Richard 1. Carlin Brown University
Providence, Rhode Island
Electronic Spectra of Transition Metal Complexes
The colors and absorption spectra of complexes of the transition metal ions have intrigued chemists for a long time. It has been possible only recently to make quantitative predictions of the spectral properties of these molecules, as well as to find a theoretical rationale for much of the experimental data. This review will deal with some of the theoretical interpretations of the visible spectra of transition metal complexes, in an attempt to show some of the recent advances in this field as well as to point out some of the details omitted in most research papers. A knowledge of crystal field theory as explained by Sutton (1) shall he assumed. McClure (B), Dunn ($), and Ballhausen (4) have given more complete reviews of the literature in this field. The basic phenomenon to be considered is the resolution of the degeneracies of eigenstates of the free metal ion by the imposed crystalline field. Once we understand which energy levels are available in a complex, it follows that the absorption spectrum will arise from electronic transitions from the ground state to the various excited states. Due to the selection rules of quantum mechanics, some of these transitions are not allowed while others are allowed only under certain conditions. T h e Spectrum of Ti3'
I n order to define terms and introduce the basic ideas to be used, it is appropriate to begin with a discussion of the spectrum of trivalent titanium. This ion is not only typical in that it forms octahedral complexes with many ligands; but it is the simplest that can be considered, since we shall have to deal with but one d electron outside the argon shell. The ground state is thus 2D. The free ion upon being placed in a crystal undergoes a spherically symmetrical repulsion Vn which causes a shift of the ground state but does not split the fivefold degeneracy. An additional potential arising from the symmetry of the cationic site will split this level. The %D term of Tia+ is split by an octahedral field, Vo,, into two levels, the lower, triply degenerate 2T2,, and the higher, doubly degenerate 2E,. The energy separation between these levels is defined as 10 Dq, and is some 20,300 cm-' for the hexacluotitanium (111) ion. See Figure 1. The energy separation between and ZE, must preserve the center of gravity of the %Dlevel. That is, the triplydegenerate set of levels is lowered in energy by 4 Dq and the donbly-degenerate set is raised by 6 Dq. Now we have an idea of the order of magnitude of the electronic splittings in complex compounds. The spectrum of Ti(HzO)6a+has yielded a value for Dq of 2030 cm-'. It is important to notice the very small
energies involved in such crystal field splittings. A perturbation of electronic levels of 20,000 cm-' in a complex is small compared to the total binding energy of such a compound. If one were now to replace the water molecules in the coordination sphere of the titanium (111) ion by chloride ion, one might expect a small change in the observed spectmm. Although this particular experiment does not seem to have been completed yet, we know from a comparison with similar systems containing other metal ions that the absorption peak would shift to lower energies. I n other words, Dq is smaller for C1- than for HIO as ligands. By studying all available ligands against one central metal ion, a spectrochemical series is found which places the ligands in order of increasing field strength. Such a series is: I- < Br-
