Knowledge Bank

The spectra of $MnF6=$ afford a favorable opportunity for the stuy of the electronic states of a $d3$ complex ion because of their sharpness and resolution and the correlation possible between infrared, ESR, visible, and near ultraviolet measurements. The ESR spectra indicate considerable covalent character of the bonds; the interatomic distances $-1.73$ {\AA} in $MnF6=$ and 1.93 {\AA} in $CrF6\equiv -$ support this view. The appearance in the optical spectra of sharp absorption and emissoin lines corresponding to the three u vibrations of an octahedral molecule show that $MnF6=$ should be considered an individual molecule and not simply $Mn4+$ surrounded by six fluoride ions which may be shared with other cations. The $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}A2g\to 2Eg$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}A2g\to 2T1g$ transitions in the region 16 000 $cm-2$ to 17 000 $cm-1$ can be simply interpreted. The polarized spectra in the case of the hexagonal environmental permit the identification of magnetic dipole transitions. The $0.7cm-1$ splitting of the ground state in the hexagonal symmery is seen in these spectra. The $\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}A2g\to 2T2g$ transition, observed at $4\circ K$ , in the region 20 500 $\to$ 24 000 $cm-2$ shows, sharp (0.5 to $1.0cm-1$ half width) magnetic dipole lines and also sharp vibronic lines, approximately $103$ times as intense. The assignments in this case are questionable. Our favored interpretation is based on a greatly reduced spin-orbit interaction, molecular and lattice vibrations and combinations of these. The effect of the environment seems due less to a distortion of the octahedron than to the changes around the octahedron which affect the vibrational frequencies markedly.