The title compound is obtained from the reaction of [Cp " Co(CO)(2)] (Cp " = eta(5)-C(5)H(3)tBu(2) with P-4 in THF under UV radiation and is structurally characterized by X-ray diffraction, In [Cp " Co(P-4){(Cp " Co)(2)(mu-CO)}], a planar P-4 ligand is eta(4)-capped by a Cp " Co unit and a [(Cp " Co)(2) mu-CO)] dimer coordinates to only three P atoms from below, resulting in a kite-like distortion of the P-4 molecule with two short and two long P-P distances. The longer distances could be considered to be on the borderline between a bonding P-P length and the van der Waals contact. The factors controlling the structure have been studied by EHT calculations.