Bond angles, VSEPR theory

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 Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

It is noteworthy that the to-bonded structure for ArF6 differs from that predicted by VSEPR theory. ArF6 is predicted to be of octahedral (Oh) symmetry, with three mutually perpendicular F i- Ar -h F triads and an s-type lone pair. In contrast, VSEPR predicts a pentagonal bipyramid (or other seven-vertex polyhedron) with some or all F-Ar-F angles less than 90°. The calculated equilibrium structure is in agreement with the co-bonding model. 

Two major theories of the covalent bond are described in this book the main features of valence bond theory are treated in terms of the VSEPR theory of molecular shapes, and MO theory which is based on the symmetry properties of the contributing atomic orbitals. The latter theory is applied qualitatively with MO diagrams being constructed and used to interpret bond orders and bond angles. The problems associated with bond angles are best treated by using the highest symmetry possible for a molecule of a particular stoichiometry. 

Throughout the book, theoretical concepts and experimental evidence are integrated An introductory chapter summarizes the principles on which the Periodic Table is established and describes the periodicity of various atomic properties which are relevant to chemical bonding. Symmetry and group theory are introduced to serve as the basis of all molecular orbital treatments of molecules. This basis is then applied to a variety of covalent molecules with discussions of bond lengths and angles and hence molecular shapes. Extensive comparisons of valence bond theory and VSEPR theory with molecular orbital theory are included Metallic bonding is related to electrical conduction and semi-conduction. 

Fig. 17.3 Molecular shapes predicted by simple VSEPR theory Bond angle values represent experimental results where known...

The reader will probably be familiar with at least one simple, qualitative explanation for the shape of the water molecule (there are several). The VSEPR approach (see Section 1.4) predicts that the bond angle should be somewhere between 90° and 109.5°, which is good enough for the purposes of most inorganic chemists. The fact that some of the underlying assumptions in this and other simple theories can be challenged does not necessarily vitiate the theory. 

VSEPR theory can successfully account for many of the fine details in a structure, especially bond angles. However, we will be mainly concerned with the gross geometries of molecules and polyatomic ions. Structural minutiae are of considerable interest to most inorganic chemists, but they are important in the study of descriptive inorganic chemistry only to the extent that they may illuminate details of bonding which are relevant to the very existence of a substance, and to its reactions. 

Valence-shell electron pair repulsion (VSEPR) theory gives reasonably accurate predictions of bond angles in a molecule. VSEPR theory uses a simple electrostatic model in which groups of electrons around a central atom repel one another and occupy positions as far apart as possible. The number of electron groups, called the steric... 

The simplest carbene CH2 ( methylene, Figure 3.14, center) is a bent molecule with an H,C,H bond angle of 135° and has a triplet ground state. The singlet CH2 is less stable by 8 kcal/mol. Its free electron pair occupies the sp AO (because in this orbital it is nearer to the nucleus and therefore more stabilized than in the 2pz AO), and the H,C,H bond angle amounts to 105° (two electrons in the sp AO as compared to one electron in the sp AO of triplet CH2 cf. the discussion of VSEPR theory in Section 1.1.1). 

Various have been developed and theories put forward to find an answer to the questions like why the molecules acquire a particular shape and what decides the bond lengths, bond angles and bond strength of the bonds that hold atoms in a molecule. One of these is Valence Shell Electron Pair Repulsion Theory (VSEPR Theory). 

By using additional rules from VSEPR theory, we can make more exact predictions about the shapes of molecules. For example, the HCO bond angle of CH20 can be pre-... 

The other approach to molecular geometry is VSEPR theory. This theory holds that the shapes of molecules are determined by the repulsion between electron pairs around a central atom. Consider the bonding angle between two hydrogen atoms in a water molecule. One would a expect a 90° angle if hydrogen formed two... 

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