Coordination Compounds

Edited By Shivani Poonia | Updated on Oct 17, 2024 06:00 PM IST

Coordination compounds, also known as complex compounds or addition compounds, are very important chemical entities from the chemistry point of view. All such types of compounds involve a central metal atom or `ion that is bonded to a number of molecules or ions called ligands. Coordination compounds find many applications due to their unique structures and properties—to act as a catalyst for chemical reactions, as a pigment for dyes, and even participate in biological processes such as oxygen transport by hemoglobin.

This paper is aimed at reviewing some of the core concepts of coordination compounds by first going through some terminologies that will be encountered in the course of the study, such as ligands, oxidation numbers, and coordination numbers. We will look at what also constitutes a ligand: whether it is a monodentate or a polydentate ligand, whether it's a positively charged ligand, a negatively charged ligand, or a neutral ligand. We'll see the concept of the effective atomic number as a measure of the stability of such compounds. We should consider what is meant by IUPAC nomenclature in regard to naming coordination compounds, complex ions, and anions. At the end of this article, the reader will be able to understand coordination compounds and their importance at an academic and practical level.

Addition Compounds or Molecular Compounds

These are those compounds that are formed by the combination of the simple addition of two or more simple salts. These compounds are of two types, i.e., Double salts and Coordination compounds.

The difference between a double salt and a coordination compound

Both double salts, as well as complexes, are formed by the combination of two or more stable compounds in a stoichiometric ratio. However, they differ in the fact that double salts such as carnallite, and KCl.MgCl₂.6H₂O, Mohr’s salt, FeSO₄.(NH₄)₂SO₄.6H₂O, potash alum, KAl(SO₄)₂.12H₂O, etc. dissociate into simple ions completely when dissolved in water. However, complex ions such as [Fe(CN)₆]^4– of K₄[Fe(CN)₆] do not dissociate into Fe²⁺ and CN^– ions.

Termonologies Pertaining to Coordination Compounds

Coordination compounds are basically chemical species resulting from the coordination of a central metal atom or ion with donors. The metal atom is usually a transition metal and acts as a Lewis acid; on the other hand, the ligands act as Lewis bases and donate electron pairs in order to form a coordinate covalent bond. Important terminologies include:

Coordination entity

A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl₃(NH₃)₃] is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions. Other examples are [Ni(CO)₄], [PtCl₂(NH₃)₂], [Fe(CN)₆]^4–, [Co(NH₃)₆]³⁺

Central atom/ion

In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it is called the central atom or ion. For example, the central atom/ion in the coordination entities: [NiCl₂(H₂O)₄], [CoCl(NH₃)₅]²⁺ and [Fe(CN)₆]^3– are Ni²⁺, Co^3+, and Fe³⁺, respectively. These central atoms/ions are also referred to as Lewis acids.

Coordination sphere

The central atom/ion and the ligands attached to it are enclosed in a square bracket and is collectively termed the coordination sphere. The ionizable groups are written outside the bracket and are called counter ions. For example, in the complex K₄[Fe(CN)₆], the coordination sphere is [Fe(CN)₆]^4– and the counter ion is K⁺

Coordination polyhedron

The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar, and tetrahedral. For example, [Co(NH₃)₆]³⁺ is octahedral, [Ni(CO)₄] is tetrahedral and [PtCl₄]^2– is square planar.

Homoleptic and heteroleptic complexes

Complexes in which a metal is bound to only one kind of donor group, e.g., [Co(NH₃)₆]³⁺, are known as homoleptic. Complexes in which a metal is bound to more than one kind of donor group, e.g., [Co(NH₃)₄Cl₂]⁺, are known as heteroleptic.

Ligand:

A molecule or ion attached to the central atom. They can be monodentate, bidentate, or polydentate based on the number of donor atoms they use.

Coordination Number:

This is the number of ligand donor atoms that are bonded to the central metal atom. It varies with the metal and size and charge of the ligands. The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. The oxidation number is represented by a Roman numeral in parenthesis following the name of the coordination entity. For example, the oxidation number of copper in [Cu(CN)₄]^3– is +1 and it is written as Cu(I).

Oxidation number

It is the charge of the central metal atom therein. This can be calculated based on the number of electrons lost or gained during bonding. The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. The oxidation number is represented by a Roman numeral in parenthesis following the name of the coordination entity. For example, the oxidation number of copper in [Cu(CN)₄]^3– is +1, and it is written as Cu(I).

