D orbital involvement

The mixed model used in ZINDO/1 is identical to that used in CNDO and INDO if there is no d-orbital involved in the quantum... 

Table 6-6. angular coefficients for the different d orbitals involved d in a a interaction with ligands... 

Although Chapter 25 does not address directly why some compounds with coordination 4 are tetrahedral and some are square planar, it is possible to surmise that the answer lies with (1) Crystal Field Theory and the energies of the d orbitals involved bonding and (2) how many unpaired electrons the metal complex has. 

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

These conclusions are not unexpected. Firstly, the free-electron approximation works best for the alkah metals, with significant deviations for divalent and trivalent metals. Secondly, the LCAO-MO TSH treatment requires a more complicated treatment of p and d orbitals, involving tensor... 

Excited states involving 3d orbitals were considered also in these extrapolations. It was found that the 3d orbitals do not appear to be involved with the lighter elements that were treated, although they begin to have some influence in the phosphorus system. Presumably any d orbital involvement will become more important with the heavier elements. 

A comparison of these compounds with the corresponding zinc derivatives should clarify the influence of the empty d orbitals involved in the bonding situation of the alkaline earth metal bis(phosphanides). Whereas zinc bis[bis(trimethylsilyl)amide] is monomeric due to the steric demand of the bulky amide ligand [6], the trimethylsilyl substituted phosphanide leads to oligomers such as dimers or trimers [7], The influence of the pnicogen atom is small, thus the phosphorus and arsenic derivatives (Fig. 5.) look very similar [8] or even crystallize isotypically. In contrast to the d metals calcium, strontium and barium, zinc derivatives solely build up monocyclic ring systems. 

The distortions we have described above, often attributed to Jahn-Teller effects are, of course another example of the busy/idle concept described earlier. The situation is a little more complex here since the two d orbitals involved do not have equal interactions with all ligands but the busy x - y and idle orbital in the axially elongated molecule may readily be compared to the metriotic situation in the octahedral molecule. 

Trend (2) is more complex, but two factors contribute to inversion of the stability of Aj-X and A2-X. One factor is repulsion between nonbonding electrons. The F-F bond is much weaker than the Cl-Cl bond for this reason. The non-bonding pairs for fluorine lie much closer to the nucleus and thus closer to the non-bonding pairs on the other nucleus, causing more repulsion and weakening the F-F bond relative to the Cl-Cl bond. The second factor is the possibility of d-orbital involvement in TT-bonding for second-row elements. The second-row elements have vacant low-lying d-orbitals which can form a drc-pjc interaction with... 

Figure 4.1. Comparison of the back-donation interactions (d 7t ) in the perpendicular (on the left) and coplanar (on the right) conformations of a

Bonding d orbitals involved in covalent dsp hybrid bonds... 

The hybridisation treatment suggests that the slopes should be 0.67 and 1.00 respectively, assuming no significant d orbital participation. The slight discrepancies may arise from some d orbital involvement, or from the neglect of the electron density in the ligand valence shdl. 

The Hiickel method is simple and has been in use for decades (Hiickel, 1931a,b). It is based on the ct-ti separation approximation while accounting for the pi-electrons only, i.e., the atomic orbitals involved refer to those 2p for Carbon atoms as well to the 2p and 3p orbitals for the second and third period elements as (N,0, F) and (S, Cl) respectively further discussion on the d-orbitals involvement may be also undertaken, yet the method essence reside in non explicitly counting on the electronic repulsion with an effective, not-defmed, mono-electronic Hamiltonian, as the most simple semi-empirical approximation. In these conditions, for the mono-electronic Hamiltonian matrix elements two basic assumptions are advanced the forthcoming discussion follows (Putz, 201 Id) ... 

Contents Introduction. - Theoretical Basis for d-Orbital Involvement. - Physical Properties Related to dp-7r Bonding. - The Effects of dp-7r Bonding on Chemical Properties and Reactivity. - Pentacovalency. - References. 

Fig. 44a-c. The way in which an electric field set up by a single electron transfer from lithium to carbon polarizes the valence electrons of the remaining Li atoms engaged into covalent bonding with carborf Arrows indicate how the Li electron will move under the infiuence of a field, (a) (RLi) case (b) CLf with no d-orbital involvement, (c) Cli with d-orbital (and higher energy orbital) involvement... 

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