Coordination model, stability constant, and kinetics study of cystamine and l-cystine with [PdCl₄]²⁻ in hydrochloric aqueous solutions

Stoichiometry, equilibria, and kinetics of [PdCl₄]²⁻interactions with l-cystine (H₄CysS²⁺) and cystamine (H₂Cyst²⁺) have been investigated spectrophotometrically in strong hydrochloric acidic media. Interactions lead to the formation of highly stable S/(S,N)-coordinated binuclear, and then with excess [PdCl₄]²⁻ trinuclear (S,S,N) or tetranuclear (S,S,N,N) species without disulfide bond cleavage. The reaction of [PdCl₄]²⁻ with H₄CysS²⁺ or H₂Cyst²⁺ at [PdCl₄]²⁻ excess has irreversible first-order kinetics, and with H₄CysS²⁺ or H₂Cyst²⁺ excess, by irreversible parallel reaction of [PdCl₄]²⁻ addition to the ligand. The influence of leaving groups on the kinetics has been explained in terms of formation of stable ionic pairs with complex species and of efficient overlap of d and π orbitals in a transition state. The reactions proceed through an associative mechanism with the first step being formation of the S-coordinated complex. Coordination models and mechanisms have been proposed. Applicability of spectrophotometry for establishment of disulfide bond state in organic disulfides in complexation processes has been demonstrated.