Infrared spectra of [(π-C₅H₅)Fe(CO)₂]₂and related compounds in hydroxylic solvents

Abstract

When [(π-C5H5)Fe(CO)2]2, [(π-MeC5H4)Fe(CO)2]2, [{π-C5H4·CH(NMe2)·CH(NMe2)·C5H4}Fe2(CO)4], [(π-C5H5)Ru(CO)2]2, [(π-C6H10)Co(CO)2]2, [(π-C5H8)Co(CO)2]2, and [(π-C5H5)Ni(CO)]2 are dissolved in hydroxylic solvents, their i.r. spectra from 1700–2100 cm.–1 are more complex than in nonprotic solvents. It is suggested that in such solvents, the bridged isomers of these compounds exist as equilibrium mixtures of ‘normal’ molecules and others where there is hydrogen bonding between the oxygen atoms of their bridging carbonyl groups and the solvent. The proportions of hydrogen-bonded species increase as the solvent pKa decreases, and also as the frequencies of the ν(CO) vibrations due to the bridging carbonyl groups of the solute decrease.