The photo-induced substitution of a CO ligand has been used to prepare the halfsandwich complexes (eta(3)-C3H5)V(CO)(4)[P(C7H7)(3)] (1), (eta(5)-C5H5)V(CO)(3)[P(C7H7)(3)] (2), (eta(7) -C7H7)V(CO)(2)[P(C7H7)(3)] (3), (eta(6)-C6H3Me3)Cr(CO)(2)[P(C7H7)(3)] (4), and (eta(5)-C5H5)Mn(CO)(2)[P(C7H7)(3)] (7), in which the olefinic phosphane is coordinated as a conventional two-electron ligand through the lone pair of electrons at phosphorus. Some analogues, which are permethylated at the aromatic ring (2*, 4*, 7*), were included for comparison. Subsequent photo-elimination of another CO group from 4 or 7 converts the olefinic phosphane into a chelating four-electron ligand, leading to (eta(6)-C6H3Me3)Cr(CO)-[P(C7H7)(2) (eta(2) -C7H7)] (5) and (eta(5)-C5H5)Mn(CO)[P(C7H7)(2) (eta(2)-C7H7] (8), respectively. The eta(2)-coordinated double bond in 5 and 8 can be displaced by trimethylphosphite to give (eta(6)-C6H3Me3)Cr(CO)[P(C7H7)(3)][P(OMe)(3)] (6) and (eta(5)-C5H5)Mn-(CO)[P(C7H7)(3)][P(OMe)(3)] (9). The P-31 and C-31 NMR spectra of all complexes are discussed, and X-ray structure analyses for 2 and 8 are presented. Prolonged irradiation of 7 and 8 led to a di(cycloheptatrienyl)phosphido-bridged dimer, {(eta(5) -C5H5)Mn(CO)-[P(C7H7)(2)]}(2) (10).