The coordinate covalent bond, also known as the dative bond, is a type of covalent bond where both electrons are from the same atom. Usually, it happens in molecules where metallic ions are bonded to ligands.
In 1916, Gilbert Newton Lewis (1875-1946) published his work on chemical bonds saying that a chemical bond is an electronic pair shared by two constituent atoms.
Each atom usually gives one electron in a bond pair. This bond pair is known as a covalent bond.
A methane molecule is made by the mutual sharing of electrons between the carbon atom and hydrogen atoms.
Coordinate (Dative) covalent bond
It is a covalent bond in which the sharing of electrons to make a bond is only observed by one of the bonded atoms.
The electron-donating species is always with an extra pair of electrons that can be donated. Although this disturbs the octet completion of that element, what better could come out of a lone electronic pair than the formation of a bond.
So, the electron-deficient species make a bond rather than staying alone.
A chemical species with the ability to accept an electronic pair is called a Lewis acid. Basically, these species have empty orbitals in these elements which make them good electron acceptors. All cations are lewis acids as they have the capacity to accept electron pairs.
- All cations e.g. Fe+2, Fe+3, Ag+2, etc.
- Atoms, ions, and molecules with incomplete octet e.g. BF, AlCl3, Me3B, etc.
- Molecules with the capacity to absorb more than eight electrons e.g. SiBr4, SiF4, etc.
- Molecules with multiple bonds between two atoms of different electronegativity values e.g. CO2, SO2, NO2, etc.
A chemical species with the ability to donate an electron pair is called a Lewis base. These species are electron efficient. Lewis bases have excess electron pairs which remain excess even after octet completion of the outermost electronic shell. So, a lewis base can also be named as an electron-pair donor.
- All anions e.g. OH–, CN–, CH3COO–, etc.
- Molecules with electrons more than required to complete the octet.
- Complex anions e.g. Sulphate, etc.
- Electron-rich pi (П) system, such as ethyne, ethene, and benzene, etc.
These electron pairs are donated from lewis bases to lewis acids to form lewis adducts. An adduct is a product formed by the addition of two or more components i.e. as the name suggests addition product, (adduct).
Examples of coordinate covalent (dative) bond
1. Ammonium ion (NH4+)
The reaction of ammonia with an acid to produce ammonium salt fits in the examples of a coordinate covalent bond formation.
The nitrogen atom of ammonia has an electron pair (Lewis base) that isn’t involved in bonding as long as its ammonia. This lone pair of electrons attack a nearby proton (Lewis acid), which are excessively present as the environment is acidic. This ammonia-hydrogen seems just like other nitrogen-hydrogen bonds of ammonia, for which this bond is called a covalent bond, except it is different in nature of the formation, so a coordinate covalent or a dative bond term comes to use.
Ammonium ion adduct can be shown as (H←N+H3).
The product is called adduct. This adduct shows the charge contained on the central dative atom. Ammonia can also share free electron pairs with other atoms than hydrogen such as metals.
2. Ammonia and Boron Trifluoride (H3N→BF3)
Ammonia, having a donatable electron pair acts as a Lewis base. Boron trifluoride has one vacant orbital which can accept an electron pair from ammonia to complete its valence shell, acting as a Lewis acid. The ammonia-boron trifluoride adduct is neutral as it has equal negative and positive charges.
Ammonia – Boron trifluoride adduct can be shown as (H3N→BF3).
3. Hydronium ion (H3O+)
The oxygen atom in water contains two lone pairs, one of which is donated to another hydrogen ion (H+) from the medium and a dative bond is created. Hydronium ion is a cationic adduct formed by a coordinate covalent bond.
H2O + H+ → H3O+
It occurs commonly in acidified water.
Hydronium ion adduct can also be represented as (H2O+→H).
Hydronium ion is also known as protonated water and is the simplest one of oxonium ions. Oxonium ions are such species that contain three atoms bonded directly to the oxygen atom. A hydronium ion explains the concept of oxonium species in the best way.
4. Metallic complexes
In metal complexes, a metal atom is bonded to several electron donors (Lewis bases). The central metal atom, acting as a Lewis acid, along with electron donors, makes the first coordination sphere. Coordination refers to the formation of coordinate (dative) covalent bonds.
The molecules present in the surroundings of metal ions are called ligands. Ligands can be neutral, cationic, or anionic but they are always the electron donors i.e. Lewis bases.
Explanation through Valence Bond Theory (VBT)
The valence bond theory, proposed by Professor L. Pauling, published in his book ‘The Nature of Chemical Bond’ clearly explains how a coordinate complex metallic atom/ion system is generated when ligands donate their lone pair of electrons toward metallic atoms or ions.
Valence bond representation of the complex Hexafluorocobaltate(III) complex ion [CoF6]3- explains that twelve electrons from six fluoride ions are shared in the hybridized (sp3d2) orbital to make the metallic complex ion hexafluorocobaltate(III).
It means that the donation of all electrons is from fluoride ions, making six-coordinate (dative) covalent bonds represented by arrows (↑).
