Lecture 7 Molecular Bonding Theories 1) Valence Shell Electron Pair Repulsion (VSEPR) theory Simple theory for qualitative prediction of geometry of polyatomic species 1)Draw a reasonable Lewis structure. Count lone pairs and atoms directly attached to the central atom (N). 2)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): 3)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 4 th and higher row elements one lone pair tends to be stereochemically inactive.

2) Most common molecular shapes Formula Lone pairs Example Geometry AB 2 0BeH 2 Linear 1CH 2 Bent 2H2OH2O 3XeF 2 Linear AB 3 0BF 3 Trigonal planar 1NH 3 Trigonal pyramid 2IF 3 T-shaped AB 4 0BH 4 - Tetrahedral 1SF 4 Butterfly 2XeF 4 Square planar Formula Lone pairs Example Geometry AB 5 0PF 5 Trigonal bipyramid 1IF 5 Tetragonal pyramid AB 6 0WF 6 Octahedral 1XeF 6 Distorted octahedral AB 7 0IF 7 Pentagonal bipyramid AB 8 0ZrF 8 4- Tetragonal antiprism 1 Mo(CN) 8 4- Trigonal dodecahedron AB 9 0ReH 9 2- Tricapped trigonal prism

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

4) sp-Hybrid orbitals Linear molecules AB 2 (BeH 2 ). sp-hybridization. Calculating the coefficients c ij : c s1 2 + c s2 2 = 1 c P1 2 + c P2 2 = 1 c S1 = c S2 = (1/2) 1/2 c P1 = c P2 = (1/2) 1/2

5) Concept of hybridization. sp 2 -Hybrid orbitals Trigonal planar molecules AB 3 (BF 3 ). sp 2 - or d 2 s-hybridization. Calculating the coefficients c ij (sp 2 ; for d 2 s use d xy instead of p x and d x2-y2 instead of p y ):  hyb, 1 =  s +  px +  py  hyb, 2 =  s +  px -  py  hyb, 3 =  s -  px -  py

6) dsp 2 -Hybrid orbitals Square planar molecules AB 4 ([PtCl 4 ] 2- ). d x2-y2 sp 2 - or d x2-y2 d z2 p 2 -hybridization. Calculating the coefficients c ij :  hyb, 1 =  s +  px +  py +  dx2-y2  hyb, 2 =  s +  px +  py -  dx2-y2  hyb, 3 =  s +  px -  py +  dx2-y2  hyb, 4 =  s -  px +  py -  dx2-y2