Aurophilicity−Coordination Interplay in the Design of Cyano-Bridged Nickel(II)−Gold(I) Bimetallic Assemblies: Structural and Computational Studies of the Gold(I)−Gold(I) Interactions

Two polymorphic cyano-bridged Au(I)−Ni(II) bimetallic complexes of formulas [Ni(en)2Au(CN)2][Au(CN)2] (1) and
[Ni(en)2{Au(CN)2}2] (2) have been prepared from the 1:2 reaction between [Au(CN)2]- and either [Ni(en)2Cl2]Cl or
[Ni(en)3]Cl2·2H2O, respectively. The structure of 1 consists of polymeric cationic chains of alternating [Au(CN)2]-
and [Ni(en)2]2+ units running along the a axis and [Au(CN)2]- anions lying between the chains. The noncoordinated
dicyanoaurate anions are aligned perpendicular to the ac plane and involved in aurophilic interactions with the
bridging dicyanoaurate groups, ultimately leading to a 2D bimetallic grid. The structure of 2 consists of trinuclear
molecules made of two [Au(CN)2]- anions linked to [Ni(en)2]2+ unit in trans configuration. Trinuclear units are
joined by aurophilic interactions to form 1D zigzag chains. The magnetic properties of these compounds are strongly
dominated by the local anisotropy of the octahedral Ni(II) ions, thus indicating that the magnetic exchange interaction
mediated by dicyanoaurate bridging groups, if it exits, is very weak. To get insight into the electronic properties of
the inter- and intramolecular interactions of the [Au(CN)2]- building blocks, the structures of different aggregates
of dicyanogold units were optimized and then analyzed by making use of atoms-in-molecules (AIM) theory. Moreover,
bond indices were calculated by methods based upon nonlinear population analysis.