Exceptional Structural Compliance of the B₁₂F₁₂^2- Superweak Anion

The single-crystal X-ray structures, thermogravimetric analyses, and/or FTIR spectra of a series of salts of the B₁₂F₁₂^2- anion and homoleptic Ag(L)_n⁺ cations are reported (L = CH₂Cl₂, n = 2; L = PhCH₃, n = 3; L = CH₃CN; n = 2-4; L = CO, n = 1, 2). The superweak-anion nature of B₁₂F₁₂^2- (Y^2-) 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(B₁₂Cl₁₂) 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(CH₃CN)₈(Y) and Ag₂(CH₃CN)₅(Y) rapidly lost coordinated CH₃CN at 25 °C to form Ag₂(CH₃CN)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(CH₂Cl₂)₄(Y) rapidly lost three of the four coordinated CH₂Cl₂ 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 Y^2- anion reported in this work. The second, more quantitative, manifestation is that Ag⁺ bond-valence sums for Ag₂(CH₃CN)_n(Y) are virtually constant, 1.20 ± 0.03, for n = 8, 5, 4, because the Y^2- anion precisely compensated for the lost CH₃CN 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(CH₃CN)₈(Y) → Ag₂(CH₃CN)₅(Y) → Ag₂(CH₃CN)₄(Y) involve close-packed layers of Y^2- anions that sandwich the Ag(CH₃CN)₄⁺ complexes splitting into staggered flat ribbons of interconnected (Y^2-)₃ triangles that surround the Ag₂(CH₃CN)₅²⁺ complexes on four sides, conceptually re-forming close-packed layers of anions that sandwich the Ag(CH₃CN)₂⁺ complexes. The interconnected (Y^2-)₃ triangle lattice of anions in Ag₂(CH₃CN)₅(Y) may be the first example of this structure type.