Which parameters determine the value of Δo? What is the difference between high spin and low spin complexes?
The parameter B is the Racah parameter which is the repulsion term which arises due to the repulsion between the terms of same spin multiplicity. The ordinate is represented as E/B and abscissa is given as Δ/B. these diagrams can be qualitative as well as quantitative in nature but mostly the results drawn are qualitative in nature. In cases where both high spin as well as low spin complexation is possible that is cases of d4 , d5 , d6 and d7 electronic configurations a vertical line is drawn in the middle of the diagram in order to separate the high spin complexes from the low spin one.
A complex can be classified as high spin or low spin. When talking about all the molecular geometries, we compare the crystal field splitting energy (Δ) and the pairing energy (P). Normally, these two quantities determine whether a certain field is low spin or high spin.
When the crystal field splitting energy is greater than the pairing energy, electrons will fill up all the lower energy orbitals first and only then pair with electrons in these orbitals before moving to the higher energy orbitals. Electrons tend to fall in the lowest possible energy state, and since the pairing energy is lower than the crystal field splitting energy, it is more energetically favourable for the electrons to pair up and completely fill up the low energy orbitals until there is no room left at all, and only then begin to fill the high energy orbitals. On the other hand, when the pairing energy is greater than the crystal field energy, the electrons will occupy all the orbitals first and then pair up, without regard to the energy of the orbitals. If every orbital of a lower energy had one electron, and the orbitals of the next higher energy had none, an electron in this case would occupy the higher energy orbital.
This follows Hund's rule that says all orbitals must be occupied before pairing begins. Remember, this situation only occurs when the pairing energy is greater than the crystal field energy. These phenomena occur because of the electron's tendency to fall into the lowest available energy state.