Oligomerization of Au(CN)₂^- and Ag(CN)₂^- Ions in Solution via Ground-State Aurophilic and Argentophilic Bonding

Abstract

Dicyanoaurate(I) and dicyanoargentate(I) ions undergo significant oligomerization in aqueous and methanolic solutions. The absorption edges of K[Au(CN) 2 ] and K[Ag(CN) 2 ] solutions undergo progressive red shifts with an increase in concentration up to near the saturation limits, whereupon total red shifts of 13.4 × 10 3 and 11.9 × 10 3 cm - 1 are obtained from the respective maxima of the corresponding lowest energy monomer bands. Two types of deviations from Beer's law are observed: a negative deviation for the monomers' MLCT bands and a positive deviation for the oligomers' bands. Increasing the concentration within a given concentration range leads to red shifts in the oligomers' absorption and/or excitation bands dominant in that range, while further increases in concentration lead to the appearance of new lower energy bands. Electronic structure calculations suggest that this behavior is attributed to metal−metal interactions between neighboring Au(CN) 2 - or Ag(CN) 2 - ions. Formation constants of 1.50 ± 0.05 and 17.9 ± 2.0 M - 1 are obtained for [Ag(CN) 2 - ] 2 and [Au(CN) 2 - ] 2 dimers, respectively, at ambient temperature.