Density functional theory calculations on 19-electron organometallic complexes:: The Mn(CO)5Cl- anion.: The difference between unpaired electron density and spin density due to spin polarization

The anisotropic hyperfine coupling (hfc) constants for the 19-electron Mn(CO)(5)Cl- complex, 1, and for the 17-electron Mn(CO)(4)Cl- complex, 2, were calculated using density functional theory (DFT). The calculated hfc values for 1 are in good agreement with the experimental ones reported in two EPR studies, which were not able to distinguish between 1 and 2. The Mn-CO(axial) bond dissociation energy in 1 was calculated to be 19 kcal/mol, which shows that 1 is stable to loss of axial CO. These data indicate that the species observed in the EPR experiments was the 19-electron complex, 1. The unpaired electron population of the manganese d(z)(2) orbital indicated by the SOMO of the DFT wave function (ca. 0.2-0.3) is significantly less than that obtained from the two EPR analyses (0.49, 0.63), which neglected spin polarization. The calculations show that spin polarization in 1 causes the spin density (as measured by EPR) to differ significantly from the unpaired electron density(SOMO). It is concluded that neglect of spin polarization in the EPR analysis of open-shell transition metal compounds may lead to an overestimate of the unpaired electron population on the metal. The standard method for estimating atomic orbital populations by ratioing the observed hfc in a molecule to the atomic hfc is not reliable for organometallic compounds.