Complexation of the symmetric cyclic diamine piperazine (1,4-diazacyclohexane) has been examined in dry dimethyl formamide by spectrophotometric titrations (with Cu²⁺, Ni²⁺) to define formation constants, and by stopped-flow kinetics to define the complexation rates and reaction pathway. Initial formation of a rarely observed η¹-piperazine intermediate occurs in a rapid second-order reactions. This intermediate then undergoes two competing reactions: formation of (chelated) η²-piperazine (ML) or the formation of (bridging) μ-piperazine (in M₂L and M₂L₃, speciation depending on relative concentrations). Protonation constants and formation constants for complexation in water of N-ethylpiperazine and thiomorpholine (1-aza-4-thiocyclohexane, tm) have been determined by potentiometric titration; 1:1 complexes with first-row M²⁺ display a log K from ~4 to 6, with speciation that suggests chelation of the heterocycles may be involved. Complexation of thiomorpholine has been further probed by the synthesis of Pd^II complexes. The N-monodentate coordination mode has been confirmed in trans-[Pd(tm)₂Br₂] by an X-ray crystal structure. Complexation of N-(2-aminoethyl)piperazine to Cu^II as a bidentate ligand involving the primary and tertiary amines is also defined by an X-ray crystal structure.