JEE Chemistry Chemical Bonding and Molecular Structure Notes Set 03

VSEPR Theory

The shape of a molecule is set by the repulsion between electron pairs in the valence shell of the central atom. Count the steric number (bond pairs plus lone pairs). The order of repulsion is lone pair-lone pair, then lone pair-bond pair, then bond pair-bond pair, so lone pairs squeeze bond angles. Lone pairs occupy equatorial positions in a trigonal bipyramid because they have more space there.

Hybridisation

1. Hybridisation mixes atomic orbitals of similar energy on the same atom to form equivalent hybrid orbitals; the number of hybrid orbitals equals the number of orbitals mixed.
2. Steric number method: sp for 2, sp2 for 3, sp3 for 4, sp3d for 5, sp3d2 for 6 and sp3d3 for 7.
3. Percentage s-character: sp 50, sp2 33, sp3 25. More s-character means a smaller orbital, shorter and stronger bonds, and higher electronegativity of the atom. The C-H bond is strongest in ethyne, then ethene, then ethane.
4. Square planar complexes (Ni2+ with strong field ligands, Pt2+) use dsp2 and use the dx2-y2 orbital, tetrahedral ones use sp3, and octahedral ones use d2sp3 or sp3d2.
5. Carbocations (CH3+) are sp2 and planar, carbanions (CH3-) are sp3 and pyramidal. In allene CH2=C=CH2 the central carbon is sp and the end carbons are sp2.

Comparing Bond Angles

1. Different hybridisation: more s-character gives a larger angle (sp 180, sp2 120, sp3 109.5).
2. Same hybridisation and same lone pair count: a more electronegative central atom pulls the bond pairs closer, so the angle is larger. Order: NH3 (107) above PH3 (93) above AsH3 (92) above SbH3 (91), and H2O (104.5) above H2S (92) above H2Se (91) above H2Te (90).
3. Same central atom: larger outer atoms give a larger angle because they repel more. Cl2O (111) is above H2O (104.5) above F2O (103).
4. Lone pairs reduce the angle: CH4 (109.5), NH3 (107) and H2O (104.5).

Back Bonding and Special Bonds

1. p pi-p pi back bonding: a lone pair on one atom is donated into an empty p orbital on a neighbour, as in BF3. This strengthens the B-F bond (646 kJ per mole against 515 for C-F). It works best between small atoms of period 2.
2. p pi-d pi back bonding: N(SiH3)3 is planar because the lone pair on nitrogen is delocalised into the empty d orbitals on silicon, whereas N(CH3)3 is pyramidal.
3. Banana (three-centre two-electron) bonds: diborane B2H6 has two bridging hydrogens and no B-B bond, and Al2H6 is similar.
4. Hyperconjugation is the delocalisation of a sigma C-H bond into an adjacent p or pi orbital (a sigma-p interaction).

Worked Examples

1. Shape of XeF4: Xe has 8 valence electrons, four are used for F, leaving two lone pairs. Steric number 6 gives an octahedral arrangement and the lone pairs sit opposite each other, so the shape is square planar.
2. Number of 90 degree angles in BrF5: a square pyramid has 4 apical-basal and 4 basal-basal angles near 90 degrees, so 8.
3. Hybridisation of N in NO2+, NO3- and NH4+: sp, sp2 and sp3 respectively.