Covalent bonds in polyatomic molecules

As in the case of a covalent bond in polyatomic molecules, a distinction should be made between the bond energy related to the bond distance and the experimentally observed dissociation energy of a hydrogen bond which includes the energy changes in the polarized systems. 

Measuring the strength of covalent bonds in polyatomic molecules is more complicated. For example, measurements show that the energy needed to break the first O H bond in H2O is different from that needed to break the second O H bond ... 

Covalent bonds in polyatomic molecules and ions are formed by the overlap of hybrid orbitals, or of hybrid orbitals with unhybridized ones. Therefore, the hybridization bonding scheme is still within the framework of valence bond theory electrons in a molecule are assumed to occupy hybrid orbitals of the individual atoms. 

The concept of hybridization is primarily employed as a theoretical model to explain covalent bonding in polyatomic molecules. 

SBB The Molecular Hamiltonian 208 an The Molecular Wavefunction 214 an Covalent Bonds in Polyatomic Molecules 223 BIOSKETCH Douglas Groljahn 225 X Non-Covalent Bonds 231 an Nuclear Magnetic Resonance Spectroscopy 233... 

According to the octet ride, atoms will lose, gain, or share electrons in order to achieve a noble gas configuration. I irs of valence electrons that are not involved in the covalent bonding in a molecule or polyatomic ion (i.e., valence electrons that are not shared) are called lone pairs. 

The energy required to break the bond between two covalently bonded atoms is called the bond dissociation energy . In polyatomic molecules this quantity varies with environment. For example, ammonia has three N—H bond dissociation energies ... 

In particular, if we have a complex that normally has n ligands when the oxidation state of the central metal is 2, but prefers (n + 2) ligands when the oxidation state is increased to (z + 2), we have the prerequisites for facile oxidative addition of a polyatomic molecule such as H2 to form two new ligands (here, hydrido ligands, H ) by breaking a covalent bond within the molecule and taking two electrons from the metal atom M (reaction 18.9). The reverse process is called reductive elimination. 

On the other hand, the discussion of covalent bonds in the last three chapters has emphasized the bonding of atoms in pairs. The covalent bonding of more than two atoms into a polyatomic molecule has been pictured as occurring by the formation of links between adjacent atoms, each welded by the localized behaviour of electrons that remain associated with no more than two atoms. 

In some molecules and polyatomic ions, both electrons to be shared come from the same atom. The covalent bond formed is known as a coordinate or dative covalent bond. In Lewis structures (electron dot diagrams), a coordinate or dative bond is often denoted by an arrow pointing from the atom which donates the lone pair to the atom which receives it. 

Up to now, we have looked at covalent bonding in molecules or polyatomic ions that have only single bonds. However, in some molecular compounds, atoms share two or three pairs of electrons to complete their octets. A double bond occurs when two pairs of electrons are shared in a triple bond, three pairs of electrons are shared. Atoms of carbon, oxygen, nitrogen, and sulfur are most likely to form multiple bonds. Atoms of hydrogen and the halogens do not form double or triple bonds. 

A bond between two different atoms cannot be purely covalent. Depending on the electronegativity of the bonded atoms, the bonded electron pair shifts toward one of the atoms. A vector showing the magnitude and direction of the shifting of this electron pair is known as a bond moment. In polyatomic molecules, each bond has an individual bond moment. The dipole moment ) of a molecule is the vectorial addition of such bond moments. In the cases where all the individual bond moments are zero (homoatomic molecules), the resultant dipole moment is always zero. However, the converse is not true. Because of equal and opposite values of individual bond moments, some molecules show a zero net dipole moment. As applied to coordination compounds, a dipole measurement can be of value in distinguishing between isomers of a compound, particularly between cis- and trans- isomers. A trans- isomer exhibits a low or zero dipole moment. 

Chapter 11 is devoted to an examination of the infrared spectra of a large number of inorganic materials. Raman data are valuable but will not be considered in this treatment To possess internal vibrations a system must be covalently bound. Many inorganic substances do exhibit covalent bonding. Thus, inorganic molecules and polyatomic ions can have normal vibrational spectra. 

This chapter is devoted to the covalent bond as it exists in molecules and polyatomic ions. We consider—... 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

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