INFRARED SPECTRA OF ISOTOPICALLY SUBSTITUTED METAL CARBONYLS .I. 13CO-SUBSTITUTED CIS-FE(CO)4I2 AND CIS-FE(CO)4BR2

All of the infrared active CO stretching absorptions including the minor peaks observed in the spectra of cis-Fe(CO)4X2 molecules (X = Br or I) have been assigned to modes of the all-12CO derivative (96% natural abundance) and the principal mono-isotopic species Fe(12CO)3(13CO)X2 (2% 3CO-cis and 2% 13CO-trans to X). The proposed assignments are supported by calculations of the five Cotton-Kraihanzel force constants in the energy factored CO stretching block of the (FG) matrix. The 13CO modes have been experimentally verified through exchange with 50% 13C-enriched CO. The CO groups cis and trans to I in cis-Fe(CO)4I2 are observed to be approximately equally enhanced by 13CO exchange in agreement with previous radio-carbon monoxide studies. © 1969.