Common ligands
Anions
Cl–
Br–
CN– ( –:C≡N:)
HO–
Molecules
H2O
NH3
CO (:C≡O:)
Transition metals (and to some extent other metals) form coordination complexes in which the metal or metal ion is associated with ligands.
Examples: [Zn(NH
3)
4]
2+, [Cu(CN)
2]
–,
[Ni(OH2)6]2+,
[CoCl
4]
2–,
[PtCl2(NH3)2]
Note that the formula for the complex is enclosed in square brackets.
Complexes may be anions, cations or have no charge.
The overall charge on complexes that are ions is given as a superscript outside the square brackets.
Ligands that have more than one atom are enclosed in parentheses.
The overall charge on the complex is equal to the sum of the charges on its component parts.
Thus the oxidation number on the metal can be deduced if the charge on the ligands is known.
Recognising the oxidation number of the metal in a complex is important because oxidation numbers are an important means of classification of inorganic species.
Examples:
Oxidation number of Ag in [Ag(NH3)2]+
+1 (overall charge) = Nox(Ag) + 2 × 0 (charge on NH3)
Nox(Ag) = +1
Oxidation number of Pt in [PtCl2(NH3)2]
0 (overall charge) = Nox(Pt) + 2 × (–1)
Nox(Pt) = +2