Use and interpretation of non-rigorous force constants derived from C–O stretching frequencies of metal carbonyls. Part I. Symmetric carbonyls

Relationships are developed between non-rigorous C–O force constants and those of a quadratic general valence force field for metal carbonyls whose ligands are symmetry-equivalent. The equations are tested by reference to full calculations on MCO molecules and to data available for group VIA hexacarbonyls. Approximations involved give errors which are of the same order as experimental frequency errors. The associated normal co-ordinate treatment leads, as part of the same approximation, to the conclusion that non-rigorous force constants are valid and realistic quantities so long as the co-ordinate system is properly defined.