Valence state promotion energies

Group IIB elements bond energies of, 11 316 heats of atomization, 11 313 ionization potentials, 11 310, 311 valence state promotion energies, 11 311, 312... 

Secondary atomic properties as those, which require, in addition to the experimentally determined quantities for the free atoms, theoretical concepts of the quantum mechanical characerisation of the electronic structure of the atoms. These are orbitals, the shell structure of atoms with emphasis of the valence shell as well as concepts like hybridisation, the definition of the valence state and the valence state promotion energy in its relation to the spectroscopic term values of the free atoms. 

Using the definitions of the orbital electronegativity presented in eqs. (3.5, 4.10 and 4.11) together with the latest experimental values for the ionisation potentials, electron affinities and term values for the atoms, an extensive reevaluation of the atomic valence state promotion energies and electronegativities has been carried out[28-30]. The results obtained are presented in Chart 1 and in more detail in Table 1. 

PbCl4 had not been destabilized by the nearly 600 kJ mol" spent to promote the metal atom to the valence state, the energy of the reactiOTi would have been about... 

Finally it has been pointed out that, within the same transition period, the trend in >m.Co is obscured by the different promotion energies to the valence state and a trend different to that in Table 5 has been calculated for the first period (Cr = 54.2 Fe = 58.7 Ni = 45.6 kcal mol" 1) 22SK A similar effect probably occurs on descending a subgroup. 

Figure 5.8. Lanthanide Ln203 oxides (cubic cI80-Mn2O3 type, on the left side) and Pb alloys (LnPb3, cubic cP4-type, on the right). The trends of the lattice parameter and of the heat of formation are shown (see the text and notice the characteristic behaviour of Eu and Yb). A schematic representation of the energy difference between the divalent and trivalent states of an ytterbium compound is shown. Apromff represents the promotion energy from di- to trivalent Yb metal, A,//11, and Ar/Ynl are the formation enthalpies of a compound in the two cases in which there is no valence change on passing from the metal to the compound the same valence state (II or III) is maintained.

Though the core expansion leads to the appropriate fit, it may not be the proper explanation for the scale factor discrepancy. Hansen et al. (1987) note that the expansion of the core would lead to a decrease of 7.5 eV in the kinetic energy of the core electrons, at variance with the HF band structure calculations of Dovesi et al. (1982), which show the decrease to be only about 1.5 eV. An alternative interpretation by von Barth and Pedroza (1985) is based on the condition of orthogonality of the core and valence wave functions. The orthogonality requirement introduces a core-like cusp in the s-like valence states, but not in the p-states. Because of the promotion of electrons from s - p in Be metal, the high-order form factor for the crystal must be lower than that for the free atom. It is this effect that can be mimicked by the apparent core expansion. 

In a) and P) the non bonding-hypothesis for 5 f electrons is retained, differences in cohesive energy being only due to promotion of outer electrons from one to another orbital state and ionization energies (or electron affinities) due to the different valence states attained. Therefore, any further discrepancy found with experimental values, is indicative of the metallic bonding introduced by delocalization of the 5f electrons (point y). 

It was pointed out in Section 4-2 that the configuration sp8, which has promotion energy about 200 kcal/mole relative to the ground configuration 2s22p2, is the basis of the quadrivalent state of the carbon atom and is shown by quantum-mechanical calculations for methane to contribute about 49 percent to this valence state. Now let us consider the iron atom, for which spectroscopic energy levels are shown on... 

In the phosphorus atom there is little initial promotion energy The ground stale is tnvalent, as is the valence state. Note that any hybridization will cost energy as a filled 3,s orbital is raised in energy and half-filled 3p orbitals are lowered in energy ... 

Divalent beryllium honds through two equivalent sp, or (Sgonal, hybrids. The appropriate ionization energy therefore is not that of ground state beryllium, ls2 2. hut an average of those energies necessary to remove electrons from the promoted, valence state ... 

---