7r symmetry

The appropriate valence atomic orbitals which must be considered are 2s, 2pj., 2p, on carbon, and the Is orbitals lsH and Ish. The orbital 2py is clearly nonbonding (n) relative to the carbon-hydrogen interactions and need not be considered further. The hydrogen orbitals can be combined into a (lsH + Ish) combination of a symmetry and a (lsH — ls ) combination of 7r symmetry. (Fig. 5). Although there are three available basic cr orbitals, only two of these belong to the C1H2 group proper. We can first eliminate the ( out )... 

The HOMO is a doubly occupied sp hybrid of a-symmetry while a degenerate pair of (orthogonal) orbitals of 7r-symmetry are the LUMO s of the carbyne fragment. 

The change to a silicon-based substituent group, e.g., SiMe3, has the opposite effect. The introduction of two more orbitals of 7r-symmetry appropriate for bonding stabilizes the metal-carbon interaction and increases the percentage electron density of the 7r-orbital on the carbyne ligand (28). 

Two more states of the 7r-symmetry ( ) and u)) on the metal ion remain unchanged as in the free metal ion and both are empty. The frontier orbitals here are the a ) (HOMO) and those in the 7r-manifold ( ) and u) - LUMO). 

Fortunately, the correct explanation of which of these effects is dominant is subject to experimental test. If a donor substituent possessing no low-lying vacant orbital of 7r symmetry, such as R2N or RO is placed on an electron-deficient alkene, then the reversal of polarization cited in the last section cannot occur. However, the secondary orbital interactions described here will still be present. Thus, cycloadditions... 

These low spin complexes produce strikingly different spectra from other low spin Co(ii) systems. In the present complexes there is a dominant contact contribution to the shifts which has been interpreted with the aid of CNINDO calculations as due to a-spin being delocalized into the ligand HOMO. In view of the 7r-symmetry of this orbital, in contrast to the c-symmetry of the d 2 orbital in which the unpaired electron is thought to reside in the ground state, the authors conclude that there must be some non-planarity of the complex in solution which will remove the orthogonality between the metal and ligand n orbitals. 

M. J. S. Dewar proposed a model (Figure 1) to describe the bonding of an olefin to silver(i) or copper(i). The model suggested that, in addition to cr-donation of olefin 7r-bonding electrons to the metal, d electrons on the metal would also interact with antibonding orbitals of 7r-symmetry on the olefin. No experimental evidence was provided to support this proposal nor, indeed, was explicit mention made of Zeise s salt or other platinum-olefin complexes. From a study of Chemical Abstracts 41 (1947)-75 (1971), it would appear that Dewar did not follow up his proposal with more detailed studies, possibly indicating that this was not a prime focus of his own interests in MO theory and its application to... 

An accurate calculation transition metal chelates would have to take into account all orbitals up to the valence-electron shell having the proper symmetry to combine either to a or to 7t bonds. But in approximate methods only the valence electrons are considered 127>. Usually the bonding in transition metal chelates like 23, 41, 42 or 43 is decribed by a back-donation model which considers only the interaction between the highest filled d orbitals of the metal and the empty ligand antibonding MO s 69,121,123) which are all of 7r symmetry. If one extends his model and considers the interaction with all bonding and antibonding tr MO s and the d orbitals of it symmetry, one obtains the jr-electron model which will be used here. 

A epoxide with a low-valent transition metal possessing an unpaired electron in a d-orbital of 7r-symmetry constitutes the decisive intermediate in their reasoning. This species can be regarded as an electronic analogue of the cyclopropylcarbinyl radical. Thus, epoxide opening by breaking of one of the C-O bonds is expected to be a fast reaction. 

The linear M—C—0 bond has conventionally been discussed in terms of two interactions (Fig. 5.7). The first involves cr-donation of electrons from carbon ( 5(j) into suitable vacant metal orbitals which are directed along the bond axis. Secondly electrons are transferred from filled metal orbitals of 7r-symmetry into the empty antibonding m.o.s (2n) of the ligand. In carbonyls the 7r-component is... 

Both MOs described in Fig. 1.10 have cylindrical symmetry about the internuclear axis. MOs of this type are called sigma (a) MOs. Since the inter-nuclear axis is usually defined as the z axis atomic orbital has a a-symmetry about this axis and thus it can be combined with other orbitals of the same symmetry (per orbitals), p and Py have however other symmetry. Rotation about the internuclear axis is not symmetric and there is a nodal plane containing this axis. These kind of orbitals are said to have 7r-symmetry. The linear combination of n atomic orbitals leads to bonding (ti ) and antibonding (tc ) MOs. Analogously atomic orbitals with symmetry 5, i.e. two nodal planes containing the internuclear axis, may be combined to give MOs with the same symmetry. That will be discussed separately in Sect. 1.3. The formation of dinuclear molecular orbitals with two different classes of symmetry is illustrated in Fig. 1.14. 

The 18 electron-rule normally does not apply to transition metal complexes with 7i-donor ligands and metal atoms in normal oxidation states. In this case the metal orbitals with 7r-symmetry lie at relatively high energy and they stay empty or are only partially filled. In this way the formation of paramagnetic species can be possible. 

The spectra show sets of peaks that can be assigned to ionization from each MO. Several closely spaced peaks are seen in most cases because of vibrational effects as the photoelectron leaves the molecule. For example, removing an electron from a bonding state will tend to weaken the molecular bond and so the nuclei will move apart, initiating vibration. These effects are different for a- and 7r-symmetry bonds and so actually help in the assignment of spectral bands. 

Since the 6 orbital carries no orbital angular momentum, the orderii of the levels according to 12 is the same as it would be vor a /7 (D i,) diatomic state (25 a) with the same number of 7r-symmetry electrons. However, the bi symmetry of d in exchanges the D41, symmetry labels A, B of the 12 = 0 and 12 = 2 spin-orbit levels relative to those expected by correlation to the diatomic triplet state [25 a] d > 17> Z, 2 (D )... 

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