In chemistry, a coordinate covalent bond, dative bond, or coordinate bond is a type of covalent bond that forms when both shared bonding electrons come from only one of the atoms. The more general term “covalent bond” covers cases when each atom contributes a bonding electron or when one atom contributes both electrons. In a coordinate covalent bond, the attraction of both nuclei to the electron pair holds the bond together. Coordinate covalent bonds form in Lewis acid and base reactions, metal ions bonding to ligands, and sometimes between nonmetals.
The IUPAC considers the term “dative bond” as obsolete and prefers “coordinate bond” over “coordinate covalent bond”. In diagrams, coordinate covalent bonds