Stabilization of the d orbitals

Application of equation (10c) to the observed single-bond radii of scandium, titanium and vanadium (1-439,1-324,1-224 A) leads to 20, 27 and 35 % of d character, respectively (table 5). The gradual increase presumably results from the increasing stability of the 3d orbitals relative to 4s and 4p. 

The symmetries of the d orbitals of the chlorine atom allow some stabilization of the two bonding orbitals, which is consistent with the assumptions made by the VSEPR approach. Both theoretical approaches offer a reason for the deviation from T-shape to an arrow shape for... 

A second factor which must be considered is the nature of the d orbitals. The size of the d orbitals is inversely related to the effective nuclear charge. Since elective overlap of d orbitals appears necessary to stabilize metal clusters, excessive contraction of them will destabilize the cluster. Hence large charges resulting from very high oxidation states arc unfavorable. For the first transition series, the d orbitals are relatively small, and even in moderately low oxidation states ( + 2 and +3) they apparently do not extend sufficiently for good overlap. 

In the (/ case discussed above, the electron occupies a t2j, orbital, which has an energy of -0.4A relative to the barycenter of the d orbitals. The complex can thus be said to be stabilized to the extent of 0.4AU compared to the hypothetical spherical-field case. 

Further examples of the jeopardy involved in casually dismissing d orbitals participation are the findings of Haddon and coworkers28 that rf-orbitat participation is especially important m S4F4, which is nonplanar, and ako that it accounts for about one-half of the delocalization energy in the one-dimensional conductor (SN)4. In the latter case, the low electronegativity of the d orbitals (see Chapter 5) increases the ionicily of the S—N bond and stabilizes the structure. 

Another effect of the coordination is that the benzylic cation is also stabilized [63]. This stabilization is explained by delocalization of the positive charge due to the interaction of the d-orbital of Cr with the 71-orbital of the benzylic carbon, caused by the coordination of Cr(CO)3. Facile stereospecific Friedel- Crafts-type cyclization of the complex of the optically active benzyl alcohol 248 gave the tetrahydrobenzazepine 249 with retention of the stereochemistry, and the free amine 250 with 98% ee was... 

The first step is a Wittig reaction18 in which the ketone is converted to the terminal olefin by reaction with a phosphorus ylide (also called a phosphorane). Phosphoranes are resonance-stabilized by overlap between the carbon p-orbital and one of the d-orbitals of the phosphorus. 

The bonding between phosphorous atoms in the P4 molecule can be described by a simple bent bond model, which is formed by the overlap of sp3 hybrids of the P atoms. Maximum overlap of each pair of sp3 orbitals does not occur along an edge of the tetrahedron. Instead, the P-P bonds are bent, as shown in Fig. 15.3.1(b). In a more elaborate model, the P4 molecule is further stabilized by the d orbitals of P atoms which also participate in the bonding. 

Figure 5 demonstrates the relativistic stabilization of the ns orbitals, as well as the destabilization of the (n-l)d orbitals for group-8 elements, as an example. One can see that trends in the relativistic and non-relativistic energies of the valence electrons are opposite from the 5d to the 6d elements. Thus, the non-relativistic description of the wave function would still give the right trend in properties from the 4d to the 5d elements, while it would result in the opposite and consequently wrong trend from the 5d to the 6d elements. 

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