The Roles of d Electrons

The effects of the bonding electrons upon the d electrons is addressed within the subjects we call crystal-field theory (CFT) or ligand-field theory (LFT). They are concerned with the J-electron properties that we observe in spectral and magnetic measurements. This subject will keep us busy for some while. We shall return to the effects of the d electrons on bonding much later, in Chapter 7. 

The Roles of t/-Electrons in Transition Metal Chemistry A New Emphasis, M. Gerloch, Coord. Chem. Rev., 1990, 99, 199. 

Atkins, Molecular Quantum Mechanics, Oxford University Press, Oxford, 1970. 

---

The synthesis of barium and strontium molybdates(IV) was first reported by Scholder, Klemm, and Brixnerd The compounds are of interest because they contain molybdenum in the 4-4 oxidation state. Both compounds have the undistorted cubic perovskite structure, which is ideal for stud3nng cation-anion-cation (superexchange) interactions. The compounds also exhibit metallic behavior, thus permitting study of the role of d electrons in the conduction process. 

The lowest plateau in our case occurs at 2.1G0, which may indicate that either the corresponding apex is not of one-atomic shape, or the atomic structure of the apex is such that it allows contribution from both s and d electrons. One may expect that the s electrons would contribute to plateaus at nG0 with roughly integer n, whereas the d electrons may give contributions differing from that corresponding to perfect transmission as the d electrons of Co atoms must couple to Ge atoms which have different electronic structure. In turn, the higher conductance plateaus, G = 2.8G0 and G = 3.5G0, may indicate the role of d electrons and the presence of spin polarized conductance channels. 

In all these discussions, we separate, as best we might, the effects of the d electrons upon the bonding electrons from the effects of the bonding electrons upon the d electrons. The latter takes us into crystal- and ligand-field theories, the former into the steric roles of d electrons and the geometries of transition-metal complexes. Both sides of the coin are relevant in the energetics of transition-metal chemistry, as is described in later chapters. 

DR. TAUBE The number of d electrons in the reducing agents does seem to be an important factor, and since above a certain number both a and n orbitals are populated, in that sense at least orbital symmetry plays a role. Perhaps the... 

Although the availability of d electrons will certainly influence the ease of formation and the strength of the covalent bonds in chemisorption phenomena on metals, we may not expect a simple relationship between the heat of chemisorption and some more or less simple property which is related to the d electrons. There will certainly be other properties of the metals which play a role. In expression (32) the last term containing the electronegativities has some relation to the availability of electrons from the metal, and it must be said that the order in which the heats of chemisorption fall (Sec. V,8,b) is nearly the same as that in which the work function of the metals rises. In the formation of the dipoles between the adsorbed atoms and the metal, work has to be done against the work function we may expect that less work will be necessary to form these dipoles and that the dipole moment will be larger, the smaller the work function. 

Electronic surface states are responsible for adsorptive and catalytic processes occurring at the surfaces of solids. Therefore, the catalytic behavior of the simple transition metal oxides such as NiO and CoO (Section I V.F) led to extensive experimental and theoretical investigations of the corresponding 3d surface states (115, 275-283). Whereas the role of d-n overlap is quite modest in the Ni2+ CO surface complexes, the same does not necessarily hold for the Ni2+ NO complexes formed upon interaction of NiO(OOl) with the stronger n-acceptor NO ligand. This is shown by the formation of... 

Ligand field, like crystal field, theory is confined to the role of d orbitals, but unlike the crystal field model, the ligand field approach is not a purely electrostatic model. It is a freely parameterized model, and uses and Racah parameters (to which we return later) which are obtained from electronic spectroscopic (i.e. experimental) data. Most importantly, although (as we showed in the last section) it is possible to approach the bonding in d-block metal complexes by using molecular orbital theory, it is incorrect to state that ligand field theory is simply the application of MO theory. ... 

In addition to the factors which we outlined on p. 2 for assessing the relative stabilities of the various coordination polyhedra, the number of d electrons present in the central ion must also play an important role. Indeed, since transition-metal ions of a given charge have approximately the... 

Dynaniical theories of unimolecuiar decomposition deal with the properties of vibrational/rotational energy levels, state preparation and intramolecular vibrational energy redistribution (IVR). Thus, the presentation in this chapter draws extensively on the previous chapters 2 and 4. Unimolecuiar decomposition d)mamics can be treated using quantum and classical mechanics, and both perspectives are considered here. The role of nonadiabatic electronic transitions in unimolecuiar dynamics is also discussed. 

Keren N, Berg A, van Kann PJM et al. Mechanism of photosystem II photoinacdvadon and D1 protein d adation at low light The role of back electron flow. Proc Natl Acad Sci USA 1997 94 1579-1584. 

---