand stretching and in-plane carbon hydrogen bending excitations. The spin orbit splitting arises from interactions of of nearly degenerate triplets and 4 pair of nearly degenerate singlets. The organolanthanide, complex is a metallacyclopropene in C-2v symmetry. The low-energy valence electron configurations of the neutral and ion species are Ce 4f(1)6s(1) and Ce 4f(1); respectively. The remaining two elections that: are associated with the isolated :Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a 5d Ce orbital and a pi* antibonding orbital of acetylene.