Knowledge Bank

The salt/molecule reaction technique, in conjunction with matrix isolation, hag been applied to the synthesis and characterization of nonlinear polyatomic anions. The reaction of alkali fluoride salt molecules with $SiF4$ yielded three moderately intense bands between 800 and $950cm-1$ as well as one or more bands below $500cm-1$. The upper bands have been assigned to the Si-F stretching vibrations of $SiF5\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$, while the lower band, or bands, have been assigned to Si-F deformation modes, similar spectra have been obtained for the mixed chlorine/fluorine species, except for the pure chlorine species $SiCl5\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ which could not be formed under these conditions. Annealing of the matrix led to two new, intense bands which are in agreement with literature values far $SiF6\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$. These results have been interpreted to indicate that the $SiF6=$ anion has been formed in a matrixion pair, but the geometry, at best, can be described as a perturbed trigonal bipyramid. The spectra can be accounted for by invoking distortion of the $SiF5\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ anion by the ion-paired cation (which could either activate the forbidden Si-F stretching mode, or. split the doubly degenerate Si-F stretching vibration), or by postulating s structure other than a trigonal bipyramid.