Electronic Structure of 3d [M(H2O)6]3+ Ions from ScIII to FeIII: A Quantum Mechanical Study Based on DFT Computations and Natural Bond Orbital Analyses.

The metal−donor atom bonding along the series of 3d [M(H2O)6]3+ ions from Sc3+ to Fe3+ has been investigated by density-functional calculations combined with natural localized bond orbital analyses. The M−OH2 bonds were considered as donor−acceptor bonds, and the contributions coming from the metal ion's 3d σ-, 3d π-, and 4s σ-interactions were treated individually. In this way, the total amount of charge transferred from the water oxygen-donor atoms toward the appropriate metal orbitals could be analyzed in a straightforward manner. One result obtained along these lines is that the overall extent of ligand-to-metal charge transfer shows a strong correlation to the hydration enthalpies of the aqua metal ions. If the contributions to the total ligand-to-metal ion charge transfer are divided into σ- and π-contributions, it turns out that Cr3+ is the best σ-acceptor, but its π-accepting abilities are the weakest along the series. Fe3+ is found to be the best π-acceptor among the 3d hexaaqua ions studied. Its...