Absolute infrared intensities in some metal carbonyls and the charge flux model of the dipole derivatives

The absolute intensities of the four infrared active modes of certain metal carbonyls, M(CO)₄ or M(CO)₆, are analyzed from the point of view of the equilibrium charge-charge flux model of the dipole derivatives. It is shown that the charges obtained from the bending modes, assuming negligible flux for these modes, are apparently identical with those reported by previous authors. However, such charges are much too sensitive to the choice of L matrix (which transforms the normal coordinates to symmetry coordinates) to permit any conclusions. The carbonyl stretching mode implies values of the dipole derivative three- to sixfold greater than in gaseous CO. It seems most reasonable to attribute most of this enhancement to charge flux between the metal and carbon atoms.