The maximum value of the sum of the interaction constants is used as an additional constraint to solve the CO-factored force fields of tricarbonyl complexes with C-s symmetry. The solution obtained led to the equations by which carbonyl stretching force constants and CO-CO interaction constants can be calculated directly from C-O stretching frequencies of the all-(CO)-C-12-O-16 molecule. The CO-factored force constants and the C-O stretching frequencies of (CO)-C-13-O-16-substituted species were calculated for Fe(CO)(3)(-) and Fe(C4H6)(CO)(3). The results obtained were found to be very close agreement with those obtained from isotopic enrichment studies. (C) 2010 Elsevier B. V. All rights reserved.