Bonding VSEPR

In still other types of molecules, a central atom is surrounded by only two regions of electron density. Figure 1.10 shows Lewis structures and ball-and-stick models of carbon dioxide (CO2) and acetylene (C2H2). As with double bonds, VSEPR treats triple bonds as one region of electron density. 

In predicting bond angles about a central atom in a covalent bond, VSEPR considers only shared pairs (pairs of electrons involved in forming covalent bonds). (1.3)... 

Chemical bonding—VSEPR and molecular geometiy (interactive). 

Vinylic substituent (Section 6.1) Refers to a substituent on a carbon atom that participates in a carbon-carbon double bond. VSEPR model (valence shell electron pair replusion) (Section 1.16) A method of predicting the geometry at a covalently bonded atom by considering the optimum geometric separation between groups of bonding and non-bonding electrons around the atom... 

The sulfur-oxygen bond is an important feature of sulfur chemistry, just as the silicon-oxygen bond is of silicon chemistry. The best-known simple oxides are the dioxide, SO2, and the trioxide, SO3 (although highly reactive lower oxides SO and S2O have been described). The bonding in SO2 and SO3 is usually described in terms of S=0 double bonds, VSEPR theory predicting the shapes shown in Structures 12.14-12.17. 

The shapes of covalent compounds are determined by the tendency for bonding pairs to be as far apart as possible whilst lone pairs have a greater effect than bonding pairs (VSEPR theory). 

The tetrahedral geometry of methane is often explained with the valence shell electron pair repulsion (VSEPR) model The VSEPR model rests on the idea that an electron pair either a bonded pair or an unshared pair associated with a particular atom will be as far away from the atom s other electron pairs as possible Thus a tetrahedral geomehy permits the four bonds of methane to be maximally separated and is charac terized by H—C—H angles of 109 5° a value referred to as the tetrahedral angle... 

The H—O—H angle m water (105°) and the H—N—H angles m ammonia (107°) are slightly smaller than the tetrahedral angle These bond angle contractions are easily accommodated by VSEPR by reasoning that electron pairs m bonds take up less space than an unshared pair The electron pair m a covalent bond feels the attractive force of... 

Multiple bonds are treated as a single unit m the VSEPR model Formaldehyde is a trigonal planar molecule m which the electrons of the double bond and those of the two single bonds are maximally separated A linear arrangement of atoms m carbon diox ide allows the electrons m one double bond to be as far away as possible from the elec Irons m the other double bond... 

The opening paragraph of this chapter emphasized that the connection between structure and properties is what chemistry is all about We have just seen one such con nection From the Lewis structure of a molecule we can use electronegativity to tell us about the polarity of bonds and combine that with VSEPR to predict whether the mol ecule has a dipole moment In the next several sections we 11 see a connection between structure and chemical reactivity as we review acids and bases... 

VSEPR model, the dihalides of Be and Mg and the heavier halides of Ca and Sr are essentially linear. However, the other dihalides are appreciably bent, e.g. Cap2 145°, Srp2 -- 120°, Bap2 108° SrCl2 - 130°, BaCh - 115° BaBri -115° Bah 105°. The uncertainties on these bond angles are often quite large ( 10°) and the molecules are rather flexible, but there seems little doubt that the equilibrium geometry is substantially non-linear. This has been interpreted in terms of sd (rather than sp) hybridization or by a suitable id hoc modification of the VSEPR theory. ... 

The VSEPR model is readly extended to species in which double or triple bonds are present A simple principle applies Insofar as molecular geometry is concerned, a multiple bond behaves like a single bond. This makes sense. The four electrons in a double bond, or the six electrons in a triple bond, must be located between the two atoms, as are the two electrons in a single bond. This means that the electron pairs in a multiple bond must occupy the same region of space as those in a single bond. Hence the extra electron pairs in a multiple bond have no effect on geometry. 

The VSEPR model applies equally well to molecules in which there is no single central atom. Consider the acetylene molecule, C2H2. Recall that here the two carbon atoms are joined by a triple bond ... 

Four-coordinate metal complexes may have either of two different geometries (Figure 15.3). The four bonds from the central metal may be directed toward the comers of a regular tetrahedron. This is what we would expect from VSEPR model (recall Chapter 7). Two common tetrahedral complexes are Zn(NH3)42+ and C0CI42. ... 

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