VSEPR Background Recall from last lecture that we had two types of electron pairs: bonding pairs and lone pairs The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding (which atoms connected to which, number of bonds), but what about the 3-d shapes, geometry? Valence Shell Electron Pair Repulsion (VSEPR): the3-D structure is determined by minimizing repulsion of electron pairs.
Example: CH 4 (bonding pairs only, no lone pairs) Key: Must consider both bonding and lone pairs in minimizing electron repulsion. Lewis Structure VSEPR Structure
Example: NH 3 (both bonding and lone pairs). Lewis Structure VSEPR Structure Molecular Shape
VSEPR Applications The previous examples illustrate the stratgey for applying VSEPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arrange the bonding and lone electron pairs in space such that repulsions are minimized.
Some useful shorthand notation: * Refer to central atom as “A” * Attached atoms are labelled “X” * Lone pairs are labelled “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 E ClF 3 : AX 3 E 2
Case: Linear Structure (AX 2 ): angle between bonds is 180° Example: BeF 2 180°
Case: Trigonal Planar Structure (AX 3 ): The angle between bonds is 120° Example: BF 3 120°
Case: Pyramidal (AX 3 E): Bond angles are <120° structure is nonplanar due to repulsion of lone-pair. Example: NH 3 107° VSEPR Structure Molecular shape Lewis
Case: Tetrahedral (AX 4 ): the angle between bonds is ~109.5° Example: CH 4 109.5°
Note: for ‘Tetrahedral’, the actual angle may vary slightly from 109.5°, due to size differences between bonding and lone pair electron densities bonding pair: more elongated, less repulsive lone pair: puffier, more repulsive
Example of distorted tetrahedron: water (AX 2 E 2 ): the angle is reduced to 104.5° by repulsion of the lone pairs “bent” VSEPR structure Molecular shape
Compare: CH 4 (AX 4 ) NH 3 (AX 3 E) H 2 O (AX 2 E 2 ) Lone pairs: none one two
Question? What is the approximate bond angle in SO 2 ? A. 90° B. 180° C. 120° D. 109.5°
Case: Trigonal Bipyramidal (AX 5 ): non-equivalent bond positions: three in-plane (equatorial, 120°), and two at 90° to plane (axial) Example, PCl 5 90° 120°
Octahedral (AX 6 ): all angles are 90°. Example SF 6 90° Lewis VSEPR
Special cases: See-Saw and Square Planar AX 4 E: ‘See Saw’AX 4 E 2 : ‘Square Planar’ 180°<120° 90° Note: error
Square Planar is more stable, and non-polar, since the lone pairs can get farther apart Square Planar: not polar, due to symmetry Obs: no dipole moment! (Would be polar) Q? Which of these two possible arrangements is preferred?
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 ): Most stable Less stable(stronger lone pair repulsion)
What is the orientation of the ClF bonds in ClF 3 (28 e - )? ABC AX 3 E 2 Always put the lone pairs in the equatorial plane
Must look at VSEPR structures for all resonance species to predict molecular properties. Example: O 3 (AX 2 E) 0 +1 0 +1 Dipole moment? Yes! bent VSEPR and Resonance Structures
Give the Lewis dot and VSEPR structures for CF 2 Cl 2. Does it have a dipole moment? (Yes) 32 e - Tetrahedral (AX 4 )
What is the expected shape of ICl 2 + ? A. linear B. bent C. tetrahedral D. square planar AX 2 E 2
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