In an octahedral complex, the d-orbitals of a transition metal ion are split into two different energy levels due to the crystal field created by the surrounding ligands. This splitting pattern is a result of the electrostatic interactions between the metal ion and the ligands.The five d-orbitals are divided into two groups: the lower energy level, called the t2g set, consists of the dxy, dyz, and dxz orbitals, while the higher energy level, called the eg set, consists of the dx^2-y^2 and dz^2 orbitals. The energy difference between these two sets of orbitals is denoted as o crystal field splitting energy in an octahedral field .The color of the complex is affected by this splitting because the energy difference between the t2g and eg orbitals corresponds to the energy of light absorbed by the complex. When an electron from the lower energy t2g orbitals absorbs a photon of light, it can be promoted to the higher energy eg orbitals. The energy of the absorbed light is equal to the energy difference between the two sets of orbitals o .The color of the complex is complementary to the color of the light absorbed. For example, if a complex absorbs green light, it will appear red because red is the complementary color of green. The specific color of a transition metal complex depends on the nature of the ligands surrounding the metal ion, as different ligands cause different degrees of splitting in the d-orbitals.Using crystal field theory, we can explain the observed color of a transition metal complex with a particular ligand set. For example, consider a hexaaquacopper II complex, [Cu H2O 6]^2+. In this complex, the copper II ion is surrounded by six water molecules in an octahedral arrangement. The water molecules cause the d-orbitals of the copper ion to split into the t2g and eg sets. The energy difference between these sets corresponds to the energy of the absorbed light, which in this case is in the red region of the spectrum. As a result, the hexaaquacopper II complex appears blue-green, which is the complementary color of red.