Question

A complex ion is a species in which a central metal ion is attached to a group of surrounding ions or molecules by coordinate covalent bonds. The surrounding species are called ligands. Within a ligand, the atoms that are directly attached to the metal ion (or atom) are called donor atoms. Most ligands contain a single donor atom (monodentate), but some contain two donor atoms (bidentate), six donor atoms (hexadentate), or another combination of donor atoms. A complex ion that is associated with one or more counter ions is called a coordination compound. For example [PdCl4]2− is a complex ion, and Na[PdCl4] is a coordination compound. The following table lists some common ligands.

Ligand	Number of donor atoms
Cl−	1
F−	1
Br−	1
CO32−	2
CN−	1
gly−	2
EDTA4−	6
NH3	1
H2O	1
CO	1

The coordination number for a complex is the total number of donor atoms surrounding the central atom. The overall charge on a complex is the sum of the charges of the metal and all the ligands.

Part A

What is the coordination number for each of the following complexes or compounds?

a) [Co(NH3)4Cl2]+

b) [Pb(EDTA)]2−

c) [Pt(NH3)4]2+

d) [Ag(NH3)2]NO3

Part B

What is the charge on each of the following complex ions?

a) hexaaquachromium(II), [Cr(H2O)6]?

b) tris(carbonato)aluminate(III), [Al(CO3)3]?

c) amminepentachloroplatinate(IV), [Pt(NH3)Cl5]?

Part C

What is the oxidation number of the central metal ion in each of the following complexes or compounds?

a) [NiCl2Br2]2−

b) [Cr(H2O)2(NH3)4]3+

c) Na[Ag(CN)2]

Answer 1

Part A:

"The coordination number for a complex is the total number of donor atoms surrounding the central atom."

a) [Co(NH3)4Cl2]⁺ : There are 6 donor atoms surronded to Co as a central atom. Hence, Coordination number is 6.

b) b) [Pb(EDTA)]^{- -} : There are 6 donor atoms from EDTA surronded to Pb as a central atom. Hence, Coordination number is 6.

c) [Pt(NH3)4]⁺⁺ : There are 4 donor atoms as NH3 surronded to Pt as a central atom. Hence, Coordination number is 4.

d) [Ag(NH3)2]NO3 : There are 2 donor atoms as NH3 surronded to Ag as a central atom. Hence, Coordination number is 2. Note: here, NO3 act as a counter anion and not as a ligand.

Part B:

"The overall charge on a complex is the sum of the charges of the metal and all the ligands."

a) hexaaquachromium(II), [Cr(H2O)6] : There is +2 charge on Cr atom and

If H = +1, then 2H = +2. If O = -2, then 2H + O = +2 - 2 = 0.

Thus, the H₂O units do not contribute to the charge on the ion. The only atom that contributes is the Cr atom.

Thus total charge = +2.

b) tris(carbonato)aluminate(III), [Al(CO3)3] : Al has +3 charge. If C = +4, O= -2 then CO3 = -2. Then total sum of charge = +3 + (-2x3) = -3.

Thus total charge = -3.

c) Amminepentachloroplatinate(IV), [Pt(NH3)Cl5] : Pt has +4 charge, If, N=-3, H=+1, NH3= -3+3 = 0, and Cl=-1, then total charge = +4-5 = -1.

Thus total charge = -1.

Part C:

a) [NiCl2Br2]^{- -} : oxidation number = total charge on compound - sum of total oxidation number of ligands.

Ni = -2 - (-2 + -2) = +2

Thus Oxidation number of Ni = +2

b) [Cr(H2O)2(NH3)4]⁺⁺⁺ : Cr = +3 - (0) = +3, Thus oxidation number of Cr = +3.

c) Na[Ag(CN)2] : Na is Na+ and (Ag(CN)2)- . Thus Ag complex has -1 charge.

Ag = -1 -(-2) = +1

Thus Oxidation number of Ag = +1.