Lecture 7 Molecular Bonding Theories 1) Valence Shell Electron Pair Repulsion (VSEPR) theory

1. Lecture 7 Molecular Bonding Theories 1) Valence Shell Electron Pair Repulsion (VSEPR) theory Simple theory for qualitative prediction of geometry of polyatomic species • Draw a reasonable Lewis structure. Count lone pairs and atoms directly attached to the central atom (N). • Point lone pairs and bonds at the central atom (A) to the vertices of a polygon or polyhedron. Start from the most symmetrical shapes shown below for 2 < N < 8 (B = an atom or a lone pair): • Consider distortions. Distortions of an initially more symmetrical shape are due to the fact that: • Lone pairs need more space than bonds. • Multiple bonds need more space than single bonds. • Bonds formed with more electronegative elements occupy less space. 4) Smaller “objects” will tend to occupy axial positions in trigonal bipyramids and equatorial positions in pentagonal bipyramids. 5) For 4th and higher row elements one lone pair tends to be stereochemically inactive.

2. 2) Most common molecular shapes

3. 3) Concept of hybridization • Describes geometry of polyatomic species ABx, but predicts degeneracy that does not exist. • Assumes that before an atom A forms x s-bonds, x non-equivalentatomic orbitals yat combine to build a set of the same number x of equivalenthybrid orbitals, yhyb. Similarly hybrid orbitals for p-bonding can be formed. Orbitals suitable for the combination can be found by applying the group theory. • Each hybrid orbital yhyb, j is a linear combination of atomic orbitals, yat, i. • The probability to find an electron on an j-th hybrid orbital yhyb, j since • The probability to find an electron on an i-th atomic orbital yat, i

4. 4) sp-Hybrid orbitals • Linear molecules AB2 (BeH2). sp-hybridization. • Calculating the coefficients cij: cs12 + cs22 = 1 cP12 + cP22 = 1 cS1 = cS2 = (1/2)1/2 cP1 = cP2 = (1/2)1/2

5. 5) Concept of hybridization. sp2-Hybrid orbitals • Trigonal planar molecules AB3 (BF3). sp2- or d2s-hybridization. • Calculating the coefficients cij (sp2; for d2s use dxy instead of px and dx2-y2 instead of py): yhyb, 1 = ys + ypx + ypy yhyb, 2 = ys + ypx - ypy yhyb, 3 = ys - ypx - ypy

6. 6) dsp2-Hybrid orbitals • Square planar molecules AB4 ([PtCl4]2-). dx2-y2sp2- or dx2-y2dz2p2-hybridization. • Calculating the coefficients cij: yhyb, 1 = ys + ypx + ypy + ydx2-y2 yhyb, 2 = ys + ypx + ypy - ydx2-y2 yhyb, 3 = ys + ypx - ypy + ydx2-y2 yhyb, 4 = ys - ypx + ypy - ydx2-y2