Editing Coordination complex (section)

===Magnetism===
{{main|magnetochemistry}}
Metal complexes that have unpaired electrons are [[paramagnetic]]. This can be due to an odd number of electrons overall, or to incomplete electron-pairing.  Thus, monomeric Ti(III) species have one "d-electron" and must be [[paramagnetism|(para)magnetic]], regardless of the geometry or the nature of the ligands.  Ti(II), with two d-electrons, forms some complexes that have two unpaired electrons and others with none. This effect is illustrated by the compounds TiX<sub>2</sub>[(CH<sub>3</sub>)<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>P(CH<sub>3</sub>)<sub>2</sub>]<sub>2</sub>: when X&nbsp;=&nbsp;[[Chlorine|Cl]], the complex is paramagnetic ([[high spin|high-spin]] configuration), whereas when X&nbsp;=&nbsp;[[methyl group|CH<sub>3</sub>]], it is diamagnetic ([[low spin|low-spin]] configuration). Ligands provide an important means of adjusting the [[ground state]] properties.

In bi- and polymetallic complexes, in which the individual centres have an odd number of electrons or that are high-spin, the situation is more complicated.  If there is interaction (either direct or through ligand) between the two (or more) metal centres, the electrons may couple ([[Antiferromagnetism|antiferromagnetic coupling]], resulting in a diamagnetic compound), or they may enhance each other ([[Ferromagnetism|ferromagnetic coupling]]).  When there is no interaction, the two (or more) individual metal centers behave as if in two separate molecules.