The reaction of Mo(CO)5(Ph2PCl) with the anion of 2,2,5-trimethyl-4H-1,3-dioxin-4-one gives [Mo(CO)5(Ph2PCH2CCHC(O)OC(CH3)2O)] (1), in 73% yield. This is only the second report of a reaction of a coordinated chlorophosphine with a carbanion that gives a good yield of the corresponding phosphine. An X-ray crystal structure of 1 shows that the coordination environment of the molybdenum is an octahedron with the MoC distance to the carbonyl trans to the phosphine shorter than those to the carbonyls trans to carbonyls. The conformation of the 1,3-diox-4-enyl group in 1 is a half chair with the isopropylidene C(CH3)2 group above the plane of the other atoms. Complex 1 reacts with alcohols and mercaptans to yield the expected [Mo(CO)5(Ph2PCH2C(O)CH2C(O)ER)] (2: ER = OCy; 3: ER = OMe; 4: ER = SPrn) complexes. These complexes have significant amounts of the enol tautomers present in chloroform-d1 at 23°C with 4 having the highest enol content. Complex 1 reacts with piperidine to give the unexpected product, [Mo(CO)5(Ph2PCH2C(O)CH3)], (5). © 1994.