Tr orbitals

Front-side approach is disfavored both because the density of the a orbital is less in the region between the carbon and the leaving group and because front-side approach would involve both a bonding and an antibonding interaction with the tr orbital since it has a nodal surface between the atoms. 

Carbon monoxide is a colorless, odorless, flammable, almost insoluble, very toxic gas that condenses to a colorless liquid at — 90°C. It is not very reactive, largely because its bond enthalpy (1074 kj-mol-1) is higher than that of any other molecule. However, it is a Lewis base, and the lone pair on the carbon atom forms covalent bonds with J-block atoms and ions. Carbon monoxide is also a Lewis acid, because its empty antibonding Tr-orbitals can accept electron density from a... 

A positive charge in an empty rc-orbital can be stabilized by the + M effect exerted by the free electron pair on an adjacent atom X, or by a filled tr-orbital of a double bond C = D. In delocalizing a positive charge into a double bond C = D, the larger orbital coefficient is again on atom D. The higher the electronegativities of the orbitals on X or on D, the less they are available for donation of electron density into the... 

FIGURE 10. Correlation diagram showing the influence of non-bonded through-space (n.bd.th.sp) interaction between the pseudo w-orbitals. t(( IU ) (circles) and the double-bond tr-orbitals tra and (ovals) on the 7r-orbital energies of the butadiene w-system. Aa is the basis energy of tra and 7Tb and SA the inductive and hyperconjugative destabilization (see equation 34)... 

The disproof of the naive expectation that the PE spectrum of a (non-planar) hydrocarbon containing z non-conjugated double bonds would show z 7r-bands, where each one corresponds to the removal of an electron from only one of the z localized two-centre Tr-orbitals, was one of the earliest successes of PE spectroscopy. In particular, if in a... 

Corresponding to this valence bond view is a molecular orbital picture. The three cr-orbitals of a CH3 group are regarded as a basis from which three group orbitals may be constructed. One of the possible combinations of the tr-orbitals has the same local symmetry as the vacant p-orbital on the cationic centre, and hence may overlap with it. Therefore, a withdrawal of electrons from the methyl group can take place. The orbital from which electron density... 

Fig. 3. Molecular orbital representations (a), the level diagram for Hjj (6), the MO s formed by the addition and subtraction of 2p-n AO s (c), the level diagram for tr orbitals of ethylene.

Compound Coupling Constants (Mc/s) A B C p-(T orbital population Total p-tr orbital Ref. population ... 

This section is intended to cover those complexes which contain unsaturated hydrocarbon groups coordinated sideways to the metal atom through their Tr-orbital systems. The substances are rather arbitrarily divided according to whether the ligand is classed as uncharged or anionic. 

Using these tr-orbitals as basis functions, construct a seven-dimensional representation of the point group. 

The other three ligands tt bond in a very similar fashion except that the acceptor orbital is a tr orbital as shown in Fig. 11,23c. The net result is the same as for ligands in which either d or cr orbitals or both serve as v acceptors The bonding level is lowered so that the quantity is increased. A molecular orbital diagram. 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

FIGURE 3.33 Two p-orbitals can overlap to give bonding (lower) and antibonding (upper) tr-orbitals. Note that the antibonding combination has a node between the two nuclei. Both orbitals have nodes passing through the nuclei. 

It is clear that introduction of the functional groups to the silicon-silicon double bond causes a large red shift of the absorption maxima of disilenes. Similar substituent effects have been known for a variety of alkenes31. Qualitatively, the reason for the red shift should arise from destabilization of the HOMO of the disilene by an appreciable interaction between the n-orbital of the heteroatoms and the tr-orbital of the disilene, although the effect of their distorted structure should also be considered as discussed later. 

Distortion of olefin tr-orbitals in dibenzobicyclo[2.2.2]octatrienes 4 has been detected37. Thus, nitro and fluoro groups give large to moderate bias with preferred syn attack (with... 

This change of character of the MLCT transition has been observed before for the mononuclear d6-complexes MX(CO)3(a-diimine)(M=Mn,Re X=Cl,Br) and M(CO)i,(a-diimine) (M=Cr,Mo,W) and ascribed to a change of interaction (tr-backbonding) between the d M) and tr -orbitals involved in this transition (3). When the tr-backbonding increases, the transition loses its... 

In the idealized ethylene-acetylene model complex the HOMOl is the olefin stabilized dxz while the HOM02 orbital, dxy, reflects alkyne w overlap. The M—C alkyne distances employed in the calculation increase overlap responsible for the alkyne-metal v interactions relative to the olefin which is further from the metal and overlaps less (60). The dir bonding contribution of the single-faced 7r-acid olefin is to stabilize the lone filled d tr orbital which is independent of the alkyne. This role is compatible with the successful incorporation of electron-poor olefins cis to the alkyne in these d4 monomers. It may well be that the HOMOl and H0M02 orbitals in isolated complexes are reversed relative to the model complex as a result of electron-withdrawing substituents present on the olefins. 

Crucial interactions occur when two carbonyls are placed in the jcz plane. The -ttx orbital is destabilized over the ir orbitals through an interaction with carbonyl tr orbitals while the tt and vy orbitals are stabilized by backbonding. The effect is to give a predicted occupancy of (v- x)2(iry)2 for... 

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

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