Silver Aggregation Caused by Stanna-closo-dodecaborate Coordination:  Syntheses, Solid-State Structures and Theoretical Studies

Stanna-closo-dodecaborate [SnB₁₁H₁₁]^2- reacts as a nucleophile with various silver electrophiles ([Ag(PMe₃)]⁺, [Ag(PEt₃)]⁺, [Ag(PPh₃)]⁺, and Ag⁺) to form silver−tin bonds. Aggregation of two, three, or four units of [{Ag(SnB₁₁H₁₁)(PR₃)}_n]^n- (PPh₃, n = 2; PEt₃, n = 3; PMe₃, n = 4) was found, depending on the size of the coordinating phosphine. The structures of the silver−tin clusters in the solid state were determined by single-crystal X-ray diffraction. In these phosphine silver coordination compounds, the tin ligand exhibits μ₂- and μ₃-coordination with the silver atoms. From the reaction with silver nitrate, an octaanionic stanna-closo-dodecaborate coordination compound, [Et₄N]₈[Ag₄(SnB₁₁H₁₁)₆], was isolated. In this cluster, arranged as butterfly, the stannaborate shows various coordination modes at four silver atoms. In the reported silver−tin complexes, the silver−silver interatomic distances are in a range of 2.6326(10)−3.1424(6) Å. Silver−tin distances were found between 2.6416(5) and 3.1460(6) Å. Analysis of the molecular orbitals calculated by means of density functional theory shows that the LUMO of the core compound without [SnB₁₁H₁₁]^2- units is always a totally symmetric combination of (mainly) s-orbitals of Ag atoms. This core is filled with electrons of the HOMOs of the [SnB₁₁H₁₁]^2- units and is leading, in this way, to a stable compound.