The most common type of complex is octahedral, in which six ligands form the vertices of an octahedron around the metal ion. In octahedral symmetry the d -orbitals split into two sets with an energy difference, Δ oct (the crystal-field splitting parameter, also commonly denoted by 10 Dq for ten times the "differential of quanta") where the d xy, d xz and d yz orbitals will be lower in energy than the d z and d x - y, which will have higher energy, because the former group is farther from the ligands than the latter and therefore experiences less repulsion. The three lower-energy orbitals are collectively referred to as t 2g, and the two higher-energy orbitals as e g. These labels are based on the theory of molecular symmetry : they are the names of irreducible representations of the octahedral point group, O h.(see the O h character table) Typical orbital energy diagrams are given below in the section High-spin and low-spin.