7t-bonding orbitals

Fig. 5.19 Diagrammatic sketches of the molecular orbitals in carbon monoxide (a) one 7t bonding orbital, (b) one rr anlibonding orbital...

Thus in ethylene the combination of two p orbitals gives a 7t bonding orbital and a 7t antibonding orbital. An important difference between the bonding and antibonding orbitals is their symmetry. 

The 7T bonding orbital of cyclobutene arises from reorganization of the butadiene tt system. This process is considered in Woodward, R. B. Hoffmann, R. The Conservation of Orbital Symmetry, Verlag Chemie Weinheim, 1970 p. 38 ff. and references cited therein. 

Anionic halide dimers of Nb(III) are prepared in quartz ampules at 600-700°C from CsX and NbsXs (X = Cl, Br, I)T The [Nb2Br9] ion displays a confacial bioctahedral geometry (Nb-Nb 277 pm) and is paramagnetic in accord with a double-bond formulation in which only two electrons occupy degenerate 7t-bonding orbitals. No description of analogous Ta(III) anions is available. 

Where a molecule has two or more double bonds separated by single bonds (a conjugated molecule), the p orbitals on several atoms can overlap to form delocalized 7t bonding orbitals. In buta-1,3-diene, H2C=CH—CH=CH2, p orbitals on all four carbons can overlap to form a n orbital covering the entire carbon framework (Figure 5.4). 

Buta-1,3-diene also has an occupied 7t bonding orbital (Figure 5.5) in which p orbitals on carbons 1 and 2 overlap, as do those on carbons 3 and 4, but those on carbon 2 do not overlap in a bonding fashion with those on carbon 3. K bonding is thus stronger between carbons 1 and 2 and between carbons 3 and 4 than between carbons 2 and 3, and this is reflected in the bond lengths (Figure 5.6). 

The oxygen 1 s and 2s orbitals will not contribute to the Figure Q.9 Delocalized 7t-bonding orbital for... 

The ground state of ethylene is referred to as being in the tt configuration, in that both electrons are in the 7T bonding orbital. The notation used to describe this excited state of ethylene is 7r 7T. This shows that one electron is in the tt antibonding orbital and one electron is in the tt bonding orbital. 

The reactivity of the active center is attributed primarily to the presence of d-orbitals in the transition metal. The initial state of the active center shows that the =CH2 group will be capable of considerable distortion from its equilibrium position, because of the availability of adjacent d-orbitals. Complexing (b) takes place when the 7t-bonding orbitals of the olefin overlap with the vacant d 2 y2 orbital of the TP+ while at the same time the 7t -antibonding orbitals can overlap the d, orbitals of the Ti +. Formation of the transition state is aided by the ability of the =CH2 group to migrate by partial overlap with the dy, and ji -orbitals (see Figures 7.1 and 7.2). 

The molecular orbitals (localized as well as canonical) can be classified as to the number of nodal surfaces going through the nuclei. A o- bond orbital has no nodal surface at all, a 7T bond orbital has a single nodal surface, a 5 bond orbital has two such surfaces. 

Isosurfaces of the HOMO and the LUMO are given in Figure 12.11. In the HOMO, the C2p orbitals are overlapping wherever there is a tt-bond. The LUMO, on the other hand, has a node in the double bonds. Excitation from the HOMO to the LUMO therefore weakens the double bonds and strengthens the single bonds considerably. The short bonds decrease in % character, and cis trans isomerization becomes possible. This is the case for all molecules with a 7t-bonding orbital as the HOMO. 

The optical spectrum of platinum—a-pyridone blue, as with the pyrimidine analogues, varies with pH, counteranions, temperature and time. Polarized single-crystal spectra in conjunction with a SCF-X Sw calculation have elucidated the major features of the electronic structure [67]. The blue color has been attributed to transitions from the inner Pt—Pt bonding o orbital to an antibonding one (a ) and from the outer Pt—Pt 7t bonding orbital to outer Pt—Pt a. The effect of alteration of the Pt—Pt distances in the platinum—ethylenediamine—pyridone blue was correlated with the optical spectrum. The delocalization of the unpaired spin density classes the pyridone blue as a Robin—Day Class III-A compound [68]. 

As orbitals spread into bands, orbitals oriented for a or a bonds spread into the widest bands. 7t orbitals form narrower bands and 5 bonding orbitals form the narrowest bands. 

Many of the reactions in which acetylene participates, as well as many properties of acetylene, can be understood in terms of the stmcture and bonding of acetylene. Acetylene is a linear molecule in which two of the atomic orbitals on the carbon are sp hybridized and two are involved in 7T bonds. The lengths and energies of the C—H O bonds and C=C<7 + 27t bonds are as follows ... 

The propylene double bond consists of a (7-bond formed by two ovedapping orbitals, and a 7t-bond formed above and below the plane by the side overlap of two p orbitals. The 7t-bond is responsible for many of the reactions that ate characteristic of alkenes. It serves as a source of electrons for electrophilic reactions such as addition reactions. Simple examples are the addition of hydrogen or a halogen, eg, chlorine ... 

Fig. 2. Overlapping of the metal, M, t5 -orbitals with the ( -bonding and 7T (antibonding) orbitals in a terrninally bonded M—CO interaction (T).

There are probably several factors which contribute to determining the endo exo ratio in any specific case. These include steric effects, dipole-dipole interactions, and London dispersion forces. MO interpretations emphasize secondary orbital interactions between the It orbitals on the dienophile substituent(s) and the developing 7t bond between C-2 and C-3 of the diene. There are quite a few exceptions to the Alder rule, and in most cases the preference for the endo isomer is relatively modest. For example, whereas cyclopentadiene reacts with methyl acrylate in decalin solution to give mainly the endo adduct (75%), the ratio is solvent-sensitive and ranges up to 90% endo in methanol. When a methyl substituent is added to the dienophile (methyl methacrylate), the exo product predominates. ... 

Figure 17.2 Schematic molecular orbital energy diagram for diatomic halogen molecules. (For F2 the order of the upper and 7T bonding MOs is inverted.).

Another measure of distortion is the shape of the highest-occupied molecular orbital (HOMO). This corresponds to the 7t bond. Is the orbital relatively undistorted in the cis compounds (as in cis-2-butene)l Is it more distorted in trans-cycloheptene than in trans-cyclooctene Explain why distortion in the HOMO is likely to be energetically unfavorable. 

What is the maximum energy structure Does it correspond to a structure that prevents 7t-type orbital overlap What is the barrier to rotation about die C2-C3 single bond ... 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

The interaction between orbitals point away from the carbon-carbon bond. This feature is not so acute for the orbitals, which involve a 2py or 2p orbital on carbon rather than a (2s, 2px) hybrid pointing away. 

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