It is possible to use multinuclear NMR spectroscopy to measure both the steric and electron donor/acceptor properties of the P‐donor ligands in Mo(CO)4[(R2″PO)2YRR′] complexes [R2″=Ph2, OCH2CMe2CH2O; YR = P(O), Si(Me); R′ = alkyl, haloalkyl, aryl]. The 13C, 29Si, 31P and 95Mo NMR data for some Mo(CO)4[(R2″PO)2YRR′] complexes [R2″ = Ph2, OCH2CMe2CH2O; YR = P(O), Si(Me); R′ = alkyl, haloalkyl, aryl] are presented and the factors affecting both the chemical shifts and coupling constants are discussed. The linear correlations between the chemical shifts of the cis and trans CO 13C, the 29Si, the 31P and the 95Mo resonances of the R2″ = Ph2 complexes and those of the similar resonances of the R2″ = OCH2CMe2CH2O complexes are also discussed. Good linear correlations have previously been found between the differences in the 13C chemical shifts of the non‐equivalent cis CO, phenyl C‐1 and phenyl C‐2,6 resonances of a series of Mo(CO)4[(Ph2PO)2Si(Me)R′] complexes [R′ = Me, Et, n‐Pr, c‐Hex, t‐Bu and CH2Cl] and the Ea parameters of the R groups. It is now reported that the chemical shift differences of similar complexes with long‐chain alkyl and chloroalkyl R′ groups do not give good correlations with the Ea parameters. In addition, similar correlations are not observed when the Ph groups are replaced with an OCH2CMe2CH2O group. The factors which affect the poor correlations are discussed. Copyright © 1986 John Wiley & Sons, Ltd.