Energetics of Hybridisation :
According to hybridisation model, bond directions are determined. By a set of hybrid orbitals on the central atom which are used to form bonds to the ligand atoms and to hold unshared pairs. Thus AB2 molecules are linear due to the use of linear sp hybrid orbitals. AB2 molecule should be equilaterally triangular, while AB₂E molecule should be angular, due to use of trigonal sp² hybrids. AB4, AB3E and AB₂E₂ molecule should be tetrahedral, pyramidal and angular, respectively, because here sp³ hybrid orbitals are used.
These cases are, of course, very familiar and involve no more than an octet of electrons.
For the AB5, AB4E, AB3E2 and AB2E3 molecules the hybrid must now include orbitals in their formation. The hybrid orbitals used must be of the sp3dz2 leading to TBP geometry and Sp3dx2-y2 leading to SP geometry. There is no way to predict with certainty which set is preferred, and know experimentally that AB5 molecules nearly all have TBP structures, the same arrangement is assumed for the AB4E cases, and so on. Even this adhoc assumption does not solve all difficulties, since the position preferred by lone pairs must be decided and there is no simple physical model here (as there was in the VSEPR approach) to guide us. A preference by lone pairs for equatorial positions has to be assumed. With these assumptions, a consistent correlation of all structures in this five electron-pair class is possible. For AB6, molecule, octahedral sp3d2 hybrids are used. For AB4E2 molecule; there is nothing in the directed valence theory itself to show whether the lone pairs should be cis or trans. The assumption that they must be trans leads to consistent results.
The most fundamental problem with the hybridisation model is that in all cases in which there are more than four electron pairs in the valence shell of the central atom, it is necessary to postulate that at least one d orbital becomes fully involved in the bonding. There are both experimental and theoretical reasons for believing that this is too drastic an assumpiton. Some recent MO calculations and other theoretical. Considerations suggest that although the valence shell d orbitals make a significant contribution to the bonding in many cases, they never play as full a part as do the valence shell p orbitals. Fairly directed experimental evidence in the form of nuclear quadruple resonance studies of the IC2-1 and ICI4-2 ions shows that in these species, d-orbitals participation is very small. This participations is probably greater in species with more electronegative ligand atoms such as PF5, SF6, and Te (OH)6, but not of equal importance with the contribution of the s and p orbitals.
Perhaps it is surprising that by going to the opposite extreme, namely by omitting all consideration of d orbitals, but still adhering to the concept of directed orbitals it is again possible to rationalize many of the principal features of the structures of main group.
Σ Es+p3 [2(-1806)]+[3(-981)] = 6555 KJ mol-1
For tetrahedral hybridised phosphours.
(3 te²2te¹3te’) the energy will be :
Σ te=5x (-1187) = -5935 KJ mol-1
In this case the hybridisation has cast 620 KJ mol of energy or roughly two bonds worth of energy. This is shown graphically in the figure 1.10.
The energy difference between the hybridised and unhybridised atom represents the increase in energy of the two electrons in the filled 3s orbital and the decrease in energy of the electrons in the half-filled 3p orbitals. The energetics of o, together with the principle of good overlap, are important in determining the electronic structure of molecules.
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