Formation of linkage isomers via the substitution of halides by selenocyanate in ruthenium(III) complexes

Summary Substitution of the halide ion of Ru(NH₃)₅X]²⁺ (X=Cl^– or Br^–) by SeCN^– was monitored spectrophotometrically between 45 and 60° C in aqueous medium. The pseudo-first order rate constants were evaluated by Guggenheim's procedure, which shows linearity for 2–3 half-lives. The rate increases linearly with SeCN^–] and all the plots have a positive intercept on the rate axis. The SeCN^–-independent path represents aquation and the SeCN^–-dependent path represents anation of the aqua complex with simultaneous formation of two isomeric (the N bonded and the Se bonded) products. The formation of the isomeric products is entropy controlled.