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Bidentate Ligand


In the whole periodic table of elements, majorly d-block elements have a tendency to form complexes or coordination compounds. The d-block elements that are also called as transition elements have an incomplete d-orbitals with completely filled or half filled valence shell. These incomplete orbitals can accept electrons from an electron donor to form coordinate bond. Therefore such metal atoms or ions act as Lewis acid or electron acceptor to form complex compounds.

The adduct species that are formed due to coordination linkage between one donor and other acceptor is called as coordination compound or coordination or may me complex ion, if carried any charge (positive or negative) on them. For example, a coordination compound of copper with ammonia is Cu(NH3)4SO4 that is formed due to reaction between copper sulphate and ammonia. Here the central metal ion is copper (II) ion that acts as an acceptor unit because of the presence of incomplete d-orbitals as discussed above. There are four donor molecules of ammonia that can donate pairs of electrons, as each ammonia molecule contains one lone pair of electrons over nitrogen atom.

Therefore four ammonia molecules can donate four pairs of electrons to the copper ion to form four coordinate bonds and arranged in certain molecular geometry. Here [Cu(NH3)4]2+ is a complex ion whose charge (2+) can be satisfied with one sulphate ion (SO42-). Here sulphate ion is called as counter ion. It does not involve in coordination entity and must write outside to the square brackets. When we dissolve a coordination compound in an appropriate solvent, it will decompose into two ions; complex ion ([Cu(NH3)4]2+) and one counter ion (SO42-).

The dissolution of coordination compound cannot decompose the coordination entity and in this coordination compound, the blue color of the solution is because of complex ion not due to counter ion. In the complex ion; there are mainly two components. One is the central metal ion that is copper (II) ion in this case and another is a donor atom or molecule or ions.

Bidentate Ligand Definition

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The donor atom or ion or molecule in the coordination compound act as Lewis base and donate its extra pair or a lone pair of electrons to the central metal atom ion to form coordination or dative bond in the coordination complex. Overall metal atom or ion acts as Lewis acid while ligands act as Lewis base in the coordination compounds.

The donor species in the coordination compound must have at least one pair of electrons to donate to the central metal atom or ions. Therefore they can be cation or anion or neutral molecules. On the basis of number of coordination bond can form between a ligand and central metal atom or ion, ligands can be classified as monodentate, bidentate, tridentate or polydentate etc.

A monodentate ligand can form one coordination bond with the central metal atom or ion. Hence there is only one donor atom in the monodentate molecule that can bind with the central metal atom or ion. For example, chloride ion is a monodentate ligand as it can form only one coordination bond with the central metal atom or ion in the coordination compound.

Similarly water is also a monodentate ligand as only an oxygen atom of this molecule can form one coordination bond with the central metal atom. Some common examples of monodentate ligands are ammonia molecule (form one coordination bond through a lone pair of nitrogen atom), hydroxide ion (OH- one coordination bond through a negative charge on the ion) and cyanide ion (CN- one coordination bond with the central metal ion/ atom), CO, CO2, SCN-, O2 etc.

Some examples of coordination compounds and complex ions with monodentate ligands are [Ni(NH3)6]2+, [Cu(NH3)4]2+, [RhI2(CO)2]-, Al(H2O)6 3+, Fe(H2O)63+ and CuCl42-¬ etc. The given image shows an octahedral coordination compound with six monodentate ligands arrange in an octahedral manner around a central metal atom or ion with six coordination bonds, one from each ligand.

  • Like monodentate ligands, there are some ligands that can form two coordination bonds with the central metal atom or ion.
  • The bidentate ligand definition says that they are donor atoms or molecules or ions that may have two donor atoms to form two coordination bonds with similar or different metal atom or ion in a coordination compound.
  • For example, ethylenediamine abbreviated as ‘en’, the oxalate ion abbreviated as ‘ox’, and glycinate ion etc.
  • The chemical formula of ethylenediamine molecule is NH2-CH2-CH2-NH2 and IUPAC name is ethane-1, 2-diamine.
  • Here two nitrogen atoms of NH2 group contain lone pair or electrons, each pair on one nitrogen atom that can donate to similar or different metal atom or ion to form a coordination compound.
  • One of the example of coordination compound with this bidentate ligand is [Cr(H2NCH2CH2NH2)3]3+ that can be abbreviated as [Cr(en)3]3+.
Remember, the presence of three ligands does not make the coordination number three. It will be six only as there are three ligands where each ligand can form two coordination bonds with the central metal atom or ion, therefore the total coordination number will be six and all of these three ligands arrange in an octahedral manner around a central metal atom or ion.

