Using an analytical approach we were able to obtain a valid solution to the CO-factored force field of Fe(CO)(5). The solution led to the relations which allow direct calculation of the CO-factored force constants from C-O stretching frequencies of the all-(CO)-C-12-O-16 molecule. The force constants calculated from these relations were used to estimate C-O stretching frequencies of (CO)-C-13-O-16- substituted species of Fe(CO)(5). The results were found to be in very close agreement with those obtained from isotopic spectra. (C) 2002 Elsevier Science B.V. All rights reserved.