IUPAC Nomenclature of Coordination Compounds

Coordination compounds are formulated and named according to the system set up by IUPAC (International Union of Pure and Applied Chemistry). It is important for writing systematic names and formulas, particularly when dealing with isomers.

1. Rules for Writing Formulae of Mononuclear Coordination Entities

It is very convenient to get information about constitution of a compound if we know the formula, mononuclear entities contain a single central atom. To write the formula, following rules are applied:

1. The central atom is listed first.

2. Ligands are then listed in alphabetical order and their placement does not depend on their charge.

3. In case of abbreviated ligands (polydentate) the first letter of abbreviation is used to determine the position of ligand in alphabetical order.

4. Entire coordination entity is enclosed in square brackets, whether charged or not. Polyatomic ligands are enclosed in paranthesis, and also their abbreviations.

5. No space is left in between names of ligands and central atom/ion.

6. When the formula of a charged coordination entity is to be written without counter ion, the charge is indicated outside square bracket as a right superscript, with the number before sign. For e.g., [Co(CN)6]3–, [Cr(H2O)6]3+, etc.

7. Charge on cation and anion is counter balanced.

2. Rules for Naming of Mononuclear Coordination Compounds

Following rules are applied for naming of coordination compounds:

1. Positive part of complex compounds will be named first, followed by negative part.

2. Ligands are named first in alphabetical order, followed by central atom (reverse in case of writing formula).

3. Prefixes mono, di, tri, tetra etc. are used to indicate number of ligands. Prefixes bis, tris, tetrakis are used for complex ligands (including a numerical prefix). For example, [NiCl2(en)2]SO4 is named as dichloridobis (ethylene diamine) nickel (II) sulphate.

4. Name of anionic ligands end with ‘o’, cationic ligands end with ‘ium’. For neutral ligands regular names are used except ‘aqua’ for H2O, ‘ammine’ for NH3, ‘nitrosyl’ for NO, ‘carbonyl’ for CO. These are placed within ( ).

5. Oxidation state of central atom/ion is indicated in roman numerals in brackett after the name of metal.

6. When coordination entity has negative charge, then name of central metal ends with ‘ate’, otherwise not. For e.g., in [Fe(CN)6]4-, ferrate is used for Fe.

The following examples will make the rules more clear:

(a) K4[Fe(CN)6] – Potassium hexacyanoferrate (II)

(b) [Ni(NH3)6]Cl2 – Hexaamminenickel (II) chloride

(c) [Ni(CO)4] – Tetracarbonylnickel (0)

(d) [NiCl2(PPh3)2] – Dichloridobis (triphenylphosphine) nickel (II)

(e) [Mn(H2O)6]2+ – Hexaaquamanganese (II) ion

(f) K2[Ni(EDTA)] – Potassium ethylenediamminetetraacetatonickelate (II)

(g) [Pt(NH3)4][PtCl4] – Tetraammine platinum (II) tetrachloridoplatinate (II)

Werner’s Theory of Coordination Compounds

To explain these properties, Werner proposed following postulates: 

1. Each metal ion possesses two types of valencies:
   (a) Primary valency (principle or ionizable)
   (b) Secondary valency (subsidiary or non-ionizable)

2. Primary valency: These are normally ionizable and are satisfied by anions only.

3. Secondary valency: These are non-ionizable, and are satisfied by ions or neutral electron pair donor molecules (i.e., ligands). It represents coordination number of central metal atom/ion.

4. Primary valencies are non-directional while secondary valencies are directional.

5. Geometry of complex is decided by secondary valency. Thus, [Co(NH3)6]3+, [CoCl(NH3)5]2+ and [CoCl2(NH3)4]+ are octahedral entities, while [Ni(CO)4] and [PtCl4]2- are tetrahedral and square planar respectively.

Effective Atomic Number

Transition metals form coordination compounds very readily because they have vacant 'd' orbitals which can accommodate electron pairs donated by ligands. Metal ion in the complex tends to attain nearest stable inert gas configuration by gaining electrons from ligands. Generally, this is krypton (Z = 36).

Effective atomic number (EAN) of metal in a complex is given by:

EAN = Z–(O.N.) + 2(C.N.) or number of lone pairs donated to central atom.

where Z = atomic number

O.N. = oxidation number

C.N. = coordination number

Bonding in Coordination Compounds

The bonding features in coordination compounds was first described by Werner’s theory. A lot of theories were introduced to explain the nature of bonding in coordination compounds, like Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital Theory (MOT).

