Coordination Compounds are also know as complex compounds. Coordination Compounds are formed by transition metal.

In a coordination compound a transition metal is surrounded by anions or neutral atoms or group of atom (ligands ) through co-ordinate covalent bond.

Important terminology related to Coordination Compounds

Ligand

Ligands are ions or neutral molecule that are bonded to central metal in coordination compound.

Examples of ligands

Anionic Ligand [latex] Cl^-, Br^- OH^-, CN^-, I^-, S^{-2}[/latex]

Neutral Ligands -[latex] H_2O, CO, NH_3, en [/latex]

Unidentate ligands:

If ligand is bonded to central metal through single donar atom then it is called unidentate ligand example: [latex] NH_3, Cl^-, H_2O, Br^- [/latex]

Didentate Ligands:

When a ligands donate electrons through two atoms to the central metal or we can say when a ligand is bonded to central metal through two atoms them it is called didentate Ligand.

Examples : : [latex] oxalato – C_2O_4^{-2}, \\ enthane-1,2- diamine \\ NH_2CH_2CH_2NH_2 [/latex] Here in case both above ligands two atoms can donate pair of electron to central metal and make coordinate bond

Polydentate Ligands :

When a ligand is bonded to central metal through more than three atoms/ bonds then it is called polydentate Ligands

Examples:: [latex] EDTA^{-4} [/latex] ethylene diamine tetra acetate can donate six pair of electrons and know as hexadentate ligand

EDTA can donate electrons from four oxygen atoms having negative charge and two lone pairs of electrons from two nitrogen atoms

Chelate Ligand

A chelate Ligand uses its two or more donar atom to make coordinate bond with same central metal ion

Chelate Ligand is always didentate or polydentate

Denticity of a ligand

Number of donar site present in a ligand is called denticity of the ligand, in other words we can say that number of coordinate bond a ligand can make with a central metal atom is called denticity of that ligand

Ambidentate ligand

If a ligand has two donar atoms and it can donates through either of them then it is called ambidentate ligand

Examples of ambidentate ligands – : [latex] NO^{-2} [/latex]Here this ligand can donate through either oxygen or nitrogen

SCN- this ligand can donate through either sulphur or nitrogen

Flexidentate Ligand

Flexidentate Ligands are the type of ligands which can vary their denticity according to the type of complex compound, for example SO4(-2) has two Oxygen atom having negative charge on it, so denticity of SO4(-2) is 2, but it can show denticity 1 also in some complex compounds.

[Co(NH3)4SO4](+2) here denticity of SO4 is 2

[Co(NH3)5SO4]+ here denticity of SO4 is 1

Other examples of Flexidentate Ligands are : CO3(-2), EDTA(-4)

Homoleptic Coordination compounds

If all ligands of a coordination compound are same then it is called homoleptic

Example : [Co(NH3)6]+3

Heteroleptic Coordination compounds

Coordination Compounds in which more than one kind of ligands are present

Examples: [ Co(NH3)4Cl2]+

Coordination Entity

Group of central metal and all ligands are collectively know as coordination Entity [Ni(CO)4], [PtCl2(NH3)2] are examples of coordination Entity

Central metal atom or ion

Central metal is the atom inside the coordination Entity which accept the pair of electrons from all the ligands.

Central metal atom/ions in [Ni(CO)4], [PtCl2(NH3)2] are Ni and Pt+2

Central metal atom/ion acts as Lewis acid because they are electron acceptor

Coordination Sphere

Central metal atom and all ligands are enclosed in square brackets along with the total charge on sphere is known as coordination Sphere example: [Fe(CN)6 ]4–

Difference between Coordination entity and Coordination Sphere

If coordination Entity is written with total charge on it then it is called coordination Sphere, for example [Fe(CN)6]4- is a coordination sphere while Fe(CN)6 is the coordination entity

Counter ion

Counter ion is written outside of square brackets to neutralise the charge of coordination Sphere. In K4[FeCN6] K+ is counter ion

Coordination Number

Number of coordinate covalent bonds formed by Central metal atom/ ion is called coordination number

In K4[FeCN6] coordination number of Fe is 6 because there are 6 co-ordinate bonds formed by 6 CN- Ligands with central metal ion Fe+2

[Co(en)3]2+ here coordination number of Co is 6 because denticity of en is 2 so one en can make two coordinate bonds with Co so 3 x 2 = 6

[Cr EDTA]3+ here coordination number of Cr is 6 because of EDTA is 6 so it can form total 6 co-ordinate bonds with Cr

Note : Only Sigma bonds are counted in coordination number, if pie bonds are formed between central metal and ligands that will not be counted in coordination number

Coordination number of some metal ions

Cu+ 2,4

Cu+2. 4,6

Ag+. 2

Ni+2. 4,6

Coordination Polyhedron

The spatial arrangements / geometry of ligands and central metal are known as coordination Polyhedron.