Effective atomic number (EAN):

This was the concept used earlier to find out the stability of the coordination compounds. It is calculated as the sum of the atomic number of the metal; and the number of electrons donated by the ligand.

To understand the structure and behavior of the coordination compounds some related terminologies are also to be understood.

Ligands are attached to the central metal ion through donor atoms. Each donor atom donates one electron pair to the central metal ion, i.e., the central metal atom or ion gains electrons from the donor atoms. In order to explain the stability of the complex, Sidgwick proposed an effective atomic number denoted as EAN, which is defined as the resultant number of electrons with the metal atom or ion after gaining electrons from the donor atoms of the ligands. The effective atomic number (EAN) generally coincides with the atomic number of the next noble gas in some cases. EAN is calculated by the given relation:

EAN = Atomic number of the metal - number of electrons lost in ion formation + number of electrons gained from the donor atoms of the ligands.

The EAN values of various metals in their respective complexes are tabulated below:

Complex	Metal(Oxidation State)	At. No. of Metal	Co-ordination number	Effective atomic number
K₄[Fe(CN)₆]	+2	26	6	(26-2) + (6x2) = 36[Kr]
[C0(NH₃)₆]Cl₃	+3	27	6	(27-3) + (6x2) = 36[Kr]
Ni(CO)₄	0	28	4	(28-0) + (4x2) = 36[Kr]
K₂[PtCl₆]	+4	78	6	(78-4) + (6x2) = 86[Rn]
[Ag(NH₃)₂]Cl	+1	47	2	(47-2) + (2x2) = 50

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Just as the octet is useful in formulating the bonding in compounds of the light elements, the notion of an EAN provides a rough guide for bonding in coordination compounds. Almost all the metals achieve the EAN of a noble gas through coordination. The EAN concept has been particularly successful for complexes of low-valent metals.

The formula of a compound is a shorthand tool used to provide basic information about the constitution of the compound in a concise and convenient manner. Mononuclear coordination entities contain a single central metal atom. The following rules are applied while writing the formulas:
(i) The central atom is listed first.
(ii) The ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge.
(iii) Polydentate ligands are also listed alphabetically. In the case of an abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in alphabetical order.
(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.
(v) There should be no space between the ligands and the metal within a coordination sphere.
(vi) When the formula of a charged coordination entity is to be written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(CN)₆]^3–, [Cr(H₂O)₆]³⁺, etc.
(vii) The charge of the cation(s) is balanced by the charge of the anion(s).

Types of Ligands

There are different types of ligands. The classification of ligands is done on the basis of their denticity and charge.

LIGANDS - types of ligands

1. Denticity:These are the ligands having one donor atom that coordinates to the metal. Some examples are water, H₂O; ammonia, NH₃; and chloride ions, Cl⁻.

2. Bidentate Ligands: These are those ligands that have two donor atoms that bind to the metal to form a chelate complex, an example is ethylenediamine or "en", having two nitrogens to coordinate the metal.

3. Polydentate Ligands: Ligands that bond through more than one donor atom. One of the best examples is ethylenediaminetetraacetic acid; it may bind through its six donor atoms and form quite stable complexes with metal ions.

Mono or Unidentate ligands
They have one donor atom, i.e., they supply only one electron pair to a central metal atom or ion. F^-, Cl^-, Br^-, H₂O, NH₃, CN^-, etc. are examples of monodentate ligands.

Bidentate ligands
Ligands that have two donor atoms and have the ability to link with the central metal ions at two positions are called bidentate ligands. Some examples include ethylenediamine(en), oxalate(ox), etc.

Tridentate ligands
The ligands having three donor atoms are called tridentate ligands. Some examples include diethylenetriamine(dien), and 2.2,2-terpyridine (terpy).

Tetradentate ligands
These ligands possess four donor atoms. Some examples include nitriloacetate, and triethylenetetramine(trien).

Pentadentate ligands
They have five donor atoms. Some examples include ethylenediaminetriacetate ion.

Hexadentate ligands
They have six donor atoms. The most important example is ethylenediaminetetraacetate ion.

Ambidentate ligands
These are those ligands that can bind to the central metal atom through two different sites. For example CN^-, NCS^-, etc.

