This is because the dominant stabilizing interaction in the solids is not the crystal field stabilization energy generated by the interaction of the ligands with the d electrons, but the perturbation that can sometimes affect the relative preferences, but which often do not.
22.
Because the number of valence electrons which actually participate in chemical reactions is difficult to predict, the concept of the valence electron is less useful for a transition metal than for a main group element; the d electron count is an alternative tool for understanding the chemistry of a transition metal.
23.
This relativistic increase in momentum for high speed electrons causes a corresponding decrease in wavelength and contraction of 6s orbitals relative to 5d orbitals ( by comparison to corresponding s and d electrons in lighter elements in the same column of the periodic table ); this results in 6s valence electrons becoming lowered in energy.
24.
The first d electron count ( special version of electron configuration ) with the possibility of holding a high spin or low spin state is octahedral d 4 since it has more than the 3 electrons to fill the non bonding d orbitals according to ligand field theory or the stabilized d orbitals according to crystal field splitting.