Similar is the case with Hexamminecobalt(III) complex ion [Co(NH3)6]3+ except that the hybridization of hexafluorocobaltate(III) uses outer ‘d’ orbital, resulting in (sp3d2) hybridization, whereas hexamminecobalt(III) ion hybridized using inner ‘d’ orbital, giving rise to (d2sp3) hybridization.
The donation of electrons from ammonia to metal (Co)3+ ion occurs giving rise to six coordinate (dative) covalent bonds represented by arrows (↑).
This is the reason why hexamminecobalt(III) is an inner orbital complex, but hexafluorocobaltate(III) is an outer orbital complex.
Difference between Covalent and Dative bond
A covalent and a coordinate covalent bond are similar because both bonds are made by sharing electrons between two atoms. Although there are some characteristics, which differ among these bonds.
Covalent vs Dative bond
Dative covalent Bond
A covalent bond is formed between two atoms by the mutual sharing of electrons
A dative bond is formed between two atoms by the sharing of electrons from one of the bonded atoms
Unpaired electrons from both atoms are responsible for covalent bond
Lone pair of one atom and a vacant orbital of another atom form a dative bond
Covalent bond may be polar or nonpolar
A dative bond is always polar
Is AlCl3 a coordinate covalent, covalent, or ionic bond?
In AlCl3, it looks like the bond is ionic because of the bonding between metal and non-metal. But in this case, the electronegativity difference is 0.55 which means it is a covalent (polar) bond (0.4 to 0.7 ≈ polar covalent bond).
There should be a lone pair to one atom and an empty orbital to another atom for the coordinate covalent bond to exist. In AlCl3, these conditions are not fulfilled, so it only has a covalent bond.
Is CO2 coordinate covalent bonded?
CO2 has no coordinate covalent bond as there are only unpaired electrons in carbon and oxygen orbitals i.e. there exists no deficiency or empty orbitals in any of these two atoms.
Why is the carbon-carbon bond in C2H4, not a coordinate bond?
In the ethene molecule, the carbon-carbon double bond is formed by sharing of electrons from both carbon atoms. As the mutual sharing of electrons doesn’t count in a coordinate covalent bond, it only forms a covalent bond.
Do we consider coordinate covalent bonds in hybridization?
The coordinate covalent bond is considered in hybridization if the bond formed does not lead to a pi bond. This explains the fact that the pi bond is not a result of coordinate covalent bonding.
What are the conditions needed for the formation of coordinate covalent bonds?
The coordinate covalent bond forms between donor and acceptor atoms. There are two conditions for this bond formation:
- The donor atom has at least one lone pair to donate.
- The acceptor atom has at least one empty orbital to accept that lone pair.
What is the meaning of a predominantly covalent bond?
A predominantly covalent bond has more covalent character than an ionic bond. It means there is less electronegativity difference between the bonded atoms.
Can coordinate bonds be ionic or covalent?
A coordinate covalent bond is actually an alternative covalent bond in which both electrons are shared from one atom.
Why can’t HCl form a coordinate bond?
In HCl, the hydrogen atom has 1 and the chlorine has 7 electrons in their valence shells. Both atoms share one electron to form a simple covalent bond. So there is no chance of either donation or accepting of a lone pair to form a coordinate covalent bond.
What is a coordination compound?
Coordination compounds are molecules in which a central metal ion is surrounded by ligands (atoms, ions, or neutral molecules that donate lone pairs) via a coordinate covalent bond.
What is a substance that donates a pair of electrons to form a coordinate covalent bond called?
Lewis base is a substance that donates an electron pair to form a coordinate covalent bond.
Does KCN have coordinate bonds?
KCN (potassium cyanide) does not have any coordinate covalent bond but in the case of KNC (Potassium isocyanide), there is. In this form, a lone pair of nitrogen is donated to carbon to make a coordinate covalent bond.
In HNO3, N makes a coordinate covalent bond with which O atom?
The oxygen atom which is attached to the nitrogen atom by a single bond and is not bonded to hydrogen is having the coordinate covalent bond.
What is covalency in chemistry?
The maximum number of bonds formed by an atom to acquire the stable electronic configuration is known as covalency in chemistry.
How many coordinate bonds are in N2O5?
There are two coordinate covalent bonds per N2O5 molecule from each nitrogen atom to an oxygen atom.
Is there a coordinate bond in an ozone molecule?
Ozone molecule has one coordinate covalent bond. The simple way to understand this is that the third oxygen in an oxygen molecule (O2) is bonded via a coordinate covalent bond.
What are ligands in coordination chemistry?
In coordination chemistry, a ligand is an ion or molecule which donates an electron pair to the central atom in coordination complexes. In fact, ligands are Lewis bases or electron donors according to the coordinate (dative) covalent bond concept.
Examples: H2O, NH3, CN–, OH–, Cl–, etc.
What is the coordination number of [CoCl3(NH3)3]
The coordination number in compound [CoCl3(NH3)3] is 6 because 3 chlorides (Cl–) and 3 ammonia (NH3) molecules donated electron pairs to cobalt in order to be neutral (oxidation state = 0).