ammonia, generating a larger ligand field split, Δ. Ligands that have occupied p orbitals are potentially π donors. These types of ligands tend to donate these electrons to the metal along with the σ bonding electrons, exhibiting stronger metal-ligand interactions and an effective decrease of Δ. Most halide ligands as well as OH− are primary examples of π donor ligands. When ligands have vacant π* and d orbitals of suitable energy, there is the possibility of pi backbonding, and the ligands may be π acceptors. This addition to the bonding scheme increases Δ. Ligands that do this very effectively include CN−, CO, and many others. Spectrochemical series of metals The metal ions can also be arranged in order of increasing Δ, and this order is largely independent of the identity of the ligand. Mn2+ < Ni2+ < Co2+ < Fe2+ < V2+ < Fe3+ < Cr3+ < V3+ < Co3+ In general, it is not possible to say whether a given ligand will exert a strong field or a weak field on a given metal ion. However, when we consider the metal ion, the following two useful trends are observed: Δ increases with increasing oxidation number, and Δ increases down a group. See also References Zumdahl, Steven S. Chemical Principles Fifth Edition. Boston: Houghton Mifflin Company, 2005. Pages 550-551 and 957-964. D. F. Shriver and P. W. Atkins Inorganic Chemistry 3rd edition, Oxford University Press, 2001. Pages: 227-236. James E. Huheey, Ellen A. Keiter, and Richard L. Keiter Inorganic Chemistry: Principles of Structure and Reactivity 4th edition, HarperCollins College Publishers, 1993. Pages 405-408. Inorganic chemistry Coordination chemistry