Multinuclear NMR data (¹³C, ³¹P, ¹³C-{³¹P}, ¹³C-(103Rh) and 31P-{¹⁰³Rh}) for a series of mono- and di-substituted derivatives of Rh₆(CO)₁₆ containing neutral two electron donor ligands [Rh₆(CO)₁₅L, (L = NCMe, py, cyclooctene, PPh₃, P(OPh)₃,1/2(μ₂,η¹η ¹-dppe)); Rh₆(CO)₁₄(LL), (LL = cis-CH₂=CMe-CMe=CH₂, dppm, dppe, (P(OPh)₃)₂)] are reported; these data show that the solid state structure is maintained in solution. Detailed assignments of the ¹³CO NMR spectra of Rh₆(CO)₁₅(PPh₃) and Rh₆(CO)₁₄(dppm) clusters have been made on the basis ¹³C-{¹⁰³Rh} double resonance measurements and the specific stereochemical features of the observed long range couplings in these clusters have been studied. The stereochemical dependence of ³J(P-C) for terminal carbonyl ligands is discussed and the values of ³J(P-C) are found to be mainly dependent on the bond angles in the P-Rh-Rh-C fragment; these data enable the fine structure of the complex multiplets in the ¹³C-{¹H} and ³¹P-{¹H} NMR spectra of Rh₆(CO)₁₄ (dppm) to be simulated. Variable temperature ¹³C-{¹H} NMR measurements on Rh₆(CO)₁₅(PPh₃) reveal the carbonyl ligands in this complex to be fluxional. The fluxional process involves exchange of all the CO ligands except the two terminal CO's associated with the rhodium trans to the substituted rhodium and can be explained by a simple oscillation of the PPh₃ on the substituted rhodium atom aided by concomitant exchange of the unique terminal CO on this rhodium with adjacent μ₃-CO's.