(1) Valence Bond Theory (VBT)

According to this theory, the metal atom or ion can use (n–1)d, ns, np, nd orbitals for hybridisation under the influence of ligands to yield a set of equivalent orbitals of definite geometry such as square planar, tetrahedral, octahedral and so on. These hybrid orbitals and ligand orbitals are allowed to overlap with each other that latter can donate electron pair for bonding.

For (n–1)d orbitals used in hybridisation, there is formation of inner orbital complex, and for np or nd orbitals the outer orbital complex formation takes place. Generally inner orbital complexes are low spin complexes with lesser number of unpaired electrons while outer orbital complexes are generally high spin complexes with more numbers of unpaired electrons.

(2) Limitations of Valence Bond Theory

The following limitations of VBT are :

1. Does not give quantitative interpretation of magnetic data.

2. Does not explain colour of coordination compounds.

3. Does not give quantitative interpretation of kinetic or thermodynamic stabilities of coordination compounds.

4. Cannot determine exact predictions regarding tetrahedral and square planar complexes in C.N. = 4

5. Does not differentiate weak and strong ligands.

(3) Crystal Field Theory (CFT)

1. Anionic ligands are treated as negative point charges and neutral ligands as point dipoles.

2. Interaction between metal atom and ligand is purely electrostatic. There is no intermixing of atomic orbitals or no insertion of electron in metal orbitals.

3. If the field produced by surrounding ligand is symmetrical then d-orbitals of metal is degenerate. But in most of complexes, degeneracy is lost because field produced by ligand is not symmetrical and all dorbitals are not equally affected by ligand field.

4. Loss of degeneracy leads to splitting of d-orbitals and in their energies, called crystal field splitting (CFS).

(i) Crystal field splitting in octahedral coordination entities Six ligands are surrounded to metal atom/ion. Since eg orbitals are more directional towards orbitals, they experience greater repulsion and their energy is higher. t2g orbitals lie away from ligands, hence they have lesser repulsion and lower energy.

(ii) Crystal field splitting in tetrahedral coordination entities Coordination number and number of ligands = 4. t2g set have higher energy than eg set as t2g orbitals are more closer to the direction of approach of ligands.

(4) Limitations of CFT

1. Assuming ligands as point charges, it follows that anionic ligands should exert the greatest splitting effect. The anionic ligands actually found at low end of spectrochemical series.

2. It does not explain the covalent character of bonding between ligand and central atom.

Importance and Application of Coordination Compounds

1. Detection and estimation of metal ions in qualitative and quantitative analysis. For example, EDTA, DMG, α-nitroso-β-naphthol, cupron, etc. give colour reactions.

2. Hardness of water is estimated by titration of Ca2+ and Mg2+ with Na2EDTA, and their estimation can be done due to difference in their stability constants.

3. Extraction of metals like silver and gold make use of complex compounds. For e.g., gold forms[Au(CN)2]- in aqueous solution and can be separated in metallic form by addition of zinc.

4. Purification of metals through formation and subsequent decomposition of coordination compounds. For example Ni (impure) is converted to [Ni(CO)4], decomposed to pure nickel.

5. In biological systems, pigments responsible for photosynthesis, chlorophyll is a complex of Magnesium. Haemoglobin, the red blood pigment of blood (oxygen carrier) is complex of iron. Vitamin B12, cyanocobalmine, the anti-pernicious anaemia factor, is complex of cobalt.

6. Complexes, like rhodium complex, [(Ph3P)3RhCl], a Wilkinson’s catalyst, is used for hydrogenation of alkenes.

7. Articles can be electroplated with silver and gold much more smoothly and evenly from solution of complexes, [Ag(CN)2]- and [Au(CN)2]- than from a solution of simple metal ions.

8. The developed film in black and white photography is fixed by washing with hyposolution which dissolves the undecomposed AgBr to form complex, [Ag(S2O3)2]3-.

9. Chelate therapy is used in medicinal industry. For example, EDTA is used in treatment of lead poisoning. Growth of tumours can be inhibited by some complexes of platinum, e.g., cis-platin [PtCl2(NH3)2] andrelated compounds.

Summary

1. Coordination compounds: Compounds formed due to combination of two or more simple stable salts, which retain their identity in solution (dissolved state) as well as solid.

