The final products of the reactions of either [Mo(CO)(6)] or [Mo(CO)(4)(NBD)] (NBD = norbornadiene) with PPy(3) (Py = 2-pyridyl) or ZPPy(3) (Z = 0, S, ClAu, C6F5Au) are fac-[Mo(CO)(3)(Py(3)P-N-3)] or fac-[Mo(CO)(3)(Py(3)PZ-N-3)], where the ligands act as tridentate N-donors. The use of ZPPhPy(2) (Z = 0, S) leads to fac-[Mo(CO)(3)(ZPPhPy(2)-Z,N-2)], the ligands acting as tridentate Z,N-2-donors. The crystal structure of fac-[Mo(CO)(3)(SPPhPy(2)-S,N-2)] was determined by area detector diffractometry and shows that the coordination at molybdenum is essentially octahedral but with trigonal elongation. some tetracarbonyl complexes which are intermediates in the formation of the above tricarbonyl complexes have been isolated (cis-[Mo(CO)(4)(PPy(3)-P)(2)] and cis-[Mo(CO)(4)(ZPPhPy(2)-Z,N)] (Z = 0, S)) or detected (cis-[Mo(CO)(4)(Py(3)P-N-2)] and cis-[Mo(CO)(4)(SPPy(3)-S,N)]). The syntheses of the new complexes [Au(C6F5)(PRPy(3)-P)] (R = Py or Ph) are also described.