Molecular bonds/orbitals highest occupied

Fig. 2. Significant molecular orbitals of terminal transition metal-oxo units in a six-coordinate 4 ligand environment. The d° configuration is a formal triple bond. The highest occupied molecular orbital in the d configuration is formally nonbonding (8 symmetry) so the metal-oxo bond order remains 3.0. However, d-electron counts above d populate orbitals that are antibonding between the metal and the terminal multiply bonded ligand (0x0 in this case, but alternatively, alkyl-imido, nitrido, sulfido, etc.). Note that all the equatorial ligand orbitals and the metal dx2 y2 orbital (hi in 4 symmetry) are ignored for simplicity.

Fig. 3. Schematic picture of a chemisorbed CO molecule, (a) As a point dipole with its image, representing the screening by the conduction electrons, (b) The spatial extension of the two molecular orbitals involved in the chemisorption bond, the highest occupied 5a and the lowest unoccupied 2n orbital, (c) The density of states of the conduction electrons and the 2n orbital, which by interaction with the metal electrons has broadened into a resonance and shifted down in energy.

These absorptions are ascribed to n-n transitions, that is, transitions of an electron from the highest occupied n molecular orbital (HOMO) to the lowest unoccupied n molecular orbital (LUMO). One can decide which orbitals are the HOMO and LUMO by filling electrons into the molecular energy level diagram from the bottom up, two electrons to each molecular orbital. The number of electrons is the number of sp carbon atoms contributing to the n system of a neuhal polyalkene, two for each double bond. In ethylene, there is only one occupied MO and one unoccupied MO. The occupied orbital in ethylene is p below the energy level represented by ot, and the unoccupied orbital is p above it. The separation between the only possibilities for the HOMO and LUMO is 2.00p. 

The Diels-Alder reaction is believed to proceed m a single step A deeper level of understanding of the bonding changes m the transition state can be obtained by examining the nodal properties of the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile... 

Draw a Lewis structure for singlet methylene, CH2 (all the electrons in singlet methylene are spin-paired). Ho many electrons remain after all bonds have been formei Where are the extra electrons located, in the plane the molecule or perpendicular to the plane Examine t highest-occupied molecular orbital (HOMO) of methyle to tell. 

What happens to electrons which are left over after all bonds have been formed Do they associate with individual atoms or are they spread uniformly throughout the molecule Draw a Lewis structure for trimethylamine. How many electrons are needed to make bonds How many are left over Where are they Display the highest-occupied molecular orbital (HOMO) for trimethylamine. Where is it located ... 

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. 

Finally, display the highest-occupied molecular orbital (HOMO) of triphenylphosphinemethylidene. Is it primarily concentrated on the methylene carbon as would be expected of a fully-developed anion, or is it delocalized over both phosphorous and carbon Does this suggest that the molecule incorporates a n bond ... 

Examine electrostatic potential maps for dimethylsulfonium and dimethylsulfoxonium ylides. Which contains the more negatively-charged carbon Do either or both of the ylides incorporate a fully formed 7U bond Compare bond distances involving methylene and methyl carbons. Also examine the highest-occupied molecular orbital (HOMO) for evidence of 7U bonding. 

Next, examine the equilibrium structure of acetamide (see also Chapter 16, Problem 8). Are the two NH protons in different chemical environments If so, would you expect interconversion to be easy or difficult Calculate the barrier to interconversion (via acetamide rotation transition state). Rationalize your result. Hint Examine the highest-occupied molecular orbital (HOMO) for both acetamide and its rotation transition state. Does the molecule incorporate a n bond. If so, is it disrupted upon rotation ... 

Woodward and Hoffmann speeulated that the preferred motion was that whieh involved constructive (bonding) overlap between the terminal lobes of the highest-occupied molecular orbital (HOMO). 

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