Coordination Compounds

Coordination Compounds are the backbone of modern inorganic and bio–inorganic chemistry and chemical industry.

In the previous Unit we learnt that the transition metals form a large number of complex compounds in which the metal atoms are bound to a number of anions or neutral molecules by sharing of electrons. In modern terminology such compounds are called coordination compounds. The chemistry of coordination compounds is an important and challenging area of modern inorganic chemistry. New concepts of chemical bonding and molecular structure have provided insights into the functioning of these compounds as vital components of biological systems. Chlorophyll, haemoglobin and vitamin $B_{12}$ are coordination compounds of magnesium, iron and cobalt respectively. Variety of metallurgical processes, industrial catalysts and analytical reagents involve the use of coordination compounds. Coordination compounds also find many applications in electroplating, textile dyeing and medicinal chemistry.

1 mol \ce{CoCl3.6NH3} (Yellow) gave 3 mol \ce{AgCl}

1 mol \ce{CoCl3.5NH3} (Purple) gave 2 mol \ce{AgCl}

1 mol \ce{CoCl3.4NH3} (Green) gave 1 mol \ce{AgCl}

1 mol \ce{CoCl3.4NH3} (Violet) gave 1 mol \ce{AgCl}

These observations, together with the results of conductivity measurements in solution can be explained if (i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the cobalt ion during the reaction and (ii) the compounds are formulated as shown in Table 5.1, where the atoms within the square brackets form a single entity which does not dissociate under the reaction conditions. Werner proposed the term secondary valence for the number of groups bound directly to the metal ion; in each of these examples the secondary valences are six.

Note that the last two compounds in Table 5.1 have identical empirical formula, \ce{CoCl3.4NH3}, but distinct properties. Such compounds are termed as isomers. Werner in 1898, propounded his theory of coordination compounds. The main postulates are: 1. In coordination compounds metals show two types of linkages (valences)-primary and secondary. 2. The primary valences are normally ionisable and are satisfied by negative ions. 3. The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. 4. The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. In modern formulations, such spatial arrangements are called coordination polyhedra. The species within the square bracket are coordination entities or complexes and the ions outside the square bracket are called counter ions. He further postulated that octahedral, tetrahedral and square planar geometrical shapes are more common in coordination compounds of transition metals. Thus, \ce{[Co(NH3)6]^3+}, \ce{[CoCl(NH3)5]^2+} and \ce{[CoCl2(NH3)4]^+}

are octahedral entities, while \ce{[Ni(CO)4]} and \ce{[PtCl4]^2-} are tetrahedral and square planar, respectively.

On the basis of the following observations made with aqueous solutions, assign secondary valences to metals in the following compounds:

Formula — Moles of \ce{AgCl} precipitated per mole of the compounds with excess \ce{AgNO3}

(i) \ce{PdCl2.4NH3} — 2

(ii) \ce{NiCl2.6H2O} — 2

(iii) \ce{PtCl4.2HCl} — 0

(iv) \ce{CoCl3.4NH3} — 1

(v) \ce{PtCl2.2NH3} — 0

(i) Secondary 4 (ii) Secondary 6 (iii) Secondary 6 (iv) Secondary 6 (v) Secondary 4