The reaction between [Pd-2(dba)(3)] (dba = dibenzylicleneacetone), tributylphosphine, and a bis(cycloalkeno)-1,4-diselenin leads to either a mononuclear diselenolene [Pd{SeC(R-1)=C(R-2) Se}(PBu3)(2)] or a dinuclear diselenolene [Pd-2{SeC(R-1)=C(R-2)Se}(2)(PBu3)(2)] [R-1, R-2 = (CH2)(n), n = 4, 5, 6] depending on the stoichiometry employed. Treatment of the dinuclear diselenolenes with 1,2-bis(diphenylphosphino)ethane (dppe) provides a high-yielding route to the mononuclear species [Pd{SeC(R-1)=C(R-2)Se}(dppe)]. All new compounds have been characterized by standard spectroscopic and analytical techniques, in particular by multinuclear NMR spectroscopy; the structure of each of the mononuclear tributylphosphine complexes has been determined by X-ray crystallography. Computational studies show that the observed asymmetry of the diselenolenes in the solid state is a result primarily of intramolecular repulsive interactions between the ligands.