Sulphur valency

TABLE 3. Coefficients (c,) and exponents (a,) of the 3G expansion for sulphur valence orbitals, and coefficients and exponents for the 6G expansion of the radial pseudopotential... 

Deformation density calculations [66] based on X-ray and neutron diffraction data indicated a substantial electron deficiency in the sulphur valence shell. Kappa refinement based on the point charge model relates a positive charge close to 1.8 to the sulphur, and about -1.0 on each of the sulphate oxygen atoms [66]. The comparison of these results to those obtained from MO cluster model calculations, provides an indication of the goodness of the applied atomic basis sets. Such a comparison is presented in Table 4. 

Metal oxides, hydroxides and sulphides are represented in minerals whose properties are substantially defined by the nature of their central atom and its valence. The metal and sulphur valence depends on the redox environment. In this case all processes of dissolution and mineral-formation are viewed in conditions of stable redox environment, in which they do not change their charge. It is assumed that oxides are in oxidation medium and sulphide in the reduction medium, with solution Eh no greater than -0.2 v. 

It is important to distinguish between the role of d-orbitals as hybridization functions and their role as polarization functions. For the former case, the d-orbital serves to increase the number of distinct orbitals utilized in the wave-function . Thus, the octet and expanded valence-shell structures (1) and (2) have Qs p, F3 -HCjSp, Fj and CjS pM, F, +C2sp d, Ft valence-state configurations for the trivalent and quadrivalent sulphur atoms. For stmctures (4) and (5) the sulphur valence-state configurations are sp, Ft and sp d, . When a 3d-orbital... 

On the basis of some bond-length data, we shall suggest that expansion of the sulphur valence-shell to generate d -p bonding is more likely to occur in either of the following situations ... 

In Refs. 23 and 24, further consideration is given to valence-bond structures that involve expansion of the valence shell for second-row and higher-row elements -for example the use of sulphur 3d orbitals as valence orbitals in SFe. See also Ref 25 for another type of SFg increased-valence structures that does not involve expansion of the sulphur valence shell. In Ref 26, increased-valence structures are presented for OSSSO (see also Ref 27) and O2SSSO2 with two tetrahedral sulphur atoms. 

Whereas some atoms have only one valency, others have several, e.g. sulphur has valencies of two, four and six and can form compounds as diverse as hydrogen sulphide, H2S (valency two), sulphur dioxide, SO2 (valency four) and sulphur hexafluoride, SF6 (valency six). Clearly some compounds comprise more than two different elements. Thus hydrogen, sulphur and oxygen can combine to produce sulphuric acid, H2SO4. From the structure it can be seen that hydrogen maintains its valency of one, oxygen two and sulphur is in a six valency state. 

Unlike oxygen, sulphur can exist in higher valency states and as a result can be incorporated into organic structures in additional ways. Examples include ... 

The effect of a particular element on the odour of its compound seems also to lend support to the residual affinity theory, for it is only the elements which possess residual affinity in certain of their compounds, which function as osmophores. Oxygen, nitrogen, sulphur, phosphorous, halogens, arsenic, antimony, bismuth, etc., whose valencies vary under certain conditions are powerfully osmophoric whereas carbon, hydrogen, and many others which have a constant valency are practically non-osmophoric, and it is very instructive to note that the element is osmophoric when it is not employing its full number of valencies and therefore has free affinity. 

According to the Stoner scheme the electronic configuration for the element sulphur, for example, is 2, 2, 2t 4, 2, 2, 2. This configuration could account successfully for the various valency states shown by the element, that is 2, 4, 6 as mentioned before. However, this new scheme did nothing to resolve the problem of the violation of quantum numbers as seen in the splitting of spectral lines in a magnetic field. 

The starting point to obtain a PP and basis set for sulphur was an accurate double-zeta STO atomic calculation4. A 24 GTO and 16 GTO expansion for core s and p orbitals, respectively, was used. For the valence functions, the STO combination resulting from the atomic calculation was contracted and re-expanded to 3G. The radial PP representation was then calculated and fitted to six gaussians, serving both for s and p valence electrons, although in principle the two expansions should be different. Table 3 gives the numerical details of all these functions. 

This reaction involves Michael addition of (28) to the enone followed by cycllsation (29) with displacement of DMSO, Sulphur y,llds react in these ways rather than removing oxygen, as phosphorus ylids do, because the SO bond is weaker than the PO bond and the lower valency states of sulphur more stable than those of phosphorus. 

The authors believe that the results are more readily explainable on the basis of a two-point absorption involving the vanadium-carbon-sulphur complex, the formation of which may be effected with little distortion of the tetrahedral valence angles. 

Other Iron Compounds of Biological Interest.—The valency of iron in a range of ferredoxin extracts has been determined by ESCA. The structure of the Fe—S complex in a bacterial ferredoxin has been determined. The iron and sulphur atoms occupy alternate corners of a cube and four more sulphur atoms project from the iron atoms.Admission of oxygen to a neutral solution of Fe and excess penicillamine gives a red bis-complex, which is relatively stable in aqueous solution at room temperature. Quantitative... 

In conclusion, the Raman spectroscopic experiments and the assignment of the SN vibrations to much lower wavenumbers than previously reported suggest a mainly electrostatic contribution in the > S —N —bond of sulphur triimides and related metal complexes. The hypothesis of valence expansion at sulphur to three covalent double bonds [S(=NR3)] seems no longer valid . 

Thus if we insert the values m = 1 and w = 2 in the first case and m = 2, K = 1 in the second, so that the hydrogen atom is held on by one elementary space and the hydrogen molecule by two elementary spaces, the conception of a uni- and divalent union is readily realised. In this way a valency of two for carbon monoxide, sulphur trioxide and ammonia on platinum two for the oxygen molecule and one for carbon monoxide on quartz can be established. 

In a way, it is rather paradoxical that the spectroscopic evidence is rather equivocal for the organic compounds having comparatively low coordination numbers of the metalloid atoms, whereas it is distinctly against strong 3d-participation in cases of high N, such as SFe. This statement may be compared with a previous review (72) on valence-shell expansion in sulphur compounds as well as a recent one 90). 

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