Chemistry 545 Inorganic Chemistry Lecture 1. VSEPR

VSEPR. The octet rule represents Valence Bond (VB) theory at its simplest level, and applies largely to compounds containing light central atoms such as B, C, N, O, and F. Heavier atoms such as Si, Ge, P, As, S, Se, Cl, Br and I are able to expand their valence shells, which leads to the familiar VSEPR (Valence Shell Electron Pair Repulsion) approach to molecular structure, which was developed by Ronald Gillespie. The basic idea is that electron pairs, whether present as lone pairs, or present in bonds between atoms, occupy space around a central atom in such a way as to minimize electrostatic repulsion between them. Ronald Gillespie.

Electron domains and molecular geometry: each pair of electrons – both the lone pair, and the pairs of electrons in each bond – constitute an electron ‘domain’ observed geometry is that where the electron domains are as far apart as possible lone pair of electrons N H H H Lewis dot diagram of ammonia Ammonia (derived from Tetrahedral geometry)

Using VSEPR In order to use VSEPR to predict molecular structure: Draw up Lewis dot diagram for the molecule or ion. The first atom (e.g. N in NH3) is usually the central atom (H2O, HF are exceptions). Place the other atoms around the central atom. If these are single bonds, contribute one electron per attached atom. Then add the valence electrons for the central atom = 5 for N. 2) Work out number of electron domains = valence electron pairs (‘n’) plus attached atoms on central atom. For NH3 n = 4. 3) Relate n to the type of structure predicted for that value of n. n = 4 = tetrahedral. 4) Place lone pairs in expected positions, maximizing separation of lone pairs. For NH3, there is one lone pair, so mol. structure = trigonal pyramidal. place 3 H atoms around central N red = 5 valence electrons for N

The structure of H2O from VSEPR: Lewis dot diagram parent structure molecular structure = bent lone pairs n = 4 from two attached atoms plus two electron pairs n = 4, parent structure = tetrahedral, but one site occupied by a lone pair molecular or final structure – disregard the two lone pairs

Parent shapes for EXn molecules (n = 2-5) Formula n shape shapes of structures EX2 2 linear EX3 3 trigonal planar EX4 4 tetrahedral

Parent shapes for EXn molecules (n = 6-8) Formula n shape shapes of structures EX5 5 trigonal bipyramidal EX6 6 octahedral

Final structures for VSEPR theory.

More final structures for VSEPR.

Examples:

Ozone – a bent molecule: The structure of the O3 (ozone) molecule can be predicted using VSEPR. First draw up the Lewis dot diagram: Note that two pairs of e’s still count as only one electron domain = one attached O-atom For the valence shell of the central oxygen atom n = 3, so parent geometry = trigonal. The final structure is thus two-coordinate bent, as seen for the ozone molecule below: Central atom (red valence electrons) Structure of the ozone molecule (oxygens = red atoms) ozone

Some more bent molecules/anions: sulfur dioxide the nitrite anion

Some linear molecules:

Trigonal planar molecules: