Abstract: | 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 (Ia) mechanism and a D3h 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**), 6.0?kcal?M?1 (M06/6-311+G**), and 4.8?kcal?M?1 (CCSD(T)/6-311+G**//MP2(full)/6-311+G**). Inclusion of an implicit solvent model (B3LYP(CPCM)/6-311+G**) leads to an activation barrier of 5.8?kcal?mol?1. Comparison of the HCN exchange mechanisms on Li(HCN)4]+ (limiting associative, A) and Cu(HCN)4]+ (associative interchange, Ia) reveals that π back donation in the equatorial Cu–N bonds in the transition state determines the mechanism. |