Flexidentate ligands
These are the polydentate ligands having many donor sites but according to the availability they change their number of donor sites

Types of Ligands - 2: Charge

Based on the charge attached to a ligand, the ligands can also be further categorized into:

1. Neutral Ligands: The ligands which have no charge on them are known as neutral ligands. Examples of such ligands may be given as molecules.

2. Anionic Ligands: Such are the negatively charged ligands that coordinate with the positively charged metal ions. Such examples are hydroxide (OH)⁻ and cyanide (CN)⁻.

3. Cationic Ligands: Besides the above, some positively charged ligands are less in number and form the coordination compounds.

The entire range of ligands and their properties should be known in order to predict correctly the behavior or nature of the coordination compounds.

Chelating ligands
Some polydentate ligands form coordinate bonds with central metal atoms through their donor sides forming a closed ring-like structure, these ligands are known as chelating ligands and the complex so formed is known as chelating complex.

The stability of these ligands can be explained on the basis of entropy change.

When a chelating ligand bonds to a central metal atom it displaces some number of monodentate ligands equal to its denticity which leads to an increase in the entropy of the system.
5 or 6-membered rings are more stable because angle strain is not there.

Significance and Applications of Coordination Compounds

The coordination compounds find application in a wide base of relevant topics, from chemistry, biochemistry, and materials science. Probably, one of the best-known uses in living systems is the case of hemoglobin, the protein that migrates oxygen in the bloodstream in order to maintain life in living organisms. This molecule is based on an iron(II) coordination complex by water that binds and carries oxygen molecules into the internal parts of the living body.

Coordination compounds have a much broader range of applications compared to industrial applications in various reactions within the sphere of industrial chemistry as catalysts. For example, transition metal complexes are applied in catalytic converters in automobiles to help reduce the by-product that comes out from their engines as emissions. Another crucial application area for coordination compounds is solving problems of pharmaceutical compounds, whereby they may be used to enhance solubility and bioavailability.

Another of the most important uses of coordination compounds is in analytical chemistry. Need one say, most of the techniques such as spectrophotometry, chromatography, etc., exploit the process of complexation in qualitative/quantitative detection and/or determination of the metal ions in the analytical sample.

The applications of coordination compounds can be developed in many aspects of materials science, including the preparation of new materials, paints, and inks. In fact, a large number of dyes selectively develop their color, arising from light wave absorption, with metal-containing coordination complexes.

In an academic sense, the course on coordination compounds forms part and parcel of inorganic chemistry. Towards this, students will get introductory knowledge of the general principles underlying the formation and stability of these compounds, and also their applied uses in a number of fields. The case studies presented by the students on medicinal, industrial, and environmental use of coordination compounds bring forth import and diversity of use for the said compounds.

Nomenclature Of Complex Ions / Molecules

IUPAC has laid down a set of rules for naming the coordination compounds. The naming of the complex ions in the coordination compounds has the ligands being named before the metal. For the name of the metal, the oxidation state of the metal is included in the name by the name of the metal through Roman numbers. The naming of the ligands is dependent on the ligand and the quantity of ligands. The prefixes tell us about how many of each type of ligand.

The names of coordination compounds are derived by following the principles of additive nomenclature. Thus, the groups that surround the central atom must be identified in the name. They are listed as prefixes to the name of the central atom along with any appropriate multipliers. The following rules are used when naming coordination compounds:
(i) The cation is named first in both positively and negatively charged coordination entities.
(ii) The ligands are named in alphabetical order before the name of the central atom/ion. (This procedure is reversed from the writing formula).
(iii) Names of the anionic ligands end in –o, those of neutral and cationic ligands are the same except aqua for H₂O, ammine for NH₃, carbonyl for CO, and nitrosyl for NO. While writing the formula of coordination entity, these are enclosed in brackets ( ).
(iv) Prefixes mono, di, tri, etc., are used to indicate the number of individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, and tetrakis are used, the ligand to which they refer is placed in parentheses. For example, [NiCl₂(PPh₃)₂] is named as dichloridobis(triphenylphosphine)nickel(II).
(v) Oxidation state of the metal in cation, anion, or neutral coordination entity is indicated by a Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named the same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix – ate. For example, Co in a complex anion, [Co(SCN)₄]²⁻ is called cobaltate. For some metals, the Latin names are used in the complex anions, e.g., ferrate for Fe.
(vii) The neutral complex molecule is named similar to that of the complex cation.

