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VSEPR.

VSEPR.

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VSEPR.

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  1. VSEPR. The familiar VSEPR (Valence Shell Electron Pair Repulsion) approach to molecular structure was developed by Ronald Gillespie. The basic idea is that lone pairs of electrons occupy space around a central atom in much the same way as do atoms that are bonded to the central atom. The lone pairs and bonded atoms then assume that geometry that minimizes electrostatic repulsion between them. Ronald Gillespie.

  2. Electron domains and molecular geometry: observed geometry is that where the electron domains are as far apart as possible each lone pair of electrons plus each atom bonded to the central atom constitute an electron ‘domain’ lone pair of electrons N H H H Lewis dot diagram of ammonia Ammonia trigonal pyramidal (derived from tetrahedral geometry)

  3. 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. Br in BrF5) is always the central atom. 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 = 7 for Br. 2) Work out number of electron domains = valence electron pairs (‘n’) plus attached atoms on central atom. For BrF5 n = 6. 3) Relate n to the type of structure predicted for that value of n. n = 6 = octahedral. 4) Place lone pairs in expected positions, maximizing separation of lone pairs. For BrF5, there is one lone pair, so mol. structure = square pyramidal. place 5 F atoms around central Br red = 7 valence electrons for Br

  4. The structure of BrF5 from VSEPR: Lewis dot diagram parent structure molecular structure = square pyramidal lone pair n = 6, parent structure = octahedral, but one site occupied by a lone pair molecular or final structure – disregard the lone pair n = 6 from five attached atoms plus one electron pair

  5. Parent shapes for EXn molecules (n = 2-5) Formula n shapeshapes of structures EX2 2 linear EX3 3 trigonal planar EX4 4 tetrahedral EX5 5 trigonal bipyramidal

  6. Parent shapes for EXn molecules (n = 6-8) Formula n shapeshapes of structures EX6 6 octahedral EX7 7 pentagonal bipyramidal EX8 8 square antiprismatic

  7. Final structures for VSEPR theory.

  8. More final structures for VSEPR.

  9. A series of derivatives of the EX4 geometry (all with n = 4) but with increasing numbers of lone pairs: lone pairs Methane ammonia water hydrogen fluoride Tetrahedral trigonal pyramid bent linear diatomic

  10. Structures derived from trigonal geometry (n = 3): lone pair trigonal planar bent boron trifluoride nitrite anion, NO2- trigonal planar bent

  11. Ozone – a bent molecule: The structure of the O3 (ozone) molecule can be predicted using VSEPR. First draw up the Lewis dot diagram: 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: Note that two pairs of e’s still count as only one electron domain = one attached O-atom Central atom (red valence electrons) Structure of the ozone molecule (oxygens = red atoms) ozone

  12. Structures derived from TBP (n = 5):(Note: Lone pairs go in the plane:)

  13. Structures derived from the octahedron (n = 6):

  14. Structures derived from the pentagonal bipyramid (n = 7) (Note: lone pairs go axial)

  15. Negative charge adds a valence electron to iodine. Example: Note: The way the number of valence electrons (= 12) on the iodine is derived is from the seven valence electrons for iodine (group 7 in the periodic table), plus one each from the F-atoms, and one from the negative charge on the complex.

  16. Example: Chlorine trifluoride NOTE: in structures derived from a TBP parent structure, the lone pairs always lie in the plane, as seen here for the T-shaped structure of ClF3.

  17. The structure of [IF5(C6H5)]-:Note: an aliphatic or aromatic group is equivalent to an F. phenyl group fluorine iodine S.Hoyer, K.Seppelt (2004) J. Fluorine Chem. ,125, 989

  18. Diphenyl(acetato)iodine(V)oxide carbon atoms from phenyls oxide oxygen iodine phenyl group two pairs of electrons = double bond oxygen from acetato group

  19. The structure of bis(pentafluorophenyl)xenon. VSEPR explains this type of structure, which is linear like XeF2. (explain the latter in terms of VSEPR) xenon pentafluoro phenyl group H.Bock, D.Hinz-Hubner, U.Ruschewitz, D.Naumann (2002) Angew.Chem.,Int.Ed. , 41, 448

  20. The [I(C6H5)2]+ cation: iodine phenyl group

  21. Bis(trifluoroacetato)phenyl-iodine(III) iodine trifluoroacetate group phenyl group

  22. The effect of lone pairs on bond angles: In VSEPR the lone pairs appear to occupy more space than electron pairs in bonds, with the result that bond angles are compressed away from the lone pairs. For example, in structures derived from tetrahedral parent geometry, such as water or ammonia, the H-O-H and H-N-H angles are compressed to be less than the 109.5º expected for a regular tetrahedron: lone pairs water ammonia

  23. Effects of lone pairs on bond angles in ClF3 and ClF5. 87.5o 86.0o chlorine trifuoride chlorine pentafluoride

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