Overlap with d orbitals

The high apicophilicity of F is indicated by its preference for apical sites in many structures (4.263), (6.534) and (7.138). Equatorial positions are favoured by n-donor groups such as NRj or Ph probably because orbital overlap with d orbitals of P is easier in that position. 

CRF=core-repulsion function MIND03 = modified intermediate neglect of differential overlap, version 3 MNDO = modified neglect of differential overlap MNDO/d = modified neglect of differential overlap with d orbitals. 

In a nutshell, the d electrons repel the bonding electrons. They get in the way of the bonds and, to a greater or lesser degree, frustrate the attraction between metal and ligands. In essence, the proposed minimal overlap of d orbitals with the ligands, but significant repulsive interaction with the bonds, is equivalent to a focus upon the two-electron operator rather than the one-electron operator that is, upon repulsions rather than overlap. 

Pi Each pi bond has one axial nodal plane (p-like shape) and the longitudinal profile, (Figs. 4.24(b) and (c)) to be expected from 7t-type overlap of d orbitals. The two pi bonds are labeled 7tw(o) (out of plane) or 7Tww(l) (in plane) to distinguish their orientation with respect to the molecular plane. 

The c/a ratio is greater than for V02, which implies that the n band (i.e. that with d-orbital lobes in the basal plane) is more occupied than in V02 (Goodenough 1971, p. 352). But we think that if it were not ferromagnetic, the n band, in contradistinction to V02, would be wholly above the Fermi energy. The Hubbard correlation term U, however, produces localized moments for the 3d2 states, as explained in Chapter 3, and these, if oriented ferromagnetically, would just fill the tjj band. The filled band (for spin-up electrons) will now overlap the n band, allowing ferromagnetic interaction of Zener or RKK Y type between the d2 moments, as described in Chapter 3. The T2 term in the resistivity could be explained as in Chapter 2, Section 6. 

However, certain phosphonium ylides, such as those with an electron-withdrawing substituent in the alkylidene moiety, are relatively unreac-tive toward certain substrates such as ketones (22, 77, 95). This led us to consider whether arsonium ylides might be preferable to phosphonium ylides in certain reactions (48, 94). The overlap of the p orbitals of carbon with d orbitals of arsenic is less effective than with d orbitals of phosphorus. Therefore the covalent canonical form (la) should make a smaller contribution to the overall structure of arsonium ylides than to that of the corresponding phosphonium ylides. 

The dinitrogen coordination has to be suitable for the overlap of both filled binding and empty antibonding dinitrogen orbitals with d-orbitals of iron atoms. 

We may similarly construct a ligand eg tr-orbital combination to overlap with d 2,... 

Fig. 11.17 Overlap of ligand orbitals in the rv plane with metal (a) and (b) orbitals. Note that an appropriate choice of sign for the ligand orbiials provides positive overlap with the orbital, however, no single sign choice for the ligand wave function produces positive overlap with ihc d,. orbital...

A possible explanation for the small change in bond distance is that, with increasing oxidation state of the metal, the d orbitals contract. This contraction may cause overlap of d orbitals in tt bonding to become less effective. Thus, as 8 electrons are removed, the tt interactions become weaker the two factors (increase in bond order and increase in oxidation state of Re) very nearly offset each other. 

The nature of the ionization is worth considering at this point. If one considers that poly(isobutylene) (PIB) with an unsaturated terminus can be ionized, and thus observed, but PIB with a saturated end group cannot. Clearly this has implications if the mode of termination in cationic polymerisation is the nature of the research project. This is an important consideration when considering the quantitative nature of the information. Thus how does a change in structural composition alter the relative ionization ability Cationisation of poly(styrene) in MALDI is usually by either Ag" or Cu, alkali metal ions proving problematic, overlap of d-orbitals with it-bonds is often used to rationalize this observation. However, ESI of poly(styrene) with transition metals gives no signal whereas results in excellent spectra. This seems difficult to rationalize at the present time. 

The stereochemistry of the alkene (H and C02Me as) will be faithfully reproduced in ms product. The stereochemistry of the OMe group comes from endo attack - we should tuck the -jas group underneath the diene so that it can overlap with the orbitals of the middle two atoms. Ifvw also said that this product will eliminate methanol on work-up and that only the stereochemistr M the ring junction really matters, you d be quite right. 

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