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 mu(2)- and mu(3)-coordination with the silver atoms. From the reaction with silver nitrate, an octaanionic stanna-closo-dodecaborate coordination compound, [Et4N](8)[Ag-4(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) A. Silver-tin distances were found between 2.6416(5) and 3.1460(6) A. 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.