Lesson 3 of 13 · 6 min
Coordination number and oxidation number
NCERT §5.2
Farhan's teacher writes [Co(en)₃]³⁺ on the board and asks how many things are touching the cobalt. Half the class says three.
The lesson in notes
In short
The coordination number (CN) is the number of ligand donor atoms bonded directly to the metal: 6 for Pt in [PtCl₆]²⁻ and 4 for Ni in [Ni(NH₃)₄]²⁺.
A didentate ligand counts twice: Fe in [Fe(C₂O₄)₃]³⁻ and Co in [Co(en)₃]³⁺ both have CN = 6.
Only σ bonds from ligand to metal fix the coordination number; any π bonds between them are not counted.
The coordination sphere is the central atom with its ligands, written in square brackets. Ionisable groups outside the bracket are counter ions: in K₄[Fe(CN)₆] the sphere is [Fe(CN)₆]⁴⁻ and the counter ion is K⁺.
The coordination polyhedron is the spatial arrangement of the donor atoms around the central atom. The usual ones are octahedral ([Co(NH₃)₆]³⁺), square planar ([PtCl₄]²⁻) and tetrahedral ([Ni(CO)₄]).
The oxidation number of the central atom is the charge left on it if every ligand is taken away together with the electron pair it shares. It is written as a Roman numeral in brackets: Cu in [Cu(CN)₄]³⁻ is +1, Cu(I).
Homoleptic complexes carry one kind of donor group, e.g. [Co(NH₃)₆]³⁺; heteroleptic complexes carry more than one, e.g. [Co(NH₃)₄Cl₂]⁺.