Knowledge Bank

The salt/molecule technique and metal atom reactions have been used in the past few years as means of generating ion pairs in inert matrices, often with the interest being directed toward the anion in the ion pair. For simple, small anions such as $HF2\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$, $F3\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ and $O3\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$, the spectroscopic effect of ion pairing with alkali metal cations has been shown to be negligible. Infrared spectra of the $M+BF4\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ ion pair, and its chlorine-fluorine analogs have been obtained to investigate the effect of the metal cation on larger, more highly symmetric anions. Two distinct, sharp, boron-fluorine stretching frequencies were detected, at 1210 and $1004cm-1$, (for the $\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}B$ isotope) indicative of splitting of the triply degenerate boron-fluorine stretch in the tetrahedral anion. Similar results, but in lesser yields, were obtained for the mixed Cl-F species, and the perchloro species. The spectra have been interpreted to indicate a $C3v$ structure for the anion, in a monodentate arrangement, based partially on analogy to the $Na+Co(CO)4\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ ion pair in solution, and Edgell’s theory of dual symmetry.