Lecture 25: VSEPR Reading: Zumdahl Outline –Concept behind VSEPR –Molecular geometries

VSEPR Background Recall from last lecture that we had two types of electron pairs: bonding and lone. The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry? Valence Shell Electron Pair Repulsion (VSEPR). 3D structure is determined by minimizing repulsion of electron pairs.

VSEPR Background (cont.) Example: CH 4 Must consider both bonding and lone pairs in minimizing repulsion. Lewis Structure VSEPR Structure

VSEPR Background (cont.) Example: NH 3 (both bonding and lone pairs). Lewis Structure VSEPR Structure

VSEPR Applications The previous examples illustrate the stratgey for applying VSEPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arranging bonding/lone electron pairs in space such that repulsions are minimized.

VSEPR Applications (cont.) Shorthand notation: 1. Refer to central atom as “A” 2. Attached atoms are referred to as “X” 3. Lone pairs are referred to as “E” Examples: H 2 O: AX 2 E 2 CH 4 : AX 4 BF 3 : AX 3 PCl 5 :AX 5 NH 3 : AX 3 EClF 3 : AX 3 E 2

VSEPR Applications Linear Structures (AX 2 ): angle between bonds is 180° Example: BeF 2 180°

VSEPR Applications Trigonal Planar Structures (AX 3 ): angle between bonds is 120° Example: BF 3 120°

VSEPR Background (cont.) Pyramidal (AX 3 E): Bond angles are <120°, and structure is nonplanar due to repulsion lone-pair. Example: NH 3 107°

VSEPR Applications Tetrahedral (AX 4 ): angle between bonds is ~109.5° Example: CH °

VSEPR Applications Tetrahedral: angle may vary from 109.5° exactly due to size differences between bonding and lone pair electron densities bonding pair lone pair

VSEPR Applications Classic example of tetrahedral angle shift from 109.5° is water (AX 2 E 2 ): “bent”

VSEPR Applications Comparison of CH 4 (AX 4 ), NH 3 (AX 3 E), and H 2 O (AX 2 E 2 ):

What is the approximate bond angle in SO 2 ? A. 90° B. 180° C. 120° D °

VSEPR Applications Trigonal Bipyramidal (AX 5 ), 120° in plane, and two orbitals at 90° to plane. Example, PCl 5 : 90° 120°

VSEPR Applications Octahedral (AX 6 ): all angles are 90°. Example, SF 6 : 90°

Advanced VSEPR Applications See-Saw versus Square Planar AX 4 E: See SawAX 4 E 2 : Square Planar 180°<120° 90°

Advanced VSEPR Applications Square Planar versus “See Saw” See Saw Square Planar No dipole moment

Advanced VSEPR Applications Driving force for last structure was to maximize the angular separation of the lone pairs. Same effect occurs in I 3 - (AX 2 E 3 ):

What is the orientation of the ClF bonds in ClF 3 (28 e - )? ABC AX 3 E 2

Advanced VSEPR Applications VSEPR and resonance structures. Must look at VSEPR structures for all resonance species to predict molecular properties. Example: O 3 (AX 2 E): Dipole moment? Yes! bent

VSEPR Applications Provide the Lewis dot and VSEPR structures for CF 2 Cl 2. Does it have a dipole moment? 32 e - Tetrahedral

What is the expected shape of ICl 2 + ? A. linear B. bent C. tetrahedral D. square planar AX 2 E 2