The photochemical reaction of [Cp′Mo(CO)3]2 (Cp′ = η5-C5H4tBu) with white phosphorus in the presence of [Cr(CO)5thf] leads to the Pn ligand complexes [Cr(CO)4 <(η4-P4){(Cr(CO)5}4>;] (1), [Cp′Mo(CO)2<(η3-P4){Cr(CO)54(H)>] (2) and [{Cp′Mo(CO)2}2<(μ,η2-P2){Cr(CO)5}2>] (3). The products were characterized by NMR, IR and partially by X-ray structure analysis. The reaction proceeds via a P---P bond cleavage leading to the bent cyclo-P4 complex [Cp′Mo(CO)2<(η3-P4){Cr(CO)5}4{Cp′Mo(CO)3}>] (2a), which hydrolysis during the chromatographic work-up to give the more stable compound 2. Furthermore, a P3/P1 fragmentation of 2a occurs to form the phosphido complex intermediate [CP′CO)2Mo---P → Cr(CO)5]. The latter dimerises resulting in the formation of the tetrahedral compound 3, containing a Mo2P2 moiety.