Bonding in Complex Ions The Localized Electron Model

5 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, althongh very simple, is a very useful model for describing the bonding in molecnles. 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, bnt there are two important points to keep in mind. 

The VSEPR model for predicting strncture does not work for complex ions. However, we can safely assnme that a complex ion with a coordination number of 6 has an octahedral arrangement of ligands and that complexes with two ligands are linear. On the other hand, complex ions with a coordination number of 4 can be either tetrahedral or square planar there is no reliable way to predict which will occur in a particular case. 

The interaction between a metal ion and a ligand can be viewed as a Lewis acid-base reaction, with the ligand donating a lone pair of electrons to an empty orbital on the metal ion to form a coordinate covalent bond  

The hybrid orbitals used by the metal ion depend on the number and arrangement of the ligands. For example, accommodating the lone pair from each ammonia molecule in the octahedral Co(NH3)g ion requires a set of six empty hybrid atomic orbitals with an octahedral arrangement. Recall that an octahedral set of orbitals is formed by the hybridization of two d, one s, and three p orbitals to give six d sp orbitals (see Fig. 19.19). 

Just as a left hand requires a left-handed glove, a left-handed receptor requires a particular enantiomer for a correct fit. Therefore, in designing pharmaceuticals, chemists must be concerned about which enantiomer is the active one—the one that fits the intended receptor. 

Ideally, the pharmaceutical should consist of the pure active isomer. One way to obtain the compound as a pure active isomer is to produce the chemical by using organisms, because the production of biomolecules in organisms is stereospecific (yields a specific stereoisomer). For example, amino acids, vitamins, and hormones are naturally produced by yeast in the fermentation of sugar and can be harvested from the ferment. Biotechnology, in which the gene for a particular molecule is inserted into the DNA of a bacterium, provides another approach. Insulin is now produced in this way. 

The alternative to using racemic mixtures is to find a way to produce the substance as a pure isomer or a way to separate the isomers from the racemic mixtures. Both of these options are difficult and thus expensive. However, it is becoming increasingly clear that many pharmaceuticals must be administered as pure isomers to produce the desired results with no side effects. Therefore, a great deal of effort is now being directed toward the synthesis and separation of chiral compounds.  

---

Although the localized electron model can account in a general way for metal-ligand bonds, it is rarely used today because it cannot predict important properties of complex ions, such as magnetism and color. Thus we will not pursue the model any further. 

---