Question

Explain with two examples each of the following: coordination entity, ligand, coordination number, coordination polyhedron, homoleptic and heteroleptic

Answer 1

Coordination entity:
A central metal atom/ion bonded to fixed number of oppositely charged ions or neutral molecules. Examples include $\left[Ni\right(CO{)}_4],[CoC{l}_3\left(N{H}_3{)}_3\right]$.

Ligand:
An atom or molecule or ion capable of donating a pair of electrons to the central metal or ion and forms a coordinate bond with it. Examples include chloride ions, water and ammonia.

Coordination number:
The total number of ligands in a coordination compound attached to the central metal atom or ion. For example, in $\left[Ni\right(CO{)}_4]$, the coordination number of Ni is 4. In $CoC{l}_3(N{H}_3{)}_3$, the coordination number of $C{o}^{3+}$ is 6.

Coordination polyhedron:
The spatial arrangement of ligand atoms which are directly attached to the central atom. For example, $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ has octahedral geometry and $Ni(CO{)}_4$ has tetrahedral geometry.

homoleptic :
Metal is bound to only one kind of donor groups. For example $\left[Ni\right(N{H}_3{)}_6{]}^{2+},\left[Co\right(N{H}_3{)}_6{]}^{3+}$.

heteroleptic :
Metal is bound to more than one kind of donor groups. For example $\left[CoC{l}_3\right(N{H}_3{)}_3],[NiC{l}_2\left(H_{2}O{)}_4\right]$.