Naming of Complex Anions

In naming negatively charged complex ions, anionic complexes, there is a modification in the name of the metal as to obtain the negative charge. The general process adopted ended with "-ate". For example, the complex ion [Fe(CN)6]3- is hexacyanoferrate(III). i.e, It is named as the complex anion containing six cyanide ligands coordinated to a ferric ion.

In the naming of complex ions, the names of ligands are written in alphabetical order followed by the name of a central metal atom with its oxidation number in Roman numerals. If the complex part contains two or more same type of ligands then di, tri, tetra, etc. are used.

For example, [Co(NH₃)₃Cl₃] is written as Triamminetrichloridocobalt(III).

Naming of Complex Anion and Complex Cation

The naming in this case starts with the cation and then followed by the anion. For example, a compound like [Cu(NH3)4]Cl2 can be named as tetraamminecopper(II) chloride because the complex cation in this case is tetraamminecopper(II) before the anion chloride.

The naming of the metal is replaced by placing the suffix - "ate".

Some special names are given to some metals.
Metals Cationic Anionic
Ag Silver Argentate
Fe Iron Ferrate
Cu Copper Cuprate
Au Gold Aurate
Pb Lead Plumbate
Sn Tin Stannate
For example, [Ag(NH₃)₂]Cl is named as diamminesilver(I)chloride.
The IUPAC of [PtCl₂(NH₃)₄]⁺²[PtCl₄]^-2 is written asTetraamminedichloridoplatinum(IV)tetrachloroplatinate(II).

Naming of Bridged Complex

If two metal centers are bridged by ligands, these are called bridged complexes. For example in the complex [RuCl2(en)(μ−Cl)]2, the name would be a specification of the bridging chloride (μ-Cl) with the ligands, which gives place to a name such as dichloro bis (ethylenediamine)ruthenium.

Use prefix -μ for ligands present in bridging.

If the charge on the complex is odd, then distribute this charge to the metal atoms and it the charge is even, then divide this charge equally to the central metal atoms.
Now, the naming can be written according to the rules discussed earlier.

Some Solved Examples

Example 1
Question:
The ratio of the number of water molecules in Mohr's salt and Potash alum is (times 10^{-1}). (Integer answer)

Solution:
The formulae of Mohr's salt and Potash alum are given as:
Mohr's Salt:($\mathrm{FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O}$)
Potash Alum:($\mathrm{KAl(SO_4)_2 \cdot 12H_2O}) (KAl(SO4)2 \cdot12H2O$)

Therefore, the ratio of water molecules is:
[ $\text{Ratio of water molecules} = \frac{6}{12} = 0.5 = 5 \times 10^{-1}$ ]

Hence, the answer is (5).

Example 2
Question:
Total number of ions in solution exhibited by the complex having a higher molar conductivity?

1. ($\mathrm{[Co(NH_3)_6]Cl_3}$ )
2. ($\mathrm{[Co(NH_3)_3Cl_3]}$ )
3. ($\mathrm{[Co(NH_3)_4Cl_2]Cl}$ )
4. ($\mathrm{K[Co(NH_3)_4Cl_2]Cl}$ )

Solution:
($\mathrm{[Co(NH_3)_6]Cl_3}$ ) gives the maximum number of ions in the solution. Thus, it shows the highest molar conductivity.

$\mathrm{[Co(NH_3)_6]Cl_3 \rightarrow [Co(NH_3)_6]^{3+} + 3Cl^-}$

Thus, the total number of ions is (4).

Hence, the answer is option (4).

Example 3
Question:
Coordination compounds have great importance in biological systems. In this context, which of the following statements is incorrect?

1. Chlorophylls are green pigments in plants and contain calcium.
2. Haemoglobin is the red pigment of blood and contains iron.
3. Cyanocobalamin is B12 and contains cobalt.
4. Carboxypeptidase-A is an enzyme that contains zinc.

Solution:
Chlorophyll is a green pigment in plants and contains magnesium, not calcium.

Hence, the answer is option (1).

Summary

The coordination compounds are the addition compounds or molecular compounds of immense importance in and different streams of science. Their uniqueness in structure and properties drives their importance. Important terminologies for understanding their behavior have to be known as ligands, coordination number, oxidation number, and effective atomic number. The ligands can be classified on the basis of dentistry and charge. Monodentate, bidentate, and polydentate are some of the important ligands that lead to the formation of stable complexes.