In the trans isomer of [Co(en)2Cl2]+, two chloride ions are placed on the trans position to the central metal ion that is cobalt (III) ion and make 180 degree bond angle with each other. Remaining two ethylene diamine ligands are arranged next to chloride ions as in the given image.

Another example of bidentate ligand is phenanthroline molecule that can abbreviate as (phen). It is a neutral molecule with three aromatic rings condense with each other and two nitrogen atoms. Here each nitrogen atom can donate a pair of electrons to the central metal atom or ion and overall one molecule of phenanthroline can form two coordination bonds with one metal atom or ion. This ligand usually finds in porphyrin molecules.
Acetylacetonate ion is also a bidentate ligand that contains two oxygen atoms, one with lone pairs of electrons and another with a negative charge on it. Both oxygen atom and oxygen ion can form a coordination bond with the central metal atom and acts as bidentate ligand. For example a coordination compound of manganese (III) ion with three acetylacetonate ions, Mn(acac)3 is shown below.

The anionic form of Glycine amino acid; NH2CH2COOH also acts as bidentate ligand and forms two coordination bonds with the central metal atom or ion through –NH2 group and hydroxyl group of –COOH group.

Bidentate Ligand Examples

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A bidentate ligand forms two coordinate bonds with the central metal atom or ion through two donor atoms. The formation of two coordination bonds of a ligand molecule or ion with same metal atom or ion results the formation of a cyclic ring with 3, 4, 5 or six members. Such rings are called as chelates and phenomenon is called as chelation.
  1. Out of all these, the six members ring is most stable. The four member ring such as with carbonato ligand is not stable. Here in the carbonate ligand (CO32-), both oxygen atoms involve in coordinate bonds formation with the metal atom or ion.
  2. The trimethylenediamine molecule (NH2-CH2-CH2-CH2-NH2) can also form two coordination bonds with the metal atom or ion through a two –NH2 groups as each nitrogen atom on -NH2 group contains a lone pair of electrons.
  3. Some other examples of bidentate ligands are diacetyldioxim (OH-N=C (CH3)-C(CH3)=N-OH), 2,2’-Dipyridyl (dipy) and oxalate (-OOC-COO-) and diphenylphosphinoethane (dppe, Ph2P-CH2-CH2-PPh2) etc.
  4. Oxalate ion, glycinate ion and ethylenediamine molecule forms five member rings with the central atom that are quite stable compare to four member chelate rings.

Some of bidentate ligands such as trimethyldiamine molecule and acetylacetonato ion form six membered chelate rings with central metal atom or ion through the formation of two coordination bonds with the metal atom/ion. These chelate rings are most stable and form stable coordination compounds.

  1. Remember some IUAPC rules for the naming of ligands in the coordination compounds such as the name of ionic ligand is ended with –o like –ide changes to –o, -ite changes "-ito", and "-ate" is changed to "-ato."
  2. The name of neutral ligand will come as usual in the coordination complex like ethylenediamine, carbonyl, aqua, nitrosyl etc.
  3. The number of polydentate ligand can write with prefix, mono, bis, tris or tetrakis etc. For monodentate ligand we are using mono, bi, tri, tetra etc.
In the name of coordination entity, the names of ligand are written in alphabetical manner followed by the name of metal atom or ion with its oxidation number in roman numerals in parenthesis.

Oxalate Bidentate Ligand

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We are familiar with the oxalic acid that is an organic acid with two carboxylic groups and written as HOOC-COOH. The dicarboxylate ion of this organic acid is known as oxalate ion that is -OOC-COO- with two negative charges, one for each oxygen atom of the carboxylic group of the molecule. These two negatively charged oxygen atoms can form coordinate bonds with the one central metal atom or ion to form a five member cyclic complex that is known as chelate complex.

This five member chelate ring is composed of one metal atom or ion, two oxygen atoms and two carbon atoms. The chelation in a coordination compound due to the presence of polydentate ligand stabilize the complex compound. For example; (NH4)2[Ni(C2O4)2(H2O)2] in the central metal ion is nickel (II) ion that is bonded with two oxalate ions and two water molecules. The overall charge on complex entity [Ni(C2O4)2(H2O)2]2- is 2- that is balanced with two ammonium ions that act as counter ion here. The name of given coordination compound would be ammonium di-aquabis(oxalato) nickelate(II) as oxalate is a bidentate ligand therefore number of it will written with bis-prefix and there is a negative charge on the complex entity therefore the name of metal will be ended with –ate suffix.

Similarly in the trisoxalatoferrate ion ([Fe(Ox)3]3-), three oxalate ions are bonded with iron (III) ion to form an anionic complex ion with 3- charge. It can exist in two enantiomeric forms which cannot separate from each other due to rapid inter-conversion in the solution.

