Electronic Spectra

BY P. DAY

1 Introduction

The most obvious difference between this Report and the one which appeared in Volume 4 of this series is its slightly greater length. Partly this is due to the fact that the period under review this time is eighteen months instead of one year, but it also reflects a genuine increase in the volume of work containing at least reference to electronic spectroscopy as a tool for characterizing new compounds, and perhaps also an increased level of activity in those laboratories which use the more elaborate refinements of low temperatures, high resolution, or unusual sample conditions such as high pressure or high magnetic fields to probe, often with great subtlety, the bonding characteristics of inorganic molecules. Such activity, and the understanding which it gives, is quite as central to the progress of inorganic chemistry as synthesizing new compounds or investigating reactivity. Indeed, the three march together.

In format this Report is like last year's. In style too it aims at conciseness, in an effort to keep the total bulk of the volume down. What we consider the most significant advances appear in the subject sections; spectra measured as part of a broader study of a group of compounds are dealt with according to the central metal atom of the complex, while papers making only passing reference to spectra, or in journals to which we have not had access, appear in the final Table.

2 Polarized and Low-temperature Crystal Spectra

Comparing the contents of this section with that in Volume 4 it is at once apparent that polarized single-crystal spectroscopy is now established as a relatively routine technique in many laboratories, and that it has become an integral part of the portfolio of physical techniques to be tried out on any newly prepared or specially interesting substance. Even the use of liquid helium, which a few years ago would have been confined to physics departments and the most avant garde physical chemistry laboratories, is now taken completely for granted in inorganic chemistry as a routine method of simplifying and improving the resolution of a wide range of spectra. In large measure this is the result of a new generation of cryogenic devices, such as continuous-flow cryostats and closed-cycle refrigerators, which are simple to set up and more or less trouble-free in operation. In part too, though, it reflects a greater readiness by inorganic chemists to get involved with more complex instrumentation in order to obtain more subtle information about the bonding in the compounds they make.

Reviews on aspects of crystal spectroscopy published in the period surveyed here include a most elegant synthesis of the intra- and inter-subshell transitions of metal impurities in ionic crystals by McClure, a useful survey of the vibronic spectra of co-ordination compounds, showing what a wealth of information is contained in such spectra, even of large molecules, and an account of work (much of it from the author's own group) on polyatomic impurities as guests in alkali halide crystal.

Discrete Complexes in Crystals. — Monoatomic Ligands. Oxide. After a gap last year the oxide ion once again figures as one of the simplest ligands in these pages, with work at low temperatures both on tetraoxo-ions and, perhaps for the first time, on substituted 0x0-species. Far less studied than the inorganic spectroscopists' favourite molecule the permanganate ion is the next member of the series, manganate. Polarized spectra of this ion doped in K2S04, Rb2S04, and Cs2S04 have now been reported over the range 10 000 — 40 000 cm-1. In tetrahedral symmetry the ground state of [MnO4]2- is 2E, and both ligand-field and charge-transfer states are well resolved (Figure 1). The Cs local symmetry of the host lattice splits the 2T2 ligand-field band into three zero-phonon components, whose polarization behaviour is quite different from that observed in the lowest-energy charge-transfer band having the same cubic symmetry. This is because spin-orbit coupling makes a contribution to the splitting of the former which is comparable to the low-symmetry field, while the orbital degeneracy in the charge-transfer state is derived from a hole localized on the oxygen, which is therefore subject to a much smaller spin-orbit interaction. This appears to be the first example of such a phenomenon. Also much less studied than permanganate, though for different reasons, is the isoelectronic but radioactive ion pertechnetate. Its spectrum in CsCIO4 between 20 000 and 47 000 cm-1 contains two band systems, each with partially resolved vibronic fine structure, though no discrete zero-phonon lines can be seen.

The substituted d0 chromate ions [CrO3X]- (X = For Cl) are attractive objects for crystal spectroscopy, since they provide a substantial trigonal perturbation on the parent tetraoxo-species, and reports on their charge-transfer spectra, in addition to that of the 5d0 ion [OsO3N]-, have come from two groups. The Copenhagen group explains the sharp line structure of the lowest-frequency band system on the assumption that 3E and 3A2 states originating from the tetrahedral parent 3T lie a few hundred cm-1 above the zero-phonon line of the lowest 1E state, originating from 1T2. The Oxford group examined [CrO3X]- and [OsO3N]- in KClO4 at 4 K and found, rather surprisingly, that the dipolar guest ions were oriented by the dipole of the Cs site in the perchlorate lattice. Assignments of the various band systems to 1E or 1A1 components of the tetrahedral 1T2 then followed from the observed dichroic ratios. The lowest-energy charge-transfer transitions result from donation of an electron from a2 and e (C3v) orbitals localized on the oxygen atoms, which correlate with the t1 shell in the parent tetraoxo-ions. Similar conclusions about nitrido-osmate were reached by the Copenhagen group, who used LiClO4,3H2O as a host. Although the highest filled levels are oxygen-localized, the π-bonding in the ion is dominated by the nitrogen. In the polarized single-crystal spectra of salts such as Ph4As[MoOCl4(H2O)], containing the MoO3+ moiety, the lowest-energy transitions are O(2pπ) -> Mo(xy) charge-transfer and Mo(xy) + Mo(x2- y2) ligand-field types.

Halide. Some of the richest ligand-field spectra to be found anywhere are those of the tetrahedral ions [MX4]2- (M = 3d ion, X = halide). The wealth of fine structure revealed at 4K is interesting, not only for the very detailed information about vibronic interactions which can be extracted from it but, more generally, for unambiguous assignments of the electronic states themselves, which then provide the starting point for searching tests of theoretical models for describing ligand-field states. An extremely detailed study of the...