键合异构是配位化合物结构异构的一种形式，配体通过不同的配位原子与中心原子配位。生成的配合物称为键合异构体，配体称为两可配体。

首先制得的键合异构体是[Co(NH₃)₅(NO₂)]Cl₂，制得的两种该钴配合物具有不同的物理性质。红色异构体中亚硝酸根以氮原子配位，黄色异构体中亚硝酸根则以一个氧原子配位，写为[Co(NH₃)₅(ONO)]。进一步的实验表明，在紫外线照射下，红色的异构体会自发转变为黄色的异构体。

虽然键合异构现象早在十九世纪晚期就已经发现，但直到1907年才对该现象做出解释。

Linkage isomerism is the existence of coordination compounds that have the same composition differing with the connectivity of the metal to a ligand.

Typical ligands that give rise to linkage isomers are:

Examples of linkage isomers are violet-colored [(NH₃)₅Co-SCN] and orange-colored [(NH₃)₅Co-NCS]. The isomerization of the S-bonded isomer to the N-bonded isomer occurs intramolecularly. In the complex, dichlorotetrakis(dimethyl sulfoxide)ruthenium(II), linkage isomerism of dimethyl sulfoxide ligands can be observed in the NMR spectrum due to the effect of S vs. O bonding on the methyl groups of DMSO. The proper notation for linkage isomerism is the kappa notation where the atom directly bonding to the metal is proceeded by the lowercase Greek letter kappa; κ. For example, NO₂ is represented as nitrito-κ-N and nitrito-κ-O, replacing the old system of trivial names such as nitro and nitroso.