Substitutions in square planar complexes tend to occur through mechanisms with associative character, although the degree of associative character may vary (eg \(A, \; I_a, \; I_d\). The outcome of square planar substitutions depend on the identities of the leaving group, the ligand trans to the leaving group, and the entering group. The reactions of Pt(II) are well studied and will serve as the primary examples in this section. Pt(II) complexes are of particular interest because of their role as effective chemotherapeutics in the treatment of several types of solid tumor cancers. The generic chemical equation for substitution in a square planar complex is shown below, where X is the leaving group, Y is the entering group, and \(\ce{[ML3X]}\) is the reactant complex: