Complexes show a variety of possible reactivities:
* Electron transfers
*: Electron transfer (ET) between metal ions can occur via two distinct mechanisms, inner and outer sphere electron transfers. In an inner sphere reaction, a bridging ligand serves as a conduit for ET.
* (Degenerate) ligand exchange
*: One important indicator of reactivity is the rate of degenerate exchange of ligands.  For example, the rate of interchange of coordinate water in [M(HO)] complexes varies over 20 orders of magnitude.  Complexes where the ligands are released and rebound rapidly are classified as labile.  Such labile complexes can be quite stable thermodynamically.  Typical labile metal complexes either have low-charge (Na), electrons in d-orbitals that are antibonding with respect to the ligands (Zn), or lack covalency (Ln, where Ln is any lanthanide).  The lability of a metal complex also depends on the high-spin vs. low-spin configurations when such is possible.  Thus, high-spin Fe(II) and Co(III) form labile complexes, whereas low-spin analogues are inert.  Cr(III) can exist only in the low-spin state (quartet), which is inert because of its high formal oxidation state, absence of electrons in orbitals that are M–L antibonding, plus some "ligand field stabilization" associated with the d configuration.
* Associative processes
*: Complexes that have unfilled or half-filled orbitals are often capable of reacting with substrates.  Most substrates have a singlet ground-state; that is, they have lone electron pairs (e.g., water, amines, ethers), so these substrates need an empty orbital to be able to react with a metal centre.  Some substrates (e.g., molecular oxygen) have a triplet ground state, which results that metals with half-filled orbitals have a tendency to react with such substrates (it must be said that the dioxygen molecule also has lone pairs, so it is also capable to react as a normal Lewis base).
If the ligands around the metal are carefully chosen, the metal can aid in (stoichiometric or catalytic) transformations of molecules or be used as a sensor.