For a free ion, e.g. gaseous Ni or Mo, the energy of the d-orbitals are equal in energy; that is, they are "degenerate". In an octahedral complex, this degeneracy is lifted. The energy of the d&minus;y set, which are aimed directly at the ligands are destabilized. On the other hand, the energy of the d, d, and d orbitals, the so-called t set, are stabilized. The labels t and e refer to irreducible representations, which describe the symmetry properties of these orbitals. The energy gap separating these two sets is the basis of crystal field theory and the more comprehensive ligand field theory. The loss of degeneracy upon the formation of an octahedral complex from a free ion is called crystal field splitting or ligand field splitting. The energy gap is labeled Δ, which varies according to the number and nature of the ligands. If the symmetry of the complex is lower than octahedral, the e and t levels can split further. For example, the t and e sets split further in trans-MLL.
Ligand strength has the following order for these electron donors:
:weak: iodine < bromine < fluorine < acetate < oxalate < water < pyridine < cyanide :strong
So called "weak field ligands" give rise to small Δ and absorb light at longer wavelengths.