Some famous coordination Polyhedron are – Tetrahedral, square plannar and octahedral

[latex] [NiCO_4] [/latex] is Tetrahedral

[latex] [PtCl_4]^{-2} [/latex] is square plannar

[Co(NH3)6]+3 is octahedral

Oxidation Number

Oxidation number is actual charge on central metal atom when all ligands along with donated electrons are removed

For example oxidation number of Fe in K4[FeCN6] is 2

To calculate oxidation number follow following steps

Let assume oxidation number be x

Total charge on compound = 0

4 + x -6 = 0

x = 2

Oxidation number of Pt in [PtCl2(NH3)2] is 2

Nomenclature of Coordination Compounds

To write IUPAC Nomenclature of Complex compounds you have remember name of ligands first

To write name of anionic Ligand replace e with o like Chloride will be converted to Chlorido or Chloro, Bromide will be converted to Bromido or Bromo

Name of some Anionic Ligand

Cl- Chlorido or Chloro

Br- Bromido or Bromo

OH- Hydroxido

H- Hydrido

CN- Cyanido/ Cyano

0(-2) Oxido

O2(-2) Peroxido

NH2- Amido

N3(-) Azido

N(-3) Nitrido

NC- Isocyanido

S(-2) Sulphido

SO3(-2) Sulphito

SO4(-2) Sulphato

CH3COO- Acetato

C2O4(-2) Oxalato

CO3(-2) Crabonato

SCN- Thioscyanato

NO2(-) Nitrito

Name of Some Neutral Ligands

NH3. Ammine

H2O Aqua

CO Carbonyl

NO Nitrosyl

C5H5N Pyridine

NH2CH2CH2NH3 Ethylenediammine

NO2 Nitryl

PPh3 Triphenyl Phosphene

Cationic Ligands

For naming of cationic Ligands add ium at the end

NH4(+) Ammonium

NO+ Nitrosylium

NO2+ Nitronium

Rules for writing name of Coordination Compounds

1. First Write the name of Ligands and use prefix di, tri, tetra, penta, hexa etc for number of ligands 2, 3, 4, 5 & 6 respectively

2. Write name of ligands in alphabetical order ignore the prefix di, tri, tetra while deciding the priority order

3. If multiple polydentate Ligands are present in the complex use prefix bis, tris, tetrakis, etc

4. After writing the name of ligands write the name of central metal atom with oxidation number in Roman numeral inside ().

5 Like we write the name of NaCl as Sodium Chloride – Cation is written First.
We also follow same rule for complex compounds Cation is written first

6. Example K4[FeCN6] here K+ is cation so that should be written first as – Potassium hexacyanoferrate(II)

So We can simplify the rules for writing name of Coordination Compounds as

1. We Write name of cation first

2. Name of Ligands should be written before writing the name of central metal ion with oxidation number

Rules for writing name of central metal of Coordination Compounds

If Central metal atom is present in anionic coordination Sphere then add ate at the end of the name

Example : [Ni(CN)4]2- tetracyanonickelate (II)

Fe Iron – Ferrate

Pb Lead – plumbate

Au Gold – Aurate

Ag Silver – Argentate

Co Cobalt – Cobaltate

Sn Tin – Stannate

Zn Zinc – Zincate

Cu Copper – Cuprate

Pt Platinum – Platinate

Al Aluminium – Aluminate

Write IUPAC Name of Following Coordination Compounds

[Fe(NH3)6]Cl3. Hexaammine Iron(III) Chloride

[CoCl(NH3)5]2+. Pentaamminechlorocobalt (III)

[Co(Cl)(NO2)(NH3)4]+. Tetraamminechloronitrocobalt(III) Ion

[Pt(NH3)2Cl(NO2) ]. Diamminechloronitrito-N-Platinum(II)

[Co(NH3)6]Cl3. Hexaammine Cobalt (III) Chloride

[Pt(NH3)2Cl(NH2CH3)]Cl Diamminechloromethaylamine Platinum(II) Chloride

[Co(NH3)4Cl(NO2)]Cl. TetraammineChloronitrito-N- Cobalt (III) Chloride

Effective Atomic Number

Effective atomic Number represents total number of electrons around central metal atom in complex compound

We denote Effective atomic number with EAN in coordination chemistry.

EAN = Z- O.N + 2 X C.N

Effective atomic number is equal to total number of electrons on central atom, we calculate it as, we subtract total number of electrons donated by it from atomic number and add total number of electrons gained by it ( 2 X total number of coordinate bond )

Examples :

Calculate EAN in following complexes

[Cr(CO)6 ] :-

Oxidation Number = 0 ( Electrons lost = 0)

Coordination Number = 6

Electrons gained = 6 x 2 = 12

EAN = 24-0+12 = 36

[Co(en)3]3+ :-

Z = 27

Oxidation Number = 3

Coordination Number = 6 ( en is bidentate ligand )

EAN = 27 – 3 + 12 = 36

[Pt(Cl)6]2- : –

Z= 78

Oxidation Number = 4