Bonding in complex ions

Until about 20 years ago, the valence bond model discussed in Chapter 7 was widely used to explain electronic structure and bonding in complex ions. It assumed that lone pairs of electrons were contributed by ligands to form covalent bonds with metal atoms. This model had two major deficiencies. It could not easily explain the magnetic properties of complex ions. 

The classical rules of valency do not apply for complex ions. To explain the particularities of chemical bonding in complex ions, various theories have been developed. As early as 1893, A. Werner suggested that, apart from normal valencies, elements possess secondary valencies which are used when complex ions are formed. He attributed directions to these secondary valencies, and thereby could explain the existence of stereoisomers, which were prepared in great numbers at that time. Later G. N. Lewis (1916), when describing his theory of chemical bonds based on the formation of electron pairs, explained the formation of complexes by the donation of a whole electron pair by an atom of the ligand to the central atom. This so-called dative bond is sometimes denoted by an arrow, showing the direction of donation of electrons. In the structural formula of the tetramminecuprate(II) ion... 

The usefulness of NQR spectroscopy is thus not confined to the study of weak secondary bonds, but can be apphed to ordinary covalent or coordinate covalent bonding as well. Hopefiilly with the advent of modem pulse-FT NQR methods, easier stmctural determination by X-ray crystallography, and more powerful computational methods to handle the many bonds in complex ions, NQR can resume its contribution in this area of inorganic chemistry as well. 

Bonding in Complex Ions The Localized Electron Model... 

By this point in your study of chemistry, you no doubt recognize that the localized electron model, although very simple, is a very useful model for describing the bonding in molecules. Recall that a central feature of the model is the formation of hybrid atomic orbitals that are used for sharing electron pairs to form cr bonds between atoms. This same model can be used to account for the bonding in complex ions, but there are two important points to keep in mind. 

Valence bond (VB) theory, which helps explain bonding and structure in main-group compounds (Section 11.1), is also used to describe bonding in complex ions. In the formation of a complex ion, the filled ligand orbital overlaps the empty metal-ion orbital. The ligand (Lewis base) donates the electron pair, and the metal ion (Lewis acid) accepts it to form one of the covalent bonds of the complex ion (Lewis adduct) (Section 18.8). Such a bond, in which one atom in the bond contributes both electrons, is called a coordinate covalent bond, although, once formed, it is identical to any covalent single bond. Recall that the... 

Valence bond theory pictures bonding in complex ions as arising from coordinate covalent bonding between Lewis bases (ligands) and Lewis acids (metal ions). Ligand lone pairs occupy hybridized metal-ion orbitals to form complex ions with characteristic shapes. 

Bonding in complex ions. The simplest view is that the ligands form dative covalent bonds by donating a lone pair of electrons into empty orbitals of the transition metal ion. These could be empty 3d, 4s, 4p or 4d orbitals in the ion (e.g. see the arrangement of electrons in boxes in Figure 5.3). 

Understand the nature of the bonding in complex ions, the shape of the ions and their colour. 

Of the various models in their simplest forms, the molecular orbital (MO) model gives the most realistic view of the bonding in complex ions. Recall from our discussions in Chapter 14 that the MO model postulates that a new set of orbitals characteristic of the molecule is formed from the atomic orbitals of the component atoms. To illustrate how this model can be applied to complex ions, we will describe the MOs in an octahedral complex of general formula MLg ". To keep things as simple as possible, we will focus only on those ligand orbitals having lone pairs that interact with the metal ion valence orbitals (3d, 4s, and 4p). There are two important considerations in predicting how atomic orbitals will interact to form MOs ... 

The model described here to account for colours of transition metal ions is a simplified version of crystal field theory. The theory is based on the idea that the bonding in complex ions is purely electrostatic and that the ligands behave as point negative charges. The most common type of complex ion is octahedral, where six ligands form an octahedron around the metal ion. 

The simplest molecule formed is that produced by the overlap of the Is orbitals of two hydrogen atoms (Figure 14.12). Later in the chapter molecular orbital theory is used to describe the bonding in the diatomic molecules in the first and second periods of the periodic table. MO theory was mentioned briefly in Chapter 13 as a description of bonding in complex ions and complexes. An alternative model of covalent bonding is provided by the valence bond (VB) theory. VB theory was used in Chapter 13 to describe the bonding in complex ions, although it is not a very satisfactory model. 

The bonding in complex ions, particularly the geometries of the ions, can be described by one of our previous bonding models, valence bond theory. Recall from Section 10.6 that in valence bond theory, a coordinate covalent bond is the overlap between a completely... 

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