Halides are X-type ligands in coordination chemistry. The halides are usually good σ- and good π-donors. These ligands are usually terminal, but they might act as bridging ligands as well. For example, the chloride ligands of aluminium chloride bridge two aluminium centers, thus the compound with the empirical formula AlCl actually has the molecular formula of AlCl under ordinary conditions. Due to their π-basicity, the halide ligands are weak field ligands. Due to a smaller crystal field splitting energy, the halide complexes of the first transition series are all high spin when possible. These complexes are low spin for the second and third row transition series. Only [CrCl] is exchange inert.
Homoleptic metal halide complexes are known with several stoichiometries, but the main ones are the hexahalometallates and the tetrahalometallates. The hexahalides adopt octahedral coordination geometry, whereas the tetrahalides are usually tetrahedral. Square planar tetrahalides are known as are examples with 2- and 3-coordination.
Alfred Werner studied hexamminecobalt(III) chloride, and was the first to propose the correct structures of coordination complexes. Cisplatin, cis-Pt(NH)Cl, is a platinum drug bearing two chloride ligands. The two chloride ligands are easily displaced, allowing the platinum center to bind to two guanine units, thus damaging DNA.
Due to the presence of filled p orbitals, halide ligands on transition metals are able to reinforce π-backbonding onto a π-acid. They are also known to labilize cis-ligands.