Question
Explain with two examples each of the following coordination entity, ligand coordination number, coordination polyhedron, homoleptic and heteroleptic.
✓
Answer
(i) Coordination Entity : The species in which a certain number of ions or molecules are attached to the central metal atom or ion by forming covalent bonds is called coordination entity. For example : $\left[ CoCl _3\left( NH _3\right)_3\right]$, the cobalt ion is surrounded by three NH3 molecules and three chloride ions. Other examples : $[Ni(CO)_4]$ and $\left[ Co \left( NH _3\right)_6\right]^{3+}$.
(ii) Ligand : The molecules or ions attached to the central metal atom or ion in the coordination entity (complex species) are called ligands. They form covalent bonds by donating an electron pair to the metal.
Example : $Cl , H _2 O$ and $NH _3$.
(iii) Coordination Number (CN) : In a complex species, the number of donor atoms of the ligands bound to the metal which are directly attached to it is called the coordination number of the metal.
Example : The coordination number of platinum Pt in the complex ion $\left[ PtCl _6\right]^{2-}$ is 6 and that of Ni in $\left[ Ni \left( NH _3\right)_4\right]^{2+}$ is 4.
(iv) Coordination polyhedra or Coordination polyhedron : The specific arrangement in space of the ligands directly attached to the central atom in a complex species is called coordination polyhedra.
Example : $\left[ Co \left( NH _3\right)_6\right]^{3+}$ is octahedral and $\left[ Ni ( CO )_4\right]$ is tetrahedral.
(v) Homoleptic and heteroleptic complexes : Complexes in which the metal atom is attached only to one type of donor groups is called homoleptic complexes, and those complexes in which the metal atom is attached to more than one type of donor groups are called heteroleptic complexes.
Example : $\left[ Co \left( NH _3\right)_4 Cl _2\right]^{+}$.
(ii) Ligand : The molecules or ions attached to the central metal atom or ion in the coordination entity (complex species) are called ligands. They form covalent bonds by donating an electron pair to the metal.
Example : $Cl , H _2 O$ and $NH _3$.
(iii) Coordination Number (CN) : In a complex species, the number of donor atoms of the ligands bound to the metal which are directly attached to it is called the coordination number of the metal.
Example : The coordination number of platinum Pt in the complex ion $\left[ PtCl _6\right]^{2-}$ is 6 and that of Ni in $\left[ Ni \left( NH _3\right)_4\right]^{2+}$ is 4.
(iv) Coordination polyhedra or Coordination polyhedron : The specific arrangement in space of the ligands directly attached to the central atom in a complex species is called coordination polyhedra.
Example : $\left[ Co \left( NH _3\right)_6\right]^{3+}$ is octahedral and $\left[ Ni ( CO )_4\right]$ is tetrahedral.
(v) Homoleptic and heteroleptic complexes : Complexes in which the metal atom is attached only to one type of donor groups is called homoleptic complexes, and those complexes in which the metal atom is attached to more than one type of donor groups are called heteroleptic complexes.
Example : $\left[ Co \left( NH _3\right)_4 Cl _2\right]^{+}$.
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