The six bonding molecular orbitals that are formed are "filled" with the electrons from the ligands, and electrons from the d-orbitals of the metal ion occupy the non-bonding and, in some cases, anti-bonding MOs. The energy difference between the latter two types of MOs is called Δ (O stands for octahedral) and is determined by the nature of the π-interaction between the ligand orbitals with the d-orbitals on the central atom. As described above, π-donor ligands lead to a small Δ and are called weak- or low-field ligands, whereas π-acceptor ligands lead to a large value of Δ and are called strong- or high-field ligands. Ligands that are neither π-donor nor π-acceptor give a value of Δ somewhere in-between.
The size of Δ determines the electronic structure of the d - d ions. In complexes of metals with these d-electron configurations, the non-bonding and anti-bonding molecular orbitals can be filled in two ways: one in which as many electrons as possible are put in the non-bonding orbitals before filling the anti-bonding orbitals, and one in which as many unpaired electrons as possible are put in. The former case is called low-spin, while the latter is called high-spin. A small Δ can be overcome by the energetic gain from not pairing the electrons, leading to high-spin. When Δ is large, however, the spin-pairing energy becomes negligible by comparison and a low-spin state arises.
The spectrochemical series is an empirically-derived list of ligands ordered by the size of the splitting Δ that they produce. It can be seen that the low-field ligands are all π-donors (such as I), the high field ligands are π-acceptors (such as CN and CO), and ligands such as HO and NH, which are neither, are in the middle.
IOO &lt; NCSCN &lt; py (pyridine) &lt;  NH &lt; en (ethylenediamine) &lt; bipy (2,2'-bipyridine) &lt; phen (1,10-phenanthroline) &lt; NO &lt; CN</sup> &lt; CO