Computed structural and energetic data indicate an associative interchange (I a ) mechanism for HCN exchange on [Cu(HCN) 4 ] + . Density functional (B3LYP, B3PW91, X3LYP, BP86, PBEPBE, PW91PW91, and M06) and ab initio (MP2, MP4sdq, CCSD, and CCSD(T)) calculations with extended basis sets (6-311+G**, TZVP, LANL2DZ+p, and SDD+p, the latter including extra polarization and diffuse functions) indicate that HCN exchange on [Cu(HCN) 4 ] + proceeds via an associative interchange (I a ) mechanism and a D 3h transition structure {[Cu(HCN) 5 ] + } ‡ . The activation barrier, relative to the model complex [Cu(HCN) 4 ] + ·HCN, varies modestly, depending on the computational level. Typical values are 8.0 kcal M −1 (B3LYP/6-311+G