Orbital overlap energy raising

This interaction raises the energy of the metal t2g orbitals, and decreases D0. On the other hand, p-acceptor ligands such as CO have empty antibonding p orbitals. Overlap with the metal in this case causes the t2g orbitals to be lowered in energy so that D0 is increased. 

The a-effect is observed not only as a kinetic effect but also as a thermodynamic a-effect, as seen in the equilibrium constants for the removal of acetyl groups from A-acctylimidazolc with hydroxylamine derivatives, which parallel their kinetics.280 For an explanation, we may compare an amide 4.31 with a hydroxamic acid 4.32. The overlap of a lone pair with the n orbital of a carbonyl group (4.31, arrows, see also pp. 104-106) is an important part of the reason why amides are stabilised relative to ketones. The effect of another lone pair is to raise the energy of the first lone pair (Fig. 2.12) and hence to make the overlap with the 7t orbital more energy-lowering. 

In the square planar M(Por), the metal orbital is highly destabilized by the strong tr-type interactions with the porphyrin N <7 orbitals. The d i orbital also has some orbital overlaps with the N a orbitals and is moderately destabilized. On the other hand, the energy levels of d and dy depend on the extent of d -p, interactions. For the first row transition metals, the porphyrin 7t (Cg) to dn (d , dy ) interactions exceed the d (n) to porphyrin n ) interactions and locate the d and rf levels slightly above the d (Figure 5a). Addition of the fifth ligand raises the d i energy level due... 

The concept of cis/trans (Z/ ) isomerism is a continuing problem for students. Errors of omission (the failure to find a possible isomer, for example) and of commission (failure to recognize that a certain double bond, so easily drawn on paper, really cannot exist) are all too common. Unlike C bonds, K bonds have substantial barriers to rotation ( 66 kcal/mol).The barrier arises because of the shape of the p orbitals making up the 7t bond. Rotation decreases overlap and raises energy. Accordingly, double bonds have sides, which are by no means easily interchanged. Substituents cannot switch sides, and are locked in position by the high barrier to rotation. It is vital to see why there are two isomers of 2-butene, cis- and /raret-2-butene, but only one isomer of 2-methyl-2-butene (Fig. 3.83). 

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