The values in the DE(M) columns are in the range of 0.5–1 eV, in
the same order of magnitude with the expected 10Dq or formal
LFSE quantities, suggesting that the role of hybridization of the d
shell with s and p ones, combined with the donation from ligand
fragments towards the outer s and p virtuals is relatively significant.
The DE(L) columns record the contribution of density rearrangements
inside the ligand, during the bond formation. The
effect is visibly smaller for the neutral aqua ligand, as compared
to the charged ones, interpreting this as the cost of negative charge
displacement. The higher DE(L) values of the fluoride systems, as
compared to the cyanides represent the high cost for the rehybridization
of fluoride ions, from spherical charge density toward a linearly
ligating status, with a lone pair formed in the direction of the
metal ion. At the formation of the bond, these energy costs are
compensated by the gain in orbital and electrostatic components.
Since the cyanide anion possesses already a lone pair, the rearrangement
of the fragment for bonding is smaller. The fact that
DE(ML) effect (combined removal of virtuals from both M and L
fragments) is higher than the DE(M) + DE(L) for [MqF6]q6 complexes,
reveals the help offered by central atom virtuals toward
the hardly deformable fluorine ions in the charge rearrangement
process, in a sort of basis set superposition interference. The
DE(ML) and DE(M) + DE(L) quantities are comparable for cyanide
and aqua complexes, suggesting that the ligand can perform the
preparation for bonding by their inner orbital resources, having
lower Rydberg states that can mix at perturbation with the
groundstate.