Cr-bond framework

The bonding in these Ru30 carboxylates can be explained by the usual MO scheme for these systems. A cr-bonding framework involves using six orbitals from each ruthenium (one s, three p, two d) to form bonds to the central O, four carboxylate oxygens and the terminal ligand (PPh3H20, etc.). 

Next, we half-fill the lone unhybridized 3p orbital on sulfur and the lone 2p orbital on the oxygen atom with a formal charge of zero (atom B). Following this, the 2p orbital of the other two oxygen atoms (atoms C and D), are filled and then lone pairs are placed in the sp2 hybrid orbitals that are still empty. At this stage, then, all 24 valence electrons have been put into atomic and hybrid orbitals on the four atoms. Now we overlap the six half-filled sp2 hybrid orbitals to generate the cr-bond framework and combine the three 2p orbitals (2 filled, one half-filled) and the 3p orbital (half-filled) to form the four 7t-molecular orbitals, as shown below ... 

A unique characteristic of polymers composed of extended chains of Group 14 elements is the delocalization of electrons through the cr-bond framework of the polymer backbone.7 These polymers are known to absorb in the ultraviolet, with absorption maxima dependent both on main chain substituents and on chain length. Several potential applications exist, such as photoconductors, photoresists in microelectronics, photoinitiators for radical reactions, and precursors to ceramic materials. 

It is seen that the cr-bond framework of the molecule is polarized to a great extent in accordance with the electronegativities of the ring atoms and the net charge distribution is dominated by this polarization. Of the three carbon atoms, C(2) is predicted to have the lowest net charge and C(4) the highest these results are in broad agreement with H and 13C NMR spectroscopy (see Section 4.18.2.3.3) furthermore, C(2) is the most reactive towards nucleophiles and the least reactive towards electrophiles. Calculated free-valence indices predict that position 5 should be the most susceptible to attack by free radicals. 

Butadiene shows features common to all conjugated molecules, that is those in which the double bonds of classical chemistry cannot be uniquely allocated. According to the aufbau approach, its electronic structure, apart from that of the cr-bonded framework, would be The extra stabilisa-... 

FIGURE 7.14 Bond formation in acetylene, (a) The cr bond framework and the two nonhybridized 2p orbitals on each carbon, (b) Overlap of two sets of parallel 2p orbitals forms two 77 bonds. 

Hybrid potentials of the type discussed in this chapter have been in use for twenty five years or so. Some of the earliest examples were those designed to study conjugated organic molecules in which the -electrons of the system were treated with a semiempirical QM method and the cr-bonding framework was described with a MM force field [8, 9], The methods were used to study the structure and spectra of the molecules [10, 11] and photoisomerization processes [12, 13], The first true combined potential in which both a and w electrons were considered in the quantum mechanical region was also developed by Warshel, in collaboration with Levitt, which they used to study the mechanism of the enzyme, lysozyme [14], They combined a semiempirical QM method to describe a portion of the enzyme and the substrate, and a standard MM force field to describe the rest of the atoms. 

The electron charge cloud associated with the two electrons making up the cr bond (in red) is concentrated between the two nuclei. The electron charge cloud associated with the two electrons of the TT bond (in blue) is concentrated in two regions above and below the cr bond framework of the molecule. 

The cr-bonding framework in a square-pyramidal species may also be described in terms of an sp d hybridization scheme. The change in spatial disposition of the five hybrid orbitals from trigonal bipyramidal to square-based pyramidal is a consequence of the participation of a different d orbital. Hybridization of s,Px,Py,Pz and dyi yi atomic orbitals generates a set of five sj d hybrid orbitals (Figure 4.7b). 

Fig. 4.8 (a) Ethene is a planar molecule with H—C—H and C—C—H bond angles close to 120°. (b) An sp hybridization scheme is appropriate to describe the cr-bonding framework, (c) This leaves a Ip atomic orbital on each C atom overlap between them gives a C—C rr-interaction. 

Table 20.1 Hybridization schemes for the cr-bonding frameworks of different geometrical configurations of ligand donor atoms.

If all five carbon atoms of the cyclopentadienyl ring interact with the metal centre, the bonding is most readily described in terms of an MO scheme. Once the cr-bonding framework of the [Cp] ligand has been formed, there is one 2p atomic orbital per C atom remaining, and five combinations are possible. The MO diagram below shows the formation of (ri -Cp)BeH (C5 ), a model compound that allows us to see how the [r -Cp] ligand interacts with an or p-block... 

The cr bond results from two sp orbitals overlapping end to end and is symmetrical about an axis linking the two carbon atoms. The tt bond results from a sideways overlap of two p orbitals it has a nodal plane like a p orbital. In the ground state the electrons of the tt bond are located between the two carbon atoms but generally above and below the plane of the cr-bond framework. 

A carbon-oxygen double bond, (a) Lewis structure of formaldehyde, CH2O, (b) the sigma (cr) bond framework and nonoverlapping parallel 2p atomic orbitals, (c) overlap of parallel 2p atomic orbitals to form a pi (77) bond, and (d) formaldehyde with its full complement of bonds and orbitals. 

Ethene, C2H4, is a planar molecule (Fig. 5.8a) with C—C—H and H—C—H bond angles of 121.3° and 117.4° respectively. Thus, each C centre is approximately trigonal planar and the cr-bonding framework within C2H4 can be described in terms of an sp hybridization scheme (Fig. 5.8b). The three... 

WThe cr-bond framework in ethylene,formed by the overlap of sp hybrid orbitals on C atoms and Is orbitals on H atoms. ... 

Intervalence transfer bands have been noted in a series of saturated spiro sulfur-bound ruthenium (III) (II) mixed-valence dimers [(NH3)sRuS C2n+2H4 S Ru(NH3)5] , wherc n = 2,3, or 4 indicates the number of spiro rings. Calculated electron transfer rates decrease with intermetal distances giving values of 8.0 x lo s at 11.3 A for n = 2, 4.9 x 10 s at 14.4 A for n = 3, and 3.5 x lO s at 17.6 A for = 4 indicating very effective tunneling even at 17.6 A with only a cr-bonded framework. 

The three bonds are distributed among the six C atoms, which amounts to 3/6, or a half-bond, between each pair of C atoms. Add to this the cr bonds in the cr-bond framework, and we have a bond order of 1.5 for each carbon-to-carbon bond. This is exactly what we also get by averaging the two Kekule structures of Figure 11-29. 

Of the 18 valence electrons in O3, assign 14 to the sp hybrid orbitals of the cr-bond framework. Four of these are bonding electrons (red) and ten are lone-pair electrons (blue). 

First, we focus on electrons associated with the cr bond framework and determine hybridization schemes for the atoms. Then, we determine the number of electrons in the tt system, and the number of 2p orbitals involved forming the tt molecular orbitals. The key to solving this problem is reasoning out the number of each type of molecular orbital (bonding, antibonding, nonbonding) in the tt system. The final step is to assign electrons in the tt system to the appropriate molecular orbitals. 

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