Exceptional Structural Compliance of the B 12 F 12 2- Superweak Anion.

The single-crystal X-ray structures, thermogravimetric analyses, and/or FTIR spectra of a series of salts of the B12 F12 2- anion and homoleptic Ag(L)n + cations are reported (L = CH2 Cl2 , n = 2; L = PhCH3 , n = 3; L = CH3 CN; n = 2-4; L = CO, n = 1, 2). The superweak-anion nature of B12 F12 2- (Y2- ) was demonstrated by the rapid reaction of microcrystalline Ag2 (Y) with 1 atm of CO to form a nonclassical silver(I) carbonyl compound with an FTIR ν(CO) band at 2198 cm-1 (and with the proposed formula [Ag(CO)n ]2 [Y]). In contrast, microcrystalline Ag2 (B12 Cl12 ) did not exhibit ν(CO) bands and therefore did not form Ag(CO)+ species, even after 32 h under 24 atm of CO. When Ag2 (Y) was treated with carbon monoxide pressures higher than 1 atm, a new ν(CO) band at 2190 cm-1 appeared, which is characteristic of a Ag(CO)2 + dicarbonyl cation. Both Ag2 (CH3 CN)8 (Y) and Ag2 (CH3 CN)5 (Y) rapidly lost coordinated CH3 CN at 25 °C to form Ag2 (CH3 CN)4 (Y), which formed solvent-free Ag2 (Y) only after heating above 100 °C. Similarly, Ag2 (PhCH3 )6 (Y) rapidly lost coordinated PhCH3 at 25 °C to form Ag2 (PhCH3 )2 (Y), which formed Ag2 (Y) after heating above 150 °C, and Ag2 (CH2 Cl2 )4 (Y) rapidly lost three of the four coordinated CH2 Cl2 ligands between 25 and 100 °C and formed Ag2 (Y) when it was heated above 200 °C. Solvent-free Ag2 (Y) was stable until it was heated above 380 °C. The rapid evaporative loss of coordinated ligands at 25 °C from nonporous crystalline solids requires equally rapid structural reorganization of the lattice and is one of three manifestations of the structural compliance of the Y2- anion reported in this work. The second, more quantitative, manifestation is that Ag+ bond-valence sums for Ag2 (CH3 CN)n (Y) are virtually constant, 1.20 ± 0.03, for n = 8, 5, 4, because the Y2- anion precisely compensated for the lost CH3 CN ligands by readily forming the necessary number of weak Ag-F(B) bonds. The third, and most exceptional, manifestation is that the idealized structural reorganization accompanying the conceptual transformations Ag2 (CH3 CN)8 (Y) → Ag2 (CH3 CN)5 (Y) → Ag2 (CH3 CN)4 (Y) involve close-packed layers of Y2- anions that sandwich the Ag(CH3 CN)4 + complexes splitting into staggered flat ribbons of interconnected (Y2- )3 triangles that surround the Ag2 (CH3 CN)5 2+ complexes on four sides, conceptually re-forming close-packed layers of anions that sandwich the Ag(CH3 CN)2 + complexes. The interconnected (Y2- )3 triangle lattice of anions in Ag2 (CH3 CN)5 (Y) may be the first example of this structure type.