Another example with the same ligand is [Co(Ox)(H2O)4], named as tetraaquaoxalatocobalt (II) complex.
Here four water molecules are bonded through oxygen atoms with one coordination bond to the central metal ion that is cobalt (II) ion.

One of the oxalate ions can form two bonds with the same central metal ion and form an octahedral coordination compound as given below. The Na4[Co(C2O4)3] coordination compound can exist in two enantiomeric forms that are non-super imposable mirror images of each other.

Ethylenediamine Bidentate Ligand

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Ethylene diamine is a colorless liquid with ammoniacal odor and 0.90 g/cm3 of the density. It is a widely used organic molecule for several synthetic purposes such as for the production of industrial chemicals and other organic compounds like fatty acid, alcohols, and carbonyl compounds.
  • It contains two functional groups (two amide groups), therefore tends to form hetero-cyclic compounds like imidazolidines.
  • It is a well known chelating agent as it can form several chelate coordination compounds as it is a bidentate ligand and can form two coordination bonds with the central metal atom or ion.
  • It involves in the formation of EDTA (ethylenediamine tetraacetic acid) with cyanide and methanal.
  • The reaction is known as Strecker synthesis.
  • Other chelating agents are hydroxyethylethylenediamine and salen ligands that are formed due to condensation reaction of salicylaldehydes with ethylene diamine.
Similarly the coordination ions of ethylene diamine with cobalt metal ion are very common such as [Co(en)2Cl2]+ that can exist in cis and trans forms. The geometrical isomerism is determined with the position of chloride ions in the complex ion. In the cis- complex ion, both chloride ions are placed next to each other in the octahedral arrangement as given below.

Another example of the same ligand is Ni (NH2CH2CH2NH2)32+ in which three ethylene diamine molecules form six coordination bonds with the nickel (II) ion. In each molecule of ethylene diamine, there are two –NH2 group.
  • Each –NH2 group contains one lone pair of electrons that can donate to the central metal atom or ion to form coordination compound or complex ion.
  • Because of the presence of a bidentate ligand in the given coordination complex, the coordination number remains six with three ligands as each ligand can form two coordination bonds with the central metal ion.
  • Ethylene diamine is a good chelating agent that can form two coordinate bonds with the central metal ion to form chelate or cyclic complexes such as the displacement reaction of [Cu(NH3)4]2+ complex ion with ethylene diamine results the formation of stable chelate complex ion as given in the reaction and replace all the four ammonia molecules from the complex ion.
Similarly the chelate complexes of cobalt can synthesis from cobalt chloride in the presence of hydrogen peroxide and hydrochloric acid. In cobalt chloride, cobalt (II) ion is chelated with two ethylene diamine molecules through an equilibrium process and further oxidize in the presence of hydrogen peroxide in acidic medium to cobalt (III) ion that acts as central metal ion from the coordination complex.

The inert complex ion of cobalt (III) ion converts to the complex with ethylene diamine and chloro ligand with other side products.

CoCl2•6H2O + 2 NH2CH2CH2NH2 + H2O2 + HCl $\rightarrow$ trans-[Co(NH2CH2CH2NH2)2Cl2]Cl•HCl + other products

This coordination complex can show geometrical isomerism and exits in -Cis and -Trans forms. The presence of asymmetry induces optical isomerism and form enantiomeric forms of complex compound that are non-super imposable on each other. The main benefit of ethylene diamine ligand is the chelation property that increases the stability of coordination compounds. It is one of the most renowned chelating agents and forms coordination complexes in stepwise processes.

For example, ethylene diamine can displace the water molecules from the coordination complex compound to form another complex in stepwise manner as given below.

[M(H2O)6]2+ + NH2CH2CH2NH2 $\rightarrow$ [M(H2O)4en]2+ + H2O
[M(H2O)4en]2++ NH2CH2CH2NH2 $\rightarrow$ [M(H2O)2(en)2]2+ + H2O
[M(H2O)2(en)2]2+ + NH2CH2CH2NH2 $\rightarrow$ [M (en)3]2+ + H2O

All of these reactions have an equilibrium constant that is also known as stability constant and represents the tendency of ethylene diamine molecule to be added on the next reactant molecule. The stability of complex ions can determine with the help of Irving-Williams order. Some of the complexes of ethylene diamine are unstable like trisethylenediaminecopper (II) that can explain with the help of Jahn-Teller effect. It is an octahedral complex which is unstable due to distortion of a six-coordinate d9 ion by either elongation or compression.

Although this complex of copper is unstable but the chelation due to three ethylene diamine molecules provides it some stability that is known as chelation.