2. Ligands: The donor atoms, molecules or anions which donate a pair of electrons to metal atom/ion.

3. Coordination entity: It constitutes a central metal atom/ion bonded to a fixed number of ligands with coordinate bonds.

4. Primary valency: Normally, it is ionizable valency and satisfied by anions only.

5. Secondary valency: Non-ionizable valency, satisfied by ligands only.

6. Structural isomerism: It is shown by compounds that have different ligands within coordination entity.

7. Stereoisomerism: It is due to different spatial arrangement of ligands around metal.

8. Valence Bond Theory (VBT): According to this theory, the metal atom/ion can use (n–1)d, ns, np, nd orbitals for hybridisation, under the influence of ligands, to yield a set of equivalent orbitals, in definite geometry.

9. Crystal Field Theory (CFT): CFT is an electrostatic model which describes electronic structure of metal ion in ionic crystals.

10. CFSE (Δ): Difference between energies of eg orbitals and t2g orbitals is called crystal field stabilization energy.

11. Spectrochemical series: Series in which ligands are arranged in order of increasing field strength.

12. Metal carbonyls: Complexes formes by most of transition metals with carbon monoxide through both σ- and π-bonds, to form homoleptic carbonyls.

13. Synergic bonding: Bonding between metal and ligand due to donation of electron pair from ligand to metal atom/ion and acceptence of electron pair from d-orbital of metal to ligand.

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Crystal Field Theory

Crystal field theory was established in 1929 and treats the interaction of metal ion and ligand as a purely electrostatic phenomenon where the ligands are considered as point charges in the vicinity of the atomic orbitals of the central atom. Development and extension of crystal field theory taken into account the partly covalent nature of bonds between the ligand and metal atom mainly through the application of molecular orbital theory. Crystal field theory is often termed ligand field theory.

Basic Concept

In Crystal Field Theory, it is assumed that the ions are simple point charges (a simplification). When applied to alkali metal ions containing a symmetric sphere of charge, calculations of bond energies are generally quite successful. The approach taken uses classical potential energy equations that take into account the attractive and repulsive interactions between charged particles (that is, Coulomb's Law interactions).

In order to understand clearly the crystal field interactions in transition metal complexes, it is necessary to have knowledge of the geometrical or spatial disposition of d orbitals. The d-orbitals are fivefold degenerate in a free gaseous metal ion. If a spherically symmetric field of negative ligand filed charge is imposed on a central metal ion, the d-orbitals will remain degenerate but followed by some changes in the energy of the free ion.

Crystal Field Splitting

Crystal field theory was proposed which described the metal-ligand bond as an ionic bond arising purely from the electrostatic interactions between the metal ions and ligands. Crystal field theory considers anions as point charges and neutral molecules as dipoles.

When transition metals are not bonded to any ligand, their d orbitals degenerate that is they have the same energy. When they start bonding with other ligands, due to different symmetries of the d orbitals and the inductive effect of the ligands on the electrons, the d orbitals split apart and become non-degenerate.

Description of d-Orbitals

To understand CFT, one must understand the description of the lobes:

• dxy: lobes lie in-between the x and the y axes.

• dxz: lobes lie in-between the x and the z axes.

• dyz: lobes lie in-between the y and the z axes.

• dx2-y2: lobes lie on the x and y axes.

• dz2: there are two lobes on the z axes and there is a donut shape ring that lies on the xy plane around the other two lobes.

High Spin and Low Spin

The complexion with the greater number of unpaired electrons is known as the high spin complex, the low spin complex contains the lesser number of unpaired electrons. High spin complexes are expected with weak field ligands whereas the crystal field splitting energy is small Δ. The opposite applies to the low spin complexes in which strong field ligands cause maximum pairing of electrons in the set of three t2 atomic orbitals due to large Δo.

• High spin – Maximum number of unpaired electrons.

• Low spin – Minimum number of unpaired electrons.

Example: [Co(NH3)6]3+ & [CoF6]3-

High Spin and Low Spin Complex

•  [Co(NH3)6]3+  – Low spin complex

• [CoF6]3- – High spin complex

Crystal Field Splitting in Octahedral Complex 

• In the case of an octahedral coordination compound having six ligands surrounding the metal atom/ion, we observe repulsion between the electrons in d orbitals and ligand electrons.

• This repulsion is experienced more in the case of dx2-y2 and dz2 orbitals as they point towards the axes along the direction of the ligand.

• Hence, they have higher energy than average energy in the spherical crystal field.

