A coordinate compound (or complex) contains one or more coordinate bonds, which are links between two electrons in which one of the atoms donates both electrons. To put it another way, it’s a chemical with a coordination complex. Except for alloys, coordination compounds include most metal complexes or compounds. Haemoglobin, chlorophyll, dyes, pigments, vitamin B12, enzymes, catalysts, and Ru3(CO)12 are just a few examples. We discuss coordinate compounds in detail in this chapter. We’ll look at how these substances can be used in the real world. 

Properties of coordinate compounds

Here are the general characteristics:

• During their electronic transitions, unpaired electrons absorb light, lending colour to the coordinate compounds generated by transition elements. For example, iron (II) complexes may be green or light green, while iron (III) coordinate compounds can be brown or yellowish-brown.
• The existence of unpaired electrons in coordinate complexes created when the coordinate centre is a metal gives rise to magnetic properties in the ensuing coordinate complexes.
• Chemically, coordinate molecules react in several ways. They can conduct electron transport activities on both the inner and outer spheres.
• Specific ligands in complex compounds may catalyse or help with molecular stoichiometry changes.

Some examples of coordinate compounds, along with their formulas, are provided in the following table:

The complexes can be classified as homoleptic or heteroleptic complexes, depending on their composition. Generally speaking, a homoleptic complex is one in which the metal is solely linked to one sort of donor group, such as: [Fe(CN)4]4- heteroleptic is a complex in which the central atom is attached to a variety of distinct donor groups, and the centre atom is the donor group.

Isomerism in coordination compounds 

Isomers are compounds with the same chemical formula but different atom configurations. As a result, coordinate compounds often exhibit two types of isomerism: stereoisomerism and structural isomerism.

Werner’s theory of coordination compounds

Alfred Werner proposed Werner’s concept in 1898 to explain the structure of coordinate chemicals. 

When AgNO3 (silver nitrate) combines with CoCl3.6NH3, all three chloride ions form AgCl (silver chloride). However, when one mixes AgNO3 and CoCl3.5NH3, it produces only two moles of AgCl.

Furthermore, the reaction of CoCl3.4NH3 with AgNO3 creates one mole of AgCl. Werner proposed the following explanation in light of this discovery.

Werner’s theory postulates

The core metal atom of the coordinate complex has two types of linkages or valencies: primary and secondary.

• Negative ions complete ionisable main connections.
• Secondary links cannot be ionised. Negative ions meet these requirements. Furthermore, the secondary valence of any metal is constant and equal to its coordinate number.
• Various spatial configurations may contain secondary connected ions corresponding to several coordinate numbers.

 Limitations of Werner’s theory

• The colour, magnetic, and optical characteristics of coordinate compounds are difficult to explain.
• The theory does not explain why all components didn’t connect to create coordinate molecules.
• It was not possible to explore the directional features of coordinate molecule bonds.
• This explanation does not account for the complex’s stability.
• This strategy disregards the complexities inherent in complexes.

Structure and properties explained using Werner’s theory

According to Werner, the following four complexes of Co (III) chloride with ammonia have the following structural and chemical properties:

• CoCl3.6NH3 has three silver chloride precipitates, which are the primary valency, and six NH3 molecules, which are the secondary valency, according to his findings. As a result, the compound is now referred to as [Co(NH3)6] Cl3.

• The remaining 1Cl– and 5NH3 ions serve as secondary valency in CoCl3. 5NH3 has primary valency, and the 2Cl– ions serve as secondary valency. So [Co(NH3)5Cl] Cl2 is the compound.

• CoCl3.4NH3 has the main valency of 1Cl– and secondary valencies of 2Cl– and 4NH3. Consequently, the chemical is [Co(NH3)5 Cl2]. Cl
• All 3Cl– and 3NH3 ions in CoCl3.3NH3 are secondary valencies. So [Co(NH3)5 Cl3] is the compound.
• He uses dotted lines (…….) to symbolise primary valencies and solid lines (—) to represent secondary valencies.

Conclusion

Coordinate complexes are formed by transition metals because of their unique ability to build these structures. The high charge to mass ratio and the availability of d-orbitals are to blame. Many complex compounds have been developed due to breakthroughs in coordinate chemistry. There are several industries where coordinated chemicals are used. 

Some examples include mining and metallurgy, as well as the medical sciences. Coordinate compounds are employed in hydrometallurgical processes to extract metals such as nickel, cobalt, and copper from their ores and in crucial catalytic processes to polymerise organic compounds such as polyethene and polypropylene.