The CO-factored force field of M(CO)(4) molecules with C-3 nu symmetry was solved using the maximum value of the sum of the interaction constants as an additional constraint. The solution obtained led to the relations which allow direct calculation of force constants from C-O stretching frequencies of the all-(CO)-C-12-O-16 molecule. The CO-factored force constants and C-O stretching frequencies of (CO)-C-13-O-16-substituted species were calculated for Co(CO)(4) and Fe(CO4-). The results were found to be in very close agreement with those obtained from isotopic enrichment studies. (C) 1999 Elsevier Science S.A. All rights reserved.