Knowledge Bank

An approximate molecular orbital treatment has been carried through for tetrahedral ions of the type $CrO4-,MnO4-,ClO4-$ in an effort to arrive at a satisfactory description of the electronic structure of these ions. The visible and near ultraviolet spectra of the transition group ions ($VO4-,CrO4-,MnO4-$, etc.) show regularities which should be accounted for qualitatively by a satisfactory theory. Such a theory should also predict that perchlorate and sulfate ions should show no near ultraviolet absorption. The approximations employed in the solution of the secular equations for the orbitals of different symmetries are radical and probably seriously in error; but it is hoped that the similarity of the molecules for which the calculations were made would allow qualitative conclusions to be drawn from the results. The theory predicts that the first transition will be due to the excitation of a $t1$ nonbonding electron to a $t2$ essentially antibonding orbital; the second transition, the excitation of a $t2$ bonding electron to a $t2$ antibonding orbital. The variation in the energy of these transitions with changing central atom is in accord with experiment as are the relative f-values for the first transition. The spectra of permanganate ions in different crystalline fields $(insolidsolutionindifferentnonabsorbingperchlorates)1$ show the existence of a number of excited states to which transition would be forbidden in the absence of proturbing fields or vibrations. The symmetries and positions (roughly) of these are in agreement with the prediction of the simplified theory. The spectra of fluorochromate and pertechnitate ions may be discussed in terms of the simplified theory and the results are again in qualitative agreement with the experiment.