The 13C, 17O, 29Si and 31P NMR spectral properties of cis-Mo(CO)4(PPh2X)2, where X is Cl, OMe, 4-MeC6H4O, NH2, NHMe, NHCH2CH2NH2, NHCH2CH2NMe2, 4-MeC6H4NH, SEt, OSiMe3 or NHSiMe3 and of cis-Mo(CO)4)(PPh2YPPh2), where Y is NMeCH2CH2CH2NMe, NHSiMe2NH, NHSiMePhNH, OSiMe2O, or OSiMePhO and of cis-[Mo(CO)4(PPh2O)2]2Si have been determined. Good to excellent correlations between the 13C and 17O chemical shifts of carbonyl ligands trans to the phosphorus donor ligands are found. Only poor correlations are seen between 13C and 17C chemical shifts for carbonyl ligands cis to the two phosphine substituted carbonyl groups. Based on only seven samples, 29Si and 13C of trans carbonyl ligands also correlate well. Correlations of chemical shift data with approximate CO stretching force constants are fair to good depending upon the selection of data. In particular, much better correlations are noted in almost all instances when complexes with monodentate ligands and bidentate ligands are examined separately. © 1983.