Orbitals and oxidation states

The systems of valent states and oxidation states introduced by chemists are not merely electron accounting systems. They are the systems which allow us to understand and predict which ratios of elements will form compounds and also suggests what are the likely structures and properties for these compounds (3). In the case of highly covalent compounds, the actual occupancy of the parent orbitals may seem to be very different than that implied from oxidation states if ionicity were high. Nonetheless, even some physicists have recognized the fundamental validity and usefulness of the chemist s oxidation state approach where the orbitals may now be described as symmetry or Wannier orbitals (6). 

The most noticeable example is that concerning Ru(bipy)32 + ions in acetonitrile solutions at a Pt electrodes with the reaction mechanism formulated as following. In the electrochemical reactions, the parent ions Ru(bipy)32+ undergo70,71 one-electron reduction (with the added electron localized on individual ligand -orbitals) and oxidation (with removal of a metal t2g electron) followed by ion s annihilation with the formation of the excited 3 Ru(bipy)32 + state and subsequent emission of light. 

ORBITAL ENERGIES, IONIZATION POTENTIALS AND OXIDATION STATES... 

Jergensen, C. K. Orbitals in Atoms and Molecules, S. 112. New York Academic Press 1962 Absorption Spectra and Chemical Bonding in Complexes, S. 120—21. New York Pergamon Press 1962 Oxidation Numbers and Oxidation States, S. 96. Berlin Springer 1969... 

The final states may be mixed with other orbitals and can be used to determine the coordination environment of an element in a compound, its electron density and oxidation state. The intensity can be used to accurately determine relative oxidation state ratios. Furthermore, XANES can be used to determine relative amounts of species by linear combination of individual compounds. In recent years, the codes for XANES calculation (especially John Rehr s FEFF code), have significantly improved and it can be expected that theoretical models of compounds will be accurately determined by XANES measurement and calculation in the future. 

L—>M 77 donation is certainly possible for complexes with ligands that have empty tt orbitals because they also have filled 77 orbitals that can donate electron density to empty metal orbitals. The experimental evidence cited earlier snggests that L—>M 77 donation is less important than M—>L 77 donation for metal carbonyls. As a general rule, L M 77 donation is more important for metals with few d electrons (high oxidation states), and M—>L 77 backbonding is more important for metals with largely occupied d orbitals (low oxidation states). 

MLCT is less common, as it requires the existence of empty ligand orbitals of suitable energy. Many of these ligands are % acceptors (see Topics H2 and H9). With changing metal ions and oxidation states, MLCT bands often follow the reverse of the trends found with LMCT. 

Two properties that are characteristic of second-row atoms in the Periodic Table, compared to the corresponding valence isoelectronic first-row atoms, are hypervalency (increased coordination) and the relative importance of d-type orbitals to their molecular electronic structure description. Hypervalency in sulphur compounds is represented by trivalent, tetravalent and hexavalent sulphur where a central sulphur atom is bonded to more than two ligand atoms or groups, compared to the oxygen atom which is almost exclusively divalent. Sulphur-containing compounds are typically classified in this manner9. Here, we have not differentiated between coordination number, valency and oxidation state. This point will be addressed later. 

XPS analyses were performed to determine the stoichiometry of the nanoparticles and oxidation states of the Ru and R (Figure 6). The spectrum of Pt can be fitted by two pairs of overlapping curves corresponding to two doublets of spin orbital interaction at Aisn and 4f7/2- The strongest doublet with binding energies of 74.42 (4f7/2)... 

In this Section our attention is directed primarily towards the d—d transitions which occur in the electronic spectra of these ions, the Laporte-allowed charge-transfer bands being discussed in more detail in Section 5, although some correlations between molecular orbital calculations, oxidation state stabilities etc., and the positions of the charge-transfer excitations are indicated. 

If the ion is specifically adsorbed, the clear distinction between reduced and oxidized states is no longer possible as a consequence of the formation of bonding orbitals and the fast electron fluctuation between the metal surface and the adsorbed ion. The solvation shell cannot achieve a stable configuration for oxidized and reduced states of the adsorbed ion. Instead, an intermediate solvent distribution develops for a mean residence probability of an electron on the adsorbed ion. 

It s time now to think about hypervalent compounds. You have encountered a few of them already, as products of A reactions and as intermediates in SN2-Si mechanisms. But what is special about such compounds Is the term hypervalent synonymous with higher-valent (No.) To better understand these issues, we ll take a step back in Section 1.24 and remind ourselves what the term valence exactly means and how it differs from related concepts such as coordination number (CN), FC, and oxidation state (OS). Confusion between these terms and incorrect usage are widespread in both textbooks and the research literature. From there we ll proceed on to some related topics such as an elementary molecular orbital description of hypervalent bonding (Section 1.25). We ll conclude this chapter with a brief discussion of the inert pair effect, an important aspect of the variable valence of the heaviest (sixth-period) p-block elements. 

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