Electronic Spectra

BY P. DAY

1 Introduction

The overall outline of this Report follows quite closely from last year's. Papers which are felt to represent particularly significant developments in inorganic spectroscopy, or which exemplify fields at present in a particularly lively state of development, are dealt with in the earlier sections, while papers whose main emphasis is on the preparation and characterization of compounds, and in which electronic spectroscopy is used as a structural tool, are referred to in the sections classified according to the metal atom in the molecule. Finally, papers in which electronic spectroscopy plays only an incidental part or which, for one reason or another, we have not been able to scan in detail, are collected in the Table of Section 14, again classified by metal atom.

A few words about the principles defining our range of coverage of inorganic materials, and classes of spectra, are relevant here. In general, no transitions of entirely delocalized (i.e. band-to-band) character are included, although one could certainly make a case that compounds such as GaP are as worthy of attention by inorganic chemical spectroscopists as KMnO4. Nevertheless, bowing to the traditional subject demarcation between inorganic chemistry and solid-state physics, almost all the materials we refer to are either discrete molecules or molecular ions or, if continuous lattice solids, are predominantly of ionic character. Also, we have refrained from including any references to emission spectra, although emission data are of course quite complementary to absorption and in many cases serve to confirm the absorption assignments. Our reason is quite simply the volume of luminescence work, to treat which properly would necessitate a separate chapter.

There have not been any books entirely devoted to inorganic electronic spectroscopy during 1972, but an important introductory reference is an account of the electronic spectra of co-ordination compounds, both ligand field and charge transfer, which appears in the American Chemical Society Monograph on 'Coordination Chemistry'. A review on stereochemical and electronic structural aspects of five-co-ordination also includes material on the electronic spectra of this important class of compound. Two new volumes have appeared in a useful compilation of literature references to electronic spectra, and reviews of nuclear hyperfine structure in the spectra of diatomics and of molecular geometries of excited states' are included in a new book on aspects of molecular spectroscopy.

2 Polarized and Low-temperature Crystal Spectra

A steadily increasing proportion of all the work on electronic spectra of inorganic molecules and complex ions is being carried out on crystals rather than solutions, thus providing the extra dimension of information concerning polarization of the transition, and removing ambiguities often found in older assignments, even of quite simple chromophores. Because of the great variety of chemical systems, and types of spectra, now being studied in the crystalline state, this section of the Report is for the first time subdivided. For this purpose we have not chosen the conventional classification of inorganic spectra into "ligand field', 'charge transfer', etc., partly because many papers report results on both kinds of spectra for the same compound and partly because we feel that the inorganic interest of the work under review is best served by concentrating on particular problems or classes of compound.

Among general trends which may be discerned in the year's activity are the continuation of definitive high-resolution work on simple octahedral and tetrahedral chromophores, mainly with oxide and halide ligands, and a marked increase in the amount of work on more complicated complex ions containing multidentate chelates, often producing unusual stereochemistries. The former activity has now produced sets of unambiguous assignments of both ligand-field and charge-transfer energies against which theoretical calculations can be subjected to stringent verification.

Much detailed work on Jahn-Teller and other vibronic interaction effects continues to appear, and warrants treatment as a separate section. We also noted in last year's Report the important resurgence of interest in the old established technique of soft-X-ray absorption and emission spectroscopy, as well as in the previously inaccessible extreme ultraviolet region, brought about by the use of synchrotron radiation from particle accelerators as a light source. Although this field still lies somewhat nearer the realm of solid-state physics than of inorganic chemistry, we again include a section devoted to new work in it because it is clear that the range of compounds being examined is widening rapidly and the results will soon warrant careful attention by inorganic chemists. Worth noting too is the way in which the new technique complements X-ray and ultraviolet photoelectron spectroscopy. This section of last year's Report also included an account of spectra measured at cryogenic temperatures on evaporated films of compounds which exist as gases at room temperature, as well as of molecules isolated in rare-gas matrices. No relevant spectra of evaporated films were noted this year, and references to matrix-isolation spectra are to be found in Section 8.

The only review entirely devoted to crystal spectra this year is by Martin, on square-planar PtII complexes. It emphasizes the work, particularly on exciton effects, carried out recently in his own laboratory.

Discrete Complexes in Crystals. — In this section we review work on both pure and doped crystals in which discrete molecules or complex ions may be distinguished. In contrast to organic molecular crystals, almost all the spectroscopic work on inorganic molecular crystals is interpreted implicitly using the 'oriented gas' model. There is no reason in principle why interactions between transition dipoles, on neighbouring molecules, giving, for example, Davydov splittings, should not be found in the inorganic crystal spectra, but scarcely anyone has ever seriously probed the question. The very occasional instances where such effects have been postulated are dealt with in Section 3.

Monatomic Ligands. Oxides. New data continue to appear on the tetra-oxo-anions which have been used so much in the past as models for testing assignments of charge-transfer spectra. Ballhausen gives some arguments based on a simplified treatment of electron repulsion for believing, as has in fact been subsequently confirmed experimentally, that the weak near infrared band of MnO4 at 14 450 cm-1 is the forbidden 1T1 state coming from the t512e1 configuration. The low-temperature spectrum of permanganate has of course been measured many times, but an interesting new host lattice for this ion is KBr, in which it substitutes for Br-. The site symmetry is...