Stepwise-displaced molybdenum carbonyl complexes Mo(CO)(6-n)L(n) (n = 1-3) of chlorosubstituted phosphine ligands PPh(3-m)Cl(m), (m = 0-3) were prepared and studied by Mo-95, P-31, and C-13 NMR spectroscopies. The extremely deshielding influence of the PCl3 ligand (and related ligands) on the transition metal NMR chemical shift and the shielding trend on the P-31 NMR resonance position indicate that this putatively strong pi-acceptor is actually a weaker pi-acceptor than all other phosphorus Ligands employed (except for PBr3). This conclusion is consistent with C-13 NMR and IR spectroscopic results for all the complexes and is also in agreement with the known very weak bond strength of M-PCl3 in Mo and Ni carbonyl complexes as well as a theoretical calculation. The traditionally used infrared parameters for phosphine-substituted metal carbonyls are not suitable for evaluating the sigma- and pi characters of trans phosphorus ligands because both a weaker sigma-donor and a stronger pi-acceptor would lead to a high carbonyl vibrational frequency. The expectation of very short Mo-PCl3 and Mo-PBr3 bond distances (consistent with literature X-ray crystallographic results for a few related transition metal complexes), coupled with the evidence for the low pi-acceptor ability of these phosphorus ligands, leads to an unconventional conclusion : a weak sigma-donor may form a short M-P bond with weak bond strength in the absence of a pi-bonding contribution. It is also concluded that the P-31 NMR chemical shift, or coordination chemical shift Delta delta(P-31), can reflect the electronic and steric variations in Mo-P bonding : both pi- acid sigma-bonding will increase the Delta delta(P-31) by inducing a deshielding effect on the P-31 nucleus, while steric effects decrease the Delta delta(P-31) by inducing a shielding effect.