Reaction of [ Mo( CO),] with 2 equivalents of tetraphenylcyclopentadienone (tetracyclone) in refluxing toluene gives [Mo(CO),(q4-C,Ph,CO),] l a in ca. 95% yield. With 1 equivalent of tetracyclone in boiling heptane, [{Mo(CO),(o:q4-CC,Ph,CO)},] 2 is produced as -well as 1 a. This dimeric complex has been crystallographically characterised [triclinic, space group Pl (no. 2), a = 8.980(3), b = 11.332(4), c = 13.526(5) A; a = 77.55(3), p = 71.94(3), y = 81.02(3)", Z = 2 monomer units]; each molybdenum is in a distorted-octahedral environment, co-ordinated by the q4-diene ligand, three carbonyl ligands, and the ketonic carbonyl of the other centrosymmetrically related tetracyclone. Another convenient route into the chemistry of tetracyclone molybdenum complexes is provided by [Mo(CO),(NCMe) (q4-C4Ph,CO)] 5 which is formed in high yield by treating [Mo(CO),(NCMe),] with tetracyclone. One (but only one) of the tetracyclone ligands of complex l a can be displaced by chelating phosphines to give [Mo(CO),(L-L)-(q4-C,Ph,CO)] (L-L = Ph,PCH,PPh, or Ph,PCH,CH,PPh,); the stereochemistry and fluxionality of these complexes is briefly discussed. Thermal reaction of l a with monodentate phosphines (L = PPh, or PPh,Me) yields [Mo(CO),L(q*-C,Ph,CO)] 7 by a carbonyl redistribution process; the same compounds can be prepared from 5 by displacement of the labile acetonitrile ligand, as can an analogous species with L = PPh,H. Thermolysis of [Mo(CO),(PPh,H)(q4-C,Ph,CO)] or thermal reaction of l a with PPh,H gives the phosphido-bridged complex [Mo,(CO),(p-PPh,) (p-o:q5-C,Ph,CO) (q5-C,Ph,COH)] 8. The crystal structure of 8 has also been determined [triclinic, space group P i (C;, no. 2), a = 12.806(10), b = 14.173(8), c = 17.958(7) A, a = 83.75(4), p = 86.92(5), y = 71.57(5)", 2 = 21 and reveals a formal Mo=Mo double bond [2.923(2) A] bridged by a diphenylphosphido group and a tetracyclone ligand bonded in a 0: q5 manner. The other tetracyclone is bonded as an q5-hydroxytetraphenylcyclopentadienyl ligand, having taken up the proton released by oxidative addition of the P-H bond.