Nomenclature of Coordination Compounds-Types of Complexes, Names of Important Ligands

The ability of the transition metals to form the complexes is referred to as coordination compounds. This property is not only applicable to transition metal only but also shown by some other metals. Werner, in 1893 was the first scientist to propose the theory of coordination compounds and also purposed the concept of the primary and secondary valence of a metal ion.

The branch of inorganic chemistry dealing with the study of coordination compounds is known as coordination chemistry.

What are Coordination Compounds?

Coordination compounds are the compounds in which the central metal atom is attached to a number of ions or neutral molecules by coordinate bonds either by donation of lone pairs of electrons by these ions or neutral molecules to the central metal atom.

Coordination compounds are used for the extraction of metals, in Analytical (qualitative or quantitative analysis), chemistry (like colour tests, titrations etc.), inorganic chemistry (can be used to remove toxics from the substances or any bodies) and even use in the treatment of chemotherapy and can be used for preparing various medicines. It can also be used as a catalyst.

e.g. Nickel tetracarbonyl, [Ni(CO)₄], in which CO molecules are linked to the central nickel atom by coordinate bonds by donating lone pairs of electrons, is a coordination compound.

Types of Complexes

There are following three types of complexes-

1. Cationic complex – The complex ion carries a net positive charge. e.g.: [Co(NH₃)₆]³⁺, [Ni(NH₃)₆]²⁺, etc.
2. Anionic complex – The complex ion carries a net negative charge. e.g.: [Ag(CN)₂]^–, [Fe(CN)₆]^4-, etc.
3. Neutral complex – A complex carrying no net charge. e.g.: [Ni(CO)₄], [Co(NH₃)₃Cl₃], etc.

Terminology used in Coordination Compounds

Before proceeding with the study of coordination compounds first we need to understand some basic terms related to it:

• Ligands and central metal atom/ion: The ion or neutral molecules which are bonded to a central metal ion or atoms is called a ligand. The atom or ion to which these ligands are attached is called a central metal atom or ion. While in the formation of the coordinate bonds, the anions or the neutral molecules act as the electron pair donors and the metal atom or ion acts as the electron pair acceptor.

For example: In [Ni(NH₃)₆]²⁺, central metal atom or ion is lewis acid. 

The ligand is attached to a single donor atom, then it is called unidentate. The number of coordinating groups attached to a ligand is called the denticity of the ligand. 

When ligands are attached to different donor atoms then it is called ambi dentate ligands. e.g. NO^–₂ can coordinate through either nitrogen or oxygen. Similarly, CN^– can coordinate through either carbon or nitrogen.

• Coordination number: The number of ligands attached to the central metal ion through coordinate bonds is called the coordination number of the metal ion.

For example, in the complex ions [Ag(CN)₂]^–, [Cu(NH₃)₄]²⁺ and [Cr(H₂O)₆]³⁺, the coordination numbers of Ag, Cu and Cr are 2, 4 and 6 respectively.

• Coordination sphere or coordination entity: A entity attached to a central atom in a fixed number of ions or molecules and is enclosed in square brackets and are called the coordination sphere or coordination entity. The ionizable groups which are written outside the square brackets are called counter ions.

For example, in the coordination compound.[Cu(NH₃)₄] SO₄, the complex ion, [Cu(NH₃)₄]²⁺, in which Cu²⁺ is the central metal ion and four NH₃ molecules are ligands, forms the coordination sphere and SO₄^2- ions are the counter ions.

• Coordination polyhedron: This spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion is called coordination polyhedron around the central atom/ion.
• Oxidation number or oxidation state: The oxidation number of the central atom is defined as the charge that it carries as if all the ligands are removed with electron pairs that are shared with the central atom. It is represented by the roman numbers, I, II, III, etc. in the parentheses after the name of the central atom.

For example, the oxidation number of copper in [Cu(NH₃)₄]²⁺ is +2 but the coordination number is 4. Similarly, the oxidation number of Fe in [Fe(CN)₆]^3- is 3 but the coordination number is 6.

• Charge on the complex ion: The charge carried by a complex ion is the algebraic sum of the charges carried by the central metal ion and the ligands coordinated to the central metal ion.

For example, in the complex ion, [Ag(CN)₂], Ag⁺ ion carries a change of +1 and each CN^– ion carries a charge of – 1. Therefore, the net charge on the complex ion [Ag(CN)₂]^– is + 1 – 2 = -1.

• Homoleptic and Heteroleptic complexes: Complexes in which the metal atom is attached to only one type of ligands are called homoleptic complexes. For example, [Co(NH₃)₆]³⁺. The complexes in which the metal atom is attached to more than one kind of ligand are called heteroleptic complexes. For example, [Co(NH₃)₄Cl₂]⁺.
• Homonuclear and polynuclear complexes: Complexes in which only one metal atom is present are known as homonuclear complexes whereas in which more than one metal atom is present are known as polynuclear complexes.

Nomenclature of coordination compounds

Coordination compounds are formulated and named according to the system set up by the inorganic nomenclature committee of the international union of pure and applied chemistry (IUPAC) while writing the formula, ligands are added after the central atom/ion.

Rule for writing formula of Mononuclear Coordination Compounds

• Formula of the cation simple or complex is written first followed by that of the anion.
• The formula of the entire coordination entity is written in square brackets either charge or neutral.
• In the coordination sphere, the symbol of the metal atom is written first followed by the symbols/formulas of the ligands arranged alphabetically according to their name irrespective of the charge present on them. For example, [Co(NH₃)₄(H₂O)Cl] Cl₂.

