A complex ion is a composite ion in which molecules or ions are bonded to a metal ion; the bond that forms it is a coordinate bond. This is one of the most characteristic chemical phenomena of transition elements.
Basic Knowledge
A ligand donates an electron pair unilaterally to a metal ion to form a bond. The number of ligands is called the coordination number; typical examples are Cu2+=4, Zn2+=4, Ag+=2, Fe3+=6, and Co3+=6.
Representative ligands and their names: H2O (aqua), NH3 (ammine), OH- (hydroxido), CN- (cyanido), and Cl- (chlorido).
Nomenclature: [coordination number + ligand name + metal-ion name + (oxidation state)] + charge. Examples: [Cu(NH3)4]2+ = tetraamminecopper(II) ion, [Ag(NH3)2]+ = diamminesilver(I) ion, and [Fe(CN)6]4- = hexacyanidoferrate(II) ion (anionic complexes use the stem + “ate” + metal name).
Structures: coordination number 2 → linear, 4 → tetrahedral or square planar, 6 → octahedral. Cu2+(d9) tends toward square planar because of the Jahn–Teller effect, while Ni2+(d8) often has a square-planar structure.
Coordinate bond (a covalent bond in which one side donates the electron pair)
Ligand (a molecule/ion that donates an electron pair to a metal)
Coordination number (the number of atoms coordinated to the central metal)
Tetraamminecopper(II) ion ([Cu(NH3)4]2+, deep blue)
Diamminesilver(I) ion ([Ag(NH3)2]+, colorless, Tollens’ reagent)
Hexacyanidoferrate(II/III) ion ([Fe(CN)6]4-/3-, extremely stable)
Further Study (Principles and Applications)
Coordination chemistry bridges analytical chemistry and biochemistry. For example, hemoglobin is an iron–porphyrin complex that transports oxygen by reversibly coordinating O2 to Fe2+. CO poisoning occurs because CO coordinates to Fe about 200 times more strongly than O2. The chlorophyll in green plants is a porphyrin complex of Mg2+, and vitamin B12 is a Co3+ complex.
Industrial applications: silver plating uses electrolysis of [Ag(CN)2]-, and dissolving gold (the cyanide process) uses 4Au + 8CN- + O2 + 2H2O → 4[Au(CN)2]- + 4OH-. EDTA (ethylenediaminetetraacetic acid) is used as a chelating agent to measure water hardness and detoxify heavy metals.
Cisplatin [PtCl2(NH3)2] is a representative example in which only the cis isomer shows anticancer activity, demonstrating powerfully that stereoisomerism determines drug efficacy.
- A coordinate bond is a covalent bond formed by donation of one electron pair
- Main coordination numbers: Ag+=2, Cu2+/Zn2+=4, Fe3+/Co3+=6
- Naming: coordination number (di/tri/tetra/penta/hexa) + ligand + metal name + (oxidation state)
- Anionic complexes end in “-ate”
- A chelate is a complex formed by a polydentate ligand (EDTA, glycine, etc.)
- Cis/trans isomers (a decisive difference in drug activity for Pt complexes)
- Hemoglobin is an Fe–porphyrin complex
Points to Note (Common Confusions and Frequent Errors)
① For a Cu ammine complex, a small amount of NH3 produces a copper hydroxide precipitate, while excess NH3 dissolves it as the deep-blue tetraammine complex. ② Ag+ + NH3 produces an AgOH precipitate, which dissolves as [Ag(NH3)2]+ in excess NH3 (colorless). ③ Zn2+ + NH3 produces a Zn(OH)2 precipitate, which dissolves as [Zn(NH3)4]2+ in excess NH3 (colorless). ④ Fe3+ + SCN- produces blood-red [Fe(SCN)]2+ (a detection reaction).
Practice
- State the English name and structure of [Cu(NH3)4]2+.
- Write the equation for adding excess aqueous NH3 to silver ions.
- Explain how hemoglobin transports oxygen from the viewpoint of coordinate bonding.