Spin states (d electrons)

[1] The Δ splitting of the d orbitals plays an important role in the electron spin state of a coordination complex.

So, one electron is put into each of the five d orbitals before any pairing occurs in accord with Hund's rule resulting in what is known as a "high-spin" complex.

Complexes such as this are called "high-spin" since populating the upper orbital avoids matches between electrons with opposite spin.

Ligands also affect the magnitude of Δ splitting of the d orbitals according to their field strength as described by the spectrochemical series.

For example, dichlorobis(triphenylphosphine)nickel(II) has been crystallized in both tetrahedral and square planar geometries.

In the case of octahedral complexes, the question of high spin vs low spin first arises for d4, 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.

In the case of octahedral complexes, electrons in the eg levels are anti-bonding with respect to the metal-ligand bonds.