Chemistry-Data processing, Metal carbonyls-Structure, Manganese compounds-Derivatives, Carbonyl compounds-Derivatives, Ligand binding (Biochemistry), Computational chemistry, Manganese pentacarbonyl derivatives, Ligand-ligand bonding
Manganese mono-ligand pentacarbonyl compounds, Mn(CO)₅R, are observed to distort from their predicted octahedral geometry. The R-Mn-CO angle varies from *the thing it varies from 90-60, with the equatorial carbonyls leaning towards the R ligand. The current model to explain this phenomenon accounts for equal distortion of all 4 carbonyls towards R, but does not account for the case when two of the carbonyls behave differently than the other two. In this work, I investigate the electronic affects behind such a distortion of only two of the four equatorial carbonyls. In compounds where the R ligand has C2v or CS symmetry, the two equatorial carbonyls eclipsed with the p orbital of the R ligand distort significantly more than the two eclipsed with the bulk of the R ligand. I obtained optimized structures for molecules with a variety of R ligands to optimize geometry through ab initio calculations in the Gaussian 09 program. I found that the two carbonyls eclipsed with the p orbital distort the most when R is a good pi donor. I propose a model of b1 interaction of donation from the filled ligand px orbital to the nearby empty carbonyl π* orbitals. Observed trends in R-Mn-CO angle in my study support this proposed donation. In addition, the C-O bond distance of a carbonyl lengthens as the corresponding R-Mn-CO angle decreases, consistent with donation into the carbonyl antibond. I investigate this interaction quantitatively using the NBO (Natural Bonding Orbital) and ADF (Amsterdam Density Functional) programs. Energies of interaction between molecular fragments calculated by these programs support my proposed model.
Valladares, Amelia Catherine, "Mn(CO)₅R : why are the carbonyls leaning?" (2012). Honors Project, Smith College, Northampton, MA.
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