Complex ions valence bond theory

The way in which oui pjesenl undeislanding of the stereochemical intricacies of Ni has evolved illustrates rather well the inteiplay of theory and cxpcruiicnt. On the basis of valence-bond theory, three types of complex of ions were anticipated. These were ... 

Note to the student The AP chemistry exam does not emphasize complex ions or coordination compounds. There is nothing on the AP exam that involves the concepts of crystal-field theory, low versus high spin, valence bond theory, or other related areas. If you understand the questions presented here, then you are basically "safe" in this area of the exam. Most high school AP chemistry programs do not focus much on this area of chemistry because of time constraints. 

Preparation of a Complex Ammonium Salt of Copper(II). Dissolve 0.5 g of finely triturated copper(II) sulphate pentahydrate in 12.5 ml of a 15% ammonia solution. If the solution is turbid, filter it. Slowly add 7.5 ml of ethanol to the filtrate and let it stand for a few hours in the cold. Filter off the formed crystals, wash them first with a mixture of ethanol and a concentrated ammonia solution (1 1), and then with ethanol and ether. Dry them at room temperature. Into what ions does the product dissociate in the solution Consider the structure of the complex ion from the viewpoint of the valence bond theory. 

As noted in Section 9.1, there are three closely related theories of the electronic structures of transition metal complexes, all making quite explicit use of the symmetry aspects of the problem but employing different physical models of the interaction of the ion with its surroundings as a basis for computations. These three theories, it will be recalled, are the crystal field, ligand field, and MO theories. There is also the valence bond theory, which makes less explicit use of symmetry but is nevertheless in accord with the essential symmetry requirements of the problem. We shall now briefly outline the crystal field and ligand field treatments and comment on their relationship to the MO theory. 

According to the valence bond theory (Section 7.10), the bonding in metal complexes arises when a filled ligand orbital containing a pair of electrons overlaps a vacant hybrid orbital on the metal ion to give a coordinate covalent bond ... 

Note the differences between crystal field theory and valence bond theory. In crystal field theory, there are no covalent bonds, no shared electrons, and no hybrid orbitals—just electrostatic interactions within an array of ions. In complexes that contain neutral dipolar ligands, such as H20 or NH3, the electrostatic interactions are of the ion-dipole type (Section 10.2). For example, in [Ti(H20)g]3+, the Ti3+ ion attracts the negative end of the water dipoles. 

For each of the following complexes, describe the bonding using valence bond theory. Include orbital diagrams for the free metal ion and the metal ion in the complex. Indicate which hybrid orbitals the metal ion uses for bonding, and specify the number of unpaired electrons. 

More important, the failure of many transition metal aqua ions to fit the correlations of Figure 8.4 highlights the influence of d electron configuration on the reactivity of metal aqua ions in substitution reactions. The importance of d electron configuration was first noted by Taube in 19521 and explained qualitatively in terms of valence bond theory. Taube, with his predilection for simple test tube demonstrations, distinguished labile metal complexes (ones which underwent substitution within the time of mixing) from inert ones, the latter being typically octahedral complexes... 

For a high-spin octahedral complex such as [FeFg], the five 3d electrons occupy the five 3d atomic orbitals (as in the free ion shown above) and the two d orbitals required for the sp d hybridization scheme must come from the Ad set. With the ligand electrons included, valence bond theory describes the bonding as follows leaving three empty Ad atomic orbitals (not shown) ... 

According to valence bond theory, the shapes of complex ions arise from hybridization of different combinations of d, s, and p orbitals. 

Application of Valence Bond Theory to Complex Ions... 

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. 

According to valence bond theory, what set of orbitals is used by a Period 4 metal ion in forming (a) a square planar complex (b) a tetrahedral complex ... 

It is in the realm of cupric square planar complexes where the valence bond theory gives not only an inadequate, but an incorrect description of the bonding. Valence bond theory ignores the presence of antibonding orbitals and invokes the use of a linear combination of d -y, Px, Py, and s orbitals to form the a bonds. For d ions, the ninth electron can only... 

Valence-bond theory has been used to describe the electronic structure of transition-metal complex ions, with such concepts as d sp hybridization of the metal orbitals. However, the simple VB treatment of complex ions is not fully satisfactory and has been replaced by ligand-field theory, which is MO theory applied to species whose atoms have d (or /) electrons (see Murrell, Kettle, and Tedder, Chapter 13 Offenhartz, Chapter 9). 

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