Slides16

Lecture #16

CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.

Lecture Outline

Molecular Geometries

VSEPR Model

Distortions from ideal geometries

effect of lone single electrons

electronegativity effects

Dipole Moments in Polyatomic Molecules

Rigidity about double bond

Geometrical isomers

The relative effectiveness of different electron pair repulsion combinations. The example illustrated shows how the HOH bond angle is distorted down to 104.5^o from its ideal tetrahedral angle of 109.5^o

An illustration of how the differences between electron pair repulsions leading to the preferred see-saw geometry of a molecule of type AB₄E.

Further distortions from the ideal VSEPR geometry of a molecule of type AB₄E.

Here is the rationale behind choosing the T-shaped molecular geometry for two lone pairs and three bonding pairs about a central atom. (This was not done in lecture). Two of the choices have six unfavorable repulsions at 90o. The other has only four such repulsions. See it?

Illustrating the imbalance when SF₄'s lone pairs distort bond angles and bond lengths from the ideal VSEPR geomtry.

In a molecule of type AB₄E₂, the lone pair repulsions make the preferred molecular geometry that corresponding to square planar. This gives minimum lone pair-lone pair repulsions

The molecular geometry of structure AB₅E and the distortions from the ideal square pyramidal molecular geometry caused by the lone pair of electrons.

Additional considerations that lead to distortions from ideal VSEPR molecular geometries. (These are not in the textbook.)

Effect of multiple bonds.

Illustration of double bond effect in POCl₃

The effect of a lone single electron.

Illustrating the lone electron effect by comparing ClO₂^- to ClO₂

The effect of electronegativity of the ligand (B) on distortions from ideal VSEPR geometries

ClF₃. What change is expected in going to BrF₃?

(Answer: The FBrF bond angle will be smaller than the FClF bond angle. The increased electronegativity of the F relative to the central atom pulls the electrons in the bonding pair further out from the center thus reducing the effectiveness of the bonding pair bonding pair repulsion.)

The effect of electronegativity of the ligand (B) on distortions from ideal VSEPR geometries

Five different ways of indicating the three-dimensional structure of a molecule, using methane as an example, are displayed here. The "dashed-wedged line" structure is the most common form used. A solid line for a bond lies in the plane of the page, the dashed line projects back behind the plane and the wedge projects out of the plane towards the observer.