Rules for Nomenclature of Mononuclear Coordination Compounds

• Order of naming ions: Simple or complex positive ion always named first followed accompany the negative ion. The name is started with a small letter and the complex part is written as one word, e.g. [Co(NH₃)₆] Cl₃, hexa amminecobalt (III) chloride. But in the non-ionic and molecular complexes, one-word name is given. e.g. [Co(NH₃)₃(NO₂)₃], triamminetrinitrocobalt.
• Naming of ligands: Different types of ligands are –
  1. Negative ligands (organic or inorganic) end in-o, e.g. CN^– (cyano), Cl^– (chloro), Br^–(bromo). If the name of anionic ligand ends in -ide, -ite or -ate, the last ‘e’ is replaced by ‘o’ giving -ido, -ito and -ato, e.g. SO₄^2- (sulphato), C₂O₄^2-(oxalato),
  2. Neutral ligands have no special ending, e.g. NH₃(ammine), H₂O (aqua*), CO (carbonyl).
  3. Positive ligands (which are very few ) end in -ium, e.g. NH₂-NH₃⁺  (hydrazinium), NO⁺ (nitrosonium), NO₂⁺ (nitronium).
  4. Organic ligands: Organic free radicals are given their own names. e.g. CH₃ (methyl), C₂H₅ (ethyl), C₆H₅(phenyl), C₅H₅ (cyclopentadienyl). For organic neutral molecules, their names are used. For example, CH₃NH₂(methylamine), P(C₆H₅)₃ (triphenylphosphine), C₅H₅N (pyridine or py), H₂NCSNH₂ (thiourea or tu), NH₂CH₂CH₂NH₂
  5. Ambidentate ligands: They are unidentate ligands with more than one coordinating atom. It is important to select the point of attachment of a ligand by placing the symbol of the donor atom attached after the name of the groups separated by a hyphen.

Thus, (NH₄)₃[Cr(SCN)₆] is named as ammonium hexathiocyanato-S-chromate (III) and (NH₄)₂[Pt(NCS)₆] is named as ammonium hexathiocyanato-N-platinate (IV).

Some Common Ligands

Actual Name (Symbol used)	Formula	Charge	Donor Atom(s)	Name given in the complex
Negative ligandsCyanide ionHalide ionperoxide ionsulphide ion	CN–X(F– , Cl– ,Br– I–)O22-S2-	-1-1-2-2	CXOS	cyano or cyanidefluoride/chloride/bromide/iodideperoxosulphido
Positive ligandsHydrazinium ionNitrosonium ionNitronium ion	NH3+NO+NO2+	+1+1+1	NNN	hydraziniumnitrosoniumnitronium
Neutral ligandsMethylamineAmmoniaWater	CH2NH2NH3H2O	ZeroZeroZero	NNO	methylamineammineaqua or aquo

• Numerical prefixes to indicate the number of ligands: If there are more than one ligand of the same type are present then use the prefixes like di, tri, tetra, penta and Hexa are used to indicate the number of ligands of that type. For example, if there are four NH₃ molecules as ligands, the name is given as tetra ammine.
• Naming of ligands: All ligands are named in alphabetical order whether positive or negative, by the name of the metal atom/ion. This is the reverse of that used for writing the formula where symbols of the metal atom are written first. The prefixes di, tri, etc. are not to be considered while writing in alphabetical order. For example, the complex, [Pt (NH₃)₄ Cl(NO₂)] SO₄ is named as tetra ammine chloridonitroplatinum (IV) sulphate.
• Naming of the complex ion and end of the central metal atom: Ligands are named first followed by metal atom. The ending of the name of the metal upon the nature of the complex ion. For example [Cu(NH₃)₄] SO₄ tetraamminecopper (II) Sulphate.

Complex Compound	Name
[Co(NH3)6] Cl3	hexaamminecobalt (III) chloride
[Cr(H2O)4Cl2] NO2	tetraaquadichloridochromium (III) nitrate
[Co(NH3)4Cl(NO2)] NO3	tetraamminechloridonitrocobalt (III) nitrate
K3 [Fe(C2O4)3]	potassium trioxalatoferrate (III)

Sample Questions(FAQs)

Question 1: Which metals form coordination compounds?

Answer:

Transition metals form coordination compounds because of the presence of empty valence shell orbitals.

Question 2: Square planner complexes show geometrical isomerism which has coordination number four but tetrahedral complexes do not. Why?  

Answer:  

The relative positions of the ligands attached to the central metal atom are the same with respect to each other, due to this tetrahedral complexes do not show geometrical isomerism in spite of having four coordination numbers.

Question 3: What do you understand by the chelate effect?

Answer:  

When a bidentate or polydentate ligand contains donor atoms located in such a way that when they coordinate with central metal ion they form five or six-membered rings, this is called the chelate effect.

For Example- Ni²⁺ complex with ‘+ion’ is more stable than NH₃.

Question 4: Is [Fe(C₂O₄)₃]^3- is a stable complex? Explain.

Answer: 

In the given complex, Fe is in +3 state and C₂O₄^2- is a bidentate ligand so it forms chelate rings and therefore it is a stable complex.

Question 5: Explain the importance of coordination compounds in industry.

Answer: 

In industry coordination compounds is used as catalysts namely Rhodium having the formula [RhCl(Ph₃P)₃] it is used for selective hydrogenation of alkenes and is also used in the pharmaceutical industry. Phthalocyanine blue it is more stable complex of copper(II) are used as pigments and dyes.

Question 6: What are the factors that affect the stability of a complex?

Answer: 

The factors that affect the stability of complex are:

• Nature of the metal ion- If the size of the ion is small and bears a greater charge that means more the charge density and hence greater is the stability of the complex.
• Nature of the ligands -Depends on the basicity of ligands means it has maximum tendency to donate electron pair and hence more stable is the complex.

Question 7: What is the denticity of ligand?

Answer: 

The number of donor atoms in a ligand that forms a coordinate bond with the central metal atom.