Ligands and Complex Ions

Key Concepts

A ligand, or complexing agent, is a polar molecule or ion bonded to a central metal ion.
A complex ion is a polyatomic species consisting of a central metal ion surrounded by several ligands.
Naming Complex Ions:

Anionic ligands have names ending in 'o'.
1. 'ide' → 'o', eg, chloride → chloro
2. 'ate' → 'ato', eg, sulfate → sulfato
Neutral ligands are named as the molecule with these notable exceptions:
1. H₂O → aquo
2. NH₃ → ammine
3. CO → carbonyl
The numbers of ligands in a complex are specified using the Greek prefixes:
1. di for 2
2. tri for 3
3. tetra for 4
4. penta for 5
5. hexa for 6
The name of a cationic complex ion ends in the name of the central metal ion with the oxidation state shown as a Roman numeral in parantheses at the end of the metal's name, eg, iron (III).
The name of an anionic complex ion ends in 'ate',
sometimes the latin name is used, eg,
chromium (II) → chromate (II)
nickel (II) → nickelate (II)
platinum (II) → platinate (II)
silver (I) → argentate (I)
iron (II) → ferrate (II)
copper (I) → cuprate (I)
lead (II) → plumbate (II)
gold (I) → aurate (I)
tin (IV) → stannate (IV)
Ligands are named before the central metal atom.

Identify the central metal ion
Identify the charge on the central metal ion (shown in parantheses)
Identify the ligands
Identify the number of ligands
Calculate the total charge on the ligands
Calculate the charge on the complex ion
= charge on metal ion + total charge on ligands
Write the formula giving the central metal ion first followed by the ligands, eg, Mn(H₂O)₆²⁺
If more than one type of ligand is present, anion ligands are given before neutral ligands, eg, CrCl₂(NH₃)₄⁺

The coordination number of the complex ion is the number of bonds formed between the central metal ion and its ligands.
The shape (geometry) of a complex ion is related to its coordination number.

Ligands can be:

Compound Name	Formula	Ligand Name	Bonding Atom
Polar Molecules
water	H₂O	aquo	O
carbon monoxide	CO	carbonyl	C
ammonia	NH₃	ammine	N
Anions
chloride	Cl^-	chloro	Cl
fluoride	F^-	fluoro	F
bromide	Br^-	bromo	Br
iodide	I^-	iodo	I
cyanide	CN^-	cyano	C
hydroxide	OH^-	hydroxo	O
thiosulfate	S₂O₃^2-	thiosulfato	O
carbonate	CO₃^2-	carbonato	O and O
oxalate	C₂O₄^2-	oxalato	O and O

Example: Name the complex ion with the forumla Fe(CN)₆^3-

Anionic ligands have names ending in 'o'.
CN^- named as cyano
The numbers of ligands in a complex are specified using a Greek prefix:
6 ligands = hexa → hexacyano
Oxidation state of the central metal atom is shown with a Roman numeral in parantheses at the end of the metal's name:
Central metal ion is iron
Charge on iron: 3- = x + (6 x 1-)
3- = x -6
x = 3+
Central metal ion: iron (III)
The complex ion is an anion, therefore the name will end in ferrate (III)
Ligands are named before central metal ion: hexacyanoferrate (III)

Formula	Ligand Name	No. of Ligands and prefix	Central Ion Name	Complex Ion Name
Ag(NH₃)₂⁺	ammine	2 → di	silver (I) (+1= x + 2(0), x = +1)	diamminesilver (I) ion (complex is a cation)
Ag(CN)₂^-	cyano	2 → di	silver (I) → argentate (I) (-1= x + 2(-1), x = +1)	dicyanoargentate (I) ion (complex is an anion)
Cu(H₂O)₆²⁺	aquo	6 → hexa	copper (II) (+2= x + 6(0), x = +2)	hexaaquocopper (II) ion (complex is a cation)
CuCl₄^2-	chloro	4 → tetra	copper (II) → cuprate (II) (-2= x + 4(-1), x = +2)	tetrachlorocuprate (II) ion (complex is an anion)

Example: write the formula for the complex ion tetraamminecopper (II)

Identify the central metal ion : copper, Cu
Identify the charge on the central metal ion (shown in parantheses): 2+
Identify the ligands: ammine = NH₃ (neutral species)
Identify the number of ligands: tetra = 4
Calculate the total charge on the ligands = 4 x 0 = 0
Calculate the charge on the complex ion = charge on metal ion + total charge on ligands = 2+ + 0 = 2+
Write the formula giving the central metal ion first followed by the ligands : Cu(NH₃)₄²⁺

Name	Central Ion Formula	Ligand Formula	No. of Ligands	Complex Ion Formula
hexaaquocobalt (II) ion	Co²⁺ (charge in parentheses)	H₂O (aquo = H₂O)	hexa = 6	Co(H₂O)₆²⁺ (4 x 0 +2 = +2)
tetrachlorocobaltate (II) ion (ate = anion)	Co²⁺ (charge in parentheses)	Cl^- (chloro = Cl^-)	tetra = 4	CoCl₄^{^2- (4 x -1 + 2 = -2)}
tetracarbonylnickel (II) ion	Ni²⁺ (charge in parentheses)	CO (carbonyl = CO)	tetra = 4	Ni(CO)₄²⁺ (4 x 0 + 2 = +2)
tetracyanonickelate (II) ion (ate = anion)	Ni²⁺ (charge in parentheses)	CN^- (cyano = CN^-)	tetra = 4	Ni(CN)₄^2- (4 x -1 +2 = -2)

Coordination number = number of ligands = 2 → linear
Coordination number = number of ligands = 4 → tetrahedral or square-planar
Coordination number = number of ligands = 6 → octahedral
(octahedral geometry is most common for transition metal complexes)