text: The Δ splitting of the d orbitals plays an important role in the electron spin state of a coordination complex. Three factors affect  Δ: the period (row in periodic table) of the metal ion, the charge of the metal ion, and the field strength of the complex's ligands as described by the spectrochemical series. Only octahedral  complexes of first row transition metals adopt high-spin states.
  In order for low spin splitting to occur, the energy cost of placing an electron into an already singly occupied orbital must be less than the cost of placing the additional electron into an e orbital at an energy cost of Δ. If the energy required to pair two electrons is greater than the energy cost of placing an electron in an e, Δ, high spin splitting occurs.
  If the separation between the orbitals is large, then the lower energy orbitals are completely filled before population of the higher orbitals according to the Aufbau principle. Complexes such as this are called "low-spin" since filling an orbital matches electrons and reduces the total electron spin.  If the separation between the orbitals is small enough then it is easier to put electrons into the higher energy orbitals than it is to put two into the same low-energy orbital, because of the repulsion resulting from matching two electrons in the same orbital.  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.
  The charge of the metal center plays a role in the ligand field and the Δ splitting. The higher the oxidation state of the metal, the stronger the ligand field that is created. In the event that there are two metals with the same d electron configuration, the one with the higher oxidation state is more likely to be low spin than the one with the lower oxidation state; for example, Fe and Co are both d; however, the higher charge of Co creates a stronger ligand field than Fe. All other things being equal, Fe is more likely to be high spin than Co.
  Ligands also affect the magnitude of Δ splitting of the d orbitals according to their field strength as described by the spectrochemical series. Strong-field ligands, such as CN and CO, increase the Δ splitting and are more likely to be low-spin. Weak-field ligands, such as I and Br cause a smaller Δ splitting and are more likely to be high-spin.
  Some octahedral complexes exhibit spin crossover, where the high and low spin states exist is dynamic equilibrium.