Solvent effects on the infrared active CO stretching frequencies of some metal carbonyl complexes—I. Dimanganese decarbonyl and dirhenium decarbonyl

Abstract

The infrared spectra of Mn 2(CO) 10 and Re 2(CO) 10 have been measured in the CO stretching region in the gas phase and in each of seventy four solvents and solvent shifts related to the parameter δ s . Using straight chained hydrocarbon solvents, there is a linear relationship between band frequency and δ s for all three ν(CO) modes for both compounds. While effects on b 2( b) and e 1 modes are similar for each compound a much larger effect occurs on the b 2( a) mode of Re 2(CO) 10 than that on the same mode of Mn 2(CO) 10. It is suggested that this is associated with greater double bond character in the axial CO groups reflected in the stronger ReRe bond for Re 2(CO) 10. Deviations that are both positive and negative are observed for all other solvents. Usually positive deviations occur with the highest frequency b 2( b) modes and negative deviations with the other modes. For polar solvents this has been explained in terms of the relative contribution of triple and double bond character to the CO groups involved and hence the sign of μ′ (the change in dipole moment with respect to the vibrational coordinate of the solute bond). As the alkyl chain is lengthened deviations become less positive (or more negative). This is explained from increase in + I effect of the alkyl group causing more polarity in the functional group which leads to a more negative value for μ′. Trends in solvent effect for branch chain hydrocarbons, alkenes and alkyl benzenes are also discussed.