Analysis of the Infrared Spectra of Metal Carbonyls

Abstract

A simplified normal coordinate treatment of the CO stretching vibrations is applied to metal carbonyls of various complexity. Infrared data to compute force constants directly are available for the following three groups of compounds: cyclopentadienyl manganese tricarbonyl and its derivatives [CpMn(CO)_3−xL_x, x=0→2], the substituted nickel carbonyls [Ni(CO)_4−xL_x, x=1→3], and some molybdenum carbonyl derivatives [sym‐Mo(CO)₃L₃ and cis‐Mo(CO)₂L₄]. When there are more force constants than observable frequencies in a molecule, the assumption is made that the effects of ligands on force constants are additive. In this way, many additional frequencies are obtained while only a few new force constants are introduced. This method has been used on two additional groups of compounds: Fe(CO)_5−x(PF₃)_x and Mn₂(CO)_10—x(PF₃)_x. Excellent agreement between calculated and experimentally observed frequencies is found in all five groups of compounds. In some cases the effective force constants obtained may be interpreted as arising from dipole—dipole interaction. The force constants are then used to calculate frequencies for ¹³C isotopically substituted species and good agreement with experiment is found.