Density functional theory (DFT) calculations have been used to find and characterize various isomers of HMn(CO)4, a coordinatively unsaturated complex. For comparison, its saturated parent compound HMn(CO)5 has been included in the study. Optimized geometries and vibrational spectra were calculated for each species. For the pentacarbonyl, the theoretical geometry allows a critical discussion of various experimental results. Its calculated vibrational spectrum agrees with experiment within an average error of less than 10 cm-1 and maximum deviation of ∼30 cm-1. In a search for isomeric forms of the tetracarbonyl complex, two minima have been found on the potential surface. The most stable form is a Cs structure, with a second isomer of C4v symmetry lying about 3 kcal/mol higher in energy. The existence of these two structures is in agreement with the conclusions of a sophisticated matrix isolation photolysis study by Church et al.18 The calculated vibrational spectrum supports the interpretation of the CO region of the infrared spectrum, a crucial part of the experimental evidence, in every detail. This refers not only to the dominant Cs form but also to the very difficult experimental proof of the presence of a small amount of the C4v isomer. We suggest, in addition, that the Mn-H stretching frequency, although difficult to observe, could be used for identification: we predict an unexpectedly large frequency shift of ∼140 cm-1 between the Cs and the C4v isomers. A third isomer, the C2v structure - suggested in some studies as an alternative to Cs - was found to be a saddle point, rather than a minimum, in our study. For the heterolytic Mn-CO bond cleavage the calculations predict a dissociation energy of 41.8 kcal/mol.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry