Introduction

Coordinate bonds play a critical role in everyday life, as many molecules are coordination compounds because they feature this unique sort of linkage. Electron-rich and electron-deficient species combine to make it.

“Coordinate chemistry” refers to the area of chemistry that focuses on the study of coordination molecules.

The example of coordination compounds are :

  1. Potassium hexa cyanide ferrate (II)
  2. Tetra cyanine Nickelate (II)

Coordination Bond: A link in which electrons are shared but not exchanged is called a weak bond. Only one atom contributes to the shared pair in this case, and the atom that donates is known as the donor, while the atom that accepts is known as the acceptor.

For the coordination compound example

NH3:  + BF3— {H3N :-> BF3}

Because it is an electron-deficient molecule, NH3 acts as a donor atom and BF3 as an acceptor.

Compounds like these are examples of coordinate compounds

  • Iron-coordination compound haemoglobin (Hb) (Fe)
  • Magnesium coordination compound chlorophyll (Mg)
  • Cobalt coordination molecule in vitamin B12 (Co)
  • Coordination compounds and double salts are conflated; however, the two have significant differences.

Double salt

A double salt is an equimolar concentration of two salts or an association of numerous chemicals.

Example: Mohr’s salt FeSO4.(NH4)2SO6H2O

Once the coordination compound is produced, its ions than the compound do not lose their identity. When dissolved in water, they show distinct properties; however, in the case of double salts, they do not display different properties but lose their identity in water.

In contrast to the solid and aqueous existence of coordination compounds, the dissociation of double salts into ions occurs only in watery conditions. A double salt has two salts, but the coordination compounds have only one salt, and the salts have just one valence.

Important factors

Central metal atom

  • It is the metal atom that accepts the coordination compound’s electron
  • It is primarily a d-block element and hence acts as an acceptor.
  • As a result of their open-mindedness, they function as Lewis acids.

Ligands

  • They can be a single atom or a group of atoms, but they are all donors.
  • A lone pair Br- or Cl- or neutral NH3, NO, CO or NH3 are examples of negatively charged or neutral species
  • There are Lewis bases here.
  • To describe ligands, we can use the term nucleophile.

Coordination sphere: The ligands and the atoms make it up.

Coordination Entity

  • The coordinating entity or complex [Co (NH3)6]
  • [Coordination entity] is written in square brackets.
  • Positively or negatively charged, this coordination sphere can be altered.
  • An anionic entity or complex has a negative charge and is referred to as a cationic entity when positively charged.

Coordination number: The number of ligands bound to the core metal atom

Oxidation number: It is the charge that remains on an atom after all other atoms have been eliminated. To compute the charge on the coordination sphere, ligands are assigned reasonable charges, and the sum of these charges is equivalent to the charge on the coordination sphere.

Denticity: Molecular donor atom count is calculated using this method.

In terms of denticity, ligands include:

  • Monodentate: Having only one donor atom in it. E.g.CO2, CO3, CO2, etc., are all examples of compounds.
  • Di-dentate: having two donor atoms. E.g. oxalate ion.
  • Polydentate: having three or more donor atoms in it. E.g. DIETHYLENE TRIAMINE (tridendate)

Chelate: In this case, the effect is referred to as chelating, and the structure that is created is called a “chelate” when the ligand has two or more donor atoms.

Application of chelates

  • Water softening is the common usage for them.
  • For the detection of metals in qualitative analysis
  • Separating lanthanoids and actinoids is an important step.

Ambidentate Ligands:  Ambidentate Ligands are those that have two donors but no chelation. One atom at a time forms this group link.

  • For instance, CN-, NO2 and so on.
  • A ligand might be symmetrical or asymmetrical.
  • If the ligands connected to the central atom are all of the same types, this is referred to as having symmetrical ligands. Example: ethylenediamine 
  • If the ligands connected are of different types, the ligands are unsymmetrical. Example: glycinate ion

Homo-leptic and heteroleptic compounds

  • Homoleptic: Homoleptic are those with the same type of ligands bound to the core of the metal atom. Example:  [Co (NH3)6] Cl3 is homoleptic as the same ligands are present. 
  • Heteroleptic: Heteroleptic in cases when the ligands connected differ in kind. Example :  [CoCl3(NH3)3]3+ is heterolytic because different ligands are present.

Applications of  coordination compounds

  • Their compounds with EDTA make them useful in the evaluation of water hardness.
  • Metal ions are estimated and detected with the help of this instrument. For example, coordination compound Dimethylglyoxime is used to estimate the number of Ni2+ ions.
  • Primary use is to extract metals.
  • Cis platin, for example, is a cancer treatment drug that contains this.
  • Haemoglobin is an iron-based complex, while chlorophyll is a magnesium-based complex, and so on.

Below some of the examples of coordination compounds are mentioned :

  • K4[Fe(CN)6]: Potassium hexa cyanide ferrate (II)
  • [Ni(CN)4]−2: Tetra cyanine Nickelate (II) ion.
  • [Zn(OH)4]−2: Tetra hydroxide zincate (II) ion.
  • [Ni(CO)4]: Tetra carbonyl Nickel (O).

Conclusion

As the name suggests, coordination compounds are composed of anions or neutral molecules linked together by covalent bonds. Coordination complexes are another name for coordination compounds. Ligands are the molecules or ions that bind to the central atom and are called such (also known as complexing agents).

The term “metal complex” refers to coordination compounds where the central atom is a metal. The core atom in these types of coordination complexes is usually a transition element. This central atom is known as the coordination centre in these complexes.