Energetic destabilization

The second (indirect) relativistic effect is the expansion of outer d and f orbitals The relativistic contraction of the s and pi/2 shells results in a more efficient screening of the nuclear charge, so that the outer orbitals which never come to the core become more expanded and energetically destabilized. While the direct relativistic effect originates in the immediate vicinity of the nucleus, the indirect relativistic effect is influenced by the outer core orbitals. It should be realized that though contracted s and pi/2 core (innermore) orbitals cause indirect destabilization of the outer orbitals, relativistically expanded d and f orbitals cause the indirect stabilization of the valence s and p-orbitals. That partially explains the very large relativistic stabilization of the 6s and 7s orbitals in Au and element 112, respectively Since d shells (it is also valid for the f shells) become fully populated at the end of the nd series, there will occur a maximum of the indirect stabilization of the valence s and p orbitals [34],... 

Fig. 4. An ortho-methyl group in diethylamino-pyrimidin induces some ground state twist and hence energetically destabilizes the B state but not yet sufficiently to make the population of the A state a najor process in supersonic jet spectroscopy. Upper panel dispersed fluorescence spectra of the jet-cooled bare molecule [36]. In clusters with methanol, the TICT state is preferentially lowered, and the majority of the ob rved red-shifted fluorescence can be assign l to arise from the TICT state (lower panel). This does not occur for the compound without an ortho-methyl group.

Thus, the main relativistic effects are (1) the radical contraction and energetic stabilization of the s and p orbitals which in turn induce the radial expansion and energetic destabilization of the outer d and f orbitals, and (2) the well-known spin-orbit splitting. These effects will be pronounced upon going from As to Sb to Bi. Associated with effect (1), it is interesting to note that the Bi atom has a tendency to form compounds in which Bi is trivalent with the 6s 6p valence configuration. For this tendency of the 6s electron pair to remain formally unoxidized in bismuth compounds (i.e. core-like nature of the 6s electrons), the term inert pair effect or nonhybridization effect has been often used for a reasonable explanation. In this context, the relatively inert 4s pair of the As atom (compared with the 5s pair of Sb) may be ascribed to the stabilization due to the d-block contraction , rather than effect (1) . On the other hand, effect (2) plays an important role in the electronic and spectroscopic properties of atoms and molecules especially in the open-shell states. It not only splits the electronic states but also mixes the states which would not mix in the absence of spin-orbit interaction. As an example, it was calculated that even the ground state ( 2 " ) of Bij is 25% contaminated by Hg. In the Pauli Hamiltonian approximation there is one more relativistic effect called the Dawin term. This will tend to counteract partially the mass-velocity effect. 

In summary, prominent features of ylide nickel complexes versus phosphane complexes have been identified an electron-rich nickel center, energetically destabilized nickel-localized occupied orbitals, a significant weakening of the Ni-O bond, the phosphoms moiety being located outside the nickel coordination plane, thus opening one axial position in the nickel coordination sphere for easy monomer landing . 

All the known intermolecular interactions can be classified within one of these three groups. Whenever there is more than one option, we suggest to look at the dominant component of the interaction energy and use it to classify the molecule (for instance the Ar- -Ar interaction is a van der Waals interaction—in fact a bond, as it is energetically stable, but K+ K" ", which is isoelectronic, is an ionic interaction— it is not a bond, as it is energetically destabilizing). 

It was not until the 1970s that the full relevance of relativistic effects in heavy-element chemistry was discovered. However, for the sixth row (W---Bi), relativistic effects are comparable to usual shell-structure effects and therefore provide an explanation for many unusual properties of gold chemistry155-159. The main effects on atomic orbitals are (i) the relativistic radial contraction and energetic stabilization of the s and p shells, (ii) the spin-orbit splitting and (iii) the relativistic radial expansion and energetic destabilization of the outer d and f shells. 

The second (indirect) relativistic effect is responsible for the expansion of outer d and f orbitals Here the relativistic contraction of the s and pi/2 shells results in a more efficient screening of the nuclear charge, and the outer high angular momentum orbitals expand and become energetically destabilized. In turn, the relativistically expanded d and f orbitals cause a small additional... 

Because of the crystal field around the metal cation and the latter s antibonding interactions with the ligands, dll five formerly degenerate d orbitals become energetically destabilized within the octahedral environment, but the f2g set is less destabilized than... 

The relativistic effects are responsible for radial contraction and energetic stabilization of the s /2 and p /2 shells and for the spin-orbit splitting of shells with Z > 0 into sub-shells with j = 1 — 1/2 and j = I + /2. The indirect relativistic effects are consequences of a screening of the d and / electrons by inner electrons occupying the contracted si/2 and pi/2 orbitals and lead to the radial expansion of the and / shells accompanied by their energetic destabilization [1,2]. 

According to the slab calculations, breaking an Fe-S bond and the loss of overlap with S 3p orbitals energetically destabilizes every Fe 3d orbital with a z-component (z defined to be normal to the surface plane) (Fig. 32). 

We will now discuss at some length the many ways in which deviations from standard bonding parameters lead to energetic destabilization of a molecule. We will focus on "stable" structures (i.e., not on reactive intermediates), but the notions we develop here also apply to reactive intermediates. We first explore acyclic systems, wherein molecular motions directly lead to strained forms. Note that we are not yet considering conventional chemical reactivity. We will be considering conformers, or conformational isomers. Recall that conformers are stereoisomers that interconvert by rotation around single bonds (see Chapter 6 for definitions of stereochemical concepts). These isomers are not to be confused with constitutional isomers, where the molecular formula is the same, but the atoms are arranged differently. 

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