• On the other hand, dxy, dyz, and dxz orbitals experience lower repulsions as they are directed between the axes.

• Hence, these three orbitals have less energy than the average energy in the spherical crystal field.

Thus, the repulsions in octahedral coordination compound yield two energy levels:

• t2g– set of three orbitals (dxy, dyz and dxz) with lower energy

• eg – set of two orbitals (dx2-y2 and dz2) with higher energy

Crystal Field Splitting in Octahedral Complex

This splitting of degenerate level in the presence of ligand is known as crystal field splitting. The difference between the energy of t2g and eg level is denoted by “Δo” (subscript o stands for octahedral). Some ligands tend to produce strong fields thereby causing large crystal field splitting whereas some ligands tend to produce weak fields thereby causing small crystal field splitting.

Crystal Field Splitting in Tetrahedral Complex 

The splitting of fivefold degenerate d orbitals of the metal ion into two levels in a tetrahedral crystal field is the representation of two sets of orbitals as Td. The electrons in dx2-y2 and dz2 orbitals are less repelled by the ligands than the electrons present in dxy, dyz, and dxz orbitals. As a result, the energy of dxy, dyz, and dxz orbital sets are raised while that of the dx2-y2 and dz2 orbitals are lowered.

• There are only four ligands in Td complexes and therefore the total negative charge of four ligands and hence the ligand field is less than that of six ligands.

• The direction of the orbitals does not coincide with the directions of the ligands approach to the metal ion.

Crystal Field Splitting in Tetrahedral Complex

Thus, the repulsions in tetrahedral coordination compound yield two energy levels:

• t2– set of three orbitals (dxy, dyz and dxz) with higher energy

• e – set of two orbitals (dx2-y2 and dz2) with lower energy

The crystal field splitting in a tetrahedral complex is intrinsically smaller in an octahedral filed because there are only two thirds as many ligands and they have a less direct effect of the d orbitals. The relative stabilizing effect of e set will be -6Dq and the destabilizing effect of t2 set will be +4Dq

Square Planar Complexes

In a square planar, there are four ligands as well. However, the difference is that the electrons of the ligands are only attracted to the xy plane. Any orbital in the xy plane has a higher energy level . There are four different energy levels for the square planar (from the highest energy level to the lowest energy level): dx2-y2, dxy, dz2, and both dxz and dyz.

Splitting of the degenerate d-orbitals (without a ligand field) due to an square planar ligand field.

The splitting energy (from highest orbital to lowest orbital) is Δsp and tends to be larger then Δo

Δsp=1.74Δo 

Crystal Field Stabilization Energy

In a chemical environment, the energy levels generally split as directed by the symmetry of the local field surrounding the metal ion. The energy difference between the eg and t2g levels is given as or 10Dq. It states that each electron that goes into the lower t2g level stabilizes the system by an amount of -4Dq and the electron that goes into eg level destabilizes the system by +6Dq. That is the t2g is lowered by 4Dq and the eg level is raised by +6Dq.

For example, the net change in energy for d5 and d10 systems will be zero as shown below.

d5 :- 3(-4Dq) + 2(+6Dq) = -12Dq + 12Dq = 0
d10 :- 6(-4Dq) + 4(+6Dq) = -24Dq + 24Dq = 0

The decrease in energy caused by the splitting of the energy levels is called the “Ligand Field Stabilization Energy (LFSE)”. 

Thus, the crystal field splitting depends on the field produced by the ligand and the charge on the metal ion. An experimentally determined series based on the absorption of light by coordination compound with different ligands known as spectrochemical series has been proposed. Spectrochemical series arranges ligands in order of their field strength as:

I– < Br– < Cl– < SCN– < F– < OH– < C2O42- < H2O < NCS– < EDTA4- < NH3 < en < CN–< CO

Filling of d-orbitals takes place in the following manner; the first three electrons are arranged in t2g level as per the Hund’s rule. The fourth electron can either enter into the t2g level giving a configuration of t2g4eg0 or can enter the eg orbital giving a configuration of t2g3eg1. This depends on two parameters magnitude of crystal field splitting, Δo and pairing energy, P. The possibilities of the two cases can better be explained as

• Δo > P – Electron enters in the t2g level giving a configuration of t2g4eg0. Ligands producing this configuration are known as strong field ligands and form low spin complexes.

• Δo < P – Electron enters in the eg level giving a configuration of t2g3eg1. Ligands producing this configuration are known as weak field ligands and form high spin complexes.