What is a dative bond?

A covalent bond is formed when two atoms share electrons ; the pair of electrons is attracted to both atomic nuclei, holding them together to form a bond. In a covalent bond by the usual definition, each atom supplies one electron to form the bond. A dative bond is a covalent bond between two atoms where one of the atoms provides all of the bond-forming electrons . This type of bond is also called a  coordinate covalent bond  or a dative covalent bond.

Bonding is indicated by drawing an arrow pointing from the electron-donating atom to the electron-accepting atom; the arrow replaces the usual line indicating that it is a chemical bond. The coordinate covalent bond can also be represented with Lewis diagrams or structures; It is usual to represent it graphically by enclosing the distribution of the atoms of the molecule in square brackets, since they are polyatomic ions.

the dative bond

Dative bonds are commonly seen in reactions involving hydrogen atoms, H; For example, when hydrogen chloride dissolves in water to produce hydrochloric acid, a dative bond is formed that generates the hydronium ion H 3 O ₊ releasing ^the chlorine anion Cl ^– .

H ₂ O + HCl → H ₃ O ⁺  + Cl ^–

The hydrogen nucleus is transferred to the water molecule to form the hydronium ion; this means that it does not contribute any electrons to the bond, and once the bond is formed there is no difference between a dative bond and an ordinary covalent bond.

Another example is the ammonium cation, NH ₄ ⁺ , which is the result of ammonia behaving as a base, capturing H ⁺ .

If we remove the only electron it has to generate a hydrogen ion or proton, H ⁺ , from an H atom, whose electronic configuration is 1s1 , it does not have any electrons left with which to form a simple covalent bond, so the only option What remains is to form a coordinate covalent bond in which the two electrons are contributed by the other species. In this way, the other species must necessarily have pairs of electrons that can be shared.

In the case of the hydronium ion mentioned, H ₃ O ⁺ , and which results from the addition of an H ⁺ to a water molecule H ₂ O, the Lewis structure is

In the same way in the case of the ammonium ion, NH ₄ ⁺ , which results from the addition of an H ⁺ to a molecule of ammonia, NH ₃ , the Lewis structure is

Fountain

Greenwood, N., Earnshaw, A. Chemistry of the Elements  Second Edition. Butterworth-Heinemann, 1997, Oxford.