Electronic Spectra

1 Introduction

The basic format of this Report is little changed from that of Volume 3. As in all the previous volumes in this series we have selected those papers which we consider to contain particularly significant new observations for more detailed coverage under individual subject headings; papers which report electronic spectral data largely in the context of preparative and characterization work on new compounds are dealt with according to the central metal atom in the molecule. We also include a Table of references to other spectra which appear either as peripheral to some other aspect of the characterization of a molecule or complex or in journals which we have not had an opportunity to examine.

One or two other general comments are in order. The Report is somewhat briefer than last year's, partly as a result of a decrease in the number of papers coming to our notice, but partly also because we have decided not to present some of our material in as much detail as in the past, so as to keep the volume's cost down to a level which individual users can still afford. It is to be hoped that this terser style of presentation does not reduce the readability of the text too much.

Two reviews contain material of interest to inorganic spectroscopists. Electronic Raman spectroscopy is now being used on a widening variety of inorganic materials, and recent work on transition-metal and rare-earth compounds has been assembled by Koningstein.' High-valent first-row transition-metal oxides and halides are among the model compounds most favoured by spectroscopists, so a comprehensive review of their preparations and properties which has appeared is welcome.

2 Polarized and Low-temperature Crystal Spectra

In Volume 3 we noted the increasing proportion of work on the electronic spectra of complex ions which is taking advantage of the extra dimension of information brought in by making polarized measurements on single crystals. This year we have noticed a continuation of the same trend with, in addition, emphasis on higher resolution yielding site-group splittings and vibronic information. On the other hand, since only relatively simple molecules yield spectra that sharpen sufficiently at low temperatures to justify increasing resolution beyond, say, 1 A, preoccupation with detail of this kind may serve to focus attention on an artificially limited range of examples within the huge available field of inorganic materials.

We shall use again the sub-divisions for this section introduced in Volume 3, including separate accounts of spectroscopy whose main emphasis is on vibronic interactions and spectra in the far and extreme ultraviolet.

No reviews devoted to polarized or low-temperature crystal spectra were noted this year.

Discrete Complexes in Crystals. — Monoatomic Ligands. In contrast to the large number of papers reported in Volume 3 on tetroxo-complexes of the 3d elements, in 1973 no such papers fall within this section.

In the field of halide complexes, a number of papers have reported fine detail in the spectra of tetrahalogeno-complexes of 3d elements, as well as hexahalides of all three transition series.

The tetragonal crystals Cs3MX5 (M = 3d ion; X = Cl or Br) are popular with electronic spectroscopists as high-symmetry hosts for [MX4]2- ions, the spin-forbidden ligand-field transitions in the cobalt(II) examples providing a particularly rich field for study. In a very elaborate series of experiments, Tsujikawa and his co-workers have measured Zeeman splittings of many of the band origins in Cs3CoCl5 and Cs3CoBr, as a function of the angle between the applied magnetic field and the crystal axes. Deriving the selection rules, they show that the anisotropy of the Zeeman splitting allows one to determine whether the excited states are Γ6 or Γ7 in D*2d, and whether g[perpendicular to] is positive or negative.

Tetrahedral nickel(II) complexes have been of interest for a number of years because of the possibility that their ground and excited states might exhibit Jahn-Teller distortions. A further contribution to this field is a study of the polarized spectra of [NiX4]2- (X = Cl or Br) in the tetragonal crystals [N(Et)4]2NiX4,4 with a special emphasis on the components of 1G. The resolved fine structure on one hand was assigned as a progression in totally symmetric stretching modes, and origins were assigned to representations of D*2d double group as follows (tetrahedral parent states in brackets):

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Part of the spectrum is shown in Figure 1.

It has been known for some time that the 3d3 ion [MnF6]2- has an elaborate sharp-line spectrum, parts of which have now been definitely assigned, using Cs2GeF6 and K2GeF6, in which the site symmetries are respectively Oh and D3d, as host lattices. Transitions gain electric dipole intensity by vibronic coupling with all three of the odd-parity intramolecular modes of the octahedral complex, and many also appear in combination with the totally symmetric modes (Figure 2). The 4A2g ->2Eg and 2T1g transitions can be assigned quite unambiguously, but for 4T2g, it is necessary to assume that the spin-orbit coupling factor is reduced about tenfold, presumably by a Ham effect. Unfortunately the higher lying 4T1g is only poorly resolved. In contrast to [MnF6]2-, the spectrum of [MnCl6]2- contains only broad transitions, and in addition there are chemical difficulties because of disproportioaation. After sorting out the MnIV band from others due to MnII and MnIII, [MnCl6]2- is assigned the parameters B, 584 cm-1 and Δ, 18 240 cm-1 in a K2SnCl6 host.

Last year we remarked on the extensive activity in the spectroscopy of 4d and 5d hexahalide complexes, referring tg work from the groups of Schatz and Patterson. The latter now contributes a study of [OsBr6]2- in the cubic host Cs2ZrBr6 at 20 K, including both absorption and luminescence. Again, vibronic absorption bands result from coupling to the odd-parity modes of the octahedral complex, the much weaker zerophonon lines being magnetic dipole in character. Jahn-Teller effects may be important in this spectrum, the major origins of which are assigned as follows:

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The salts [EtNH3]2SnX6 (X = Cl or Br) are hexagonal, and provide trigonally distorted sites for [MX6]2- ions. Values of Δ and the trigonal distortion parameter have been calculated from the 4A2g ->2T2g transitions of [TcX6]2- and [ReX6]2- in this host at 77 and 298 K. Optical electronegativities derived from the charge-transfer spectra are 2.3 and 2.1 for TcIV and ReIV respectively.

The lowest-energy absorption band of the dimeric anion [Re2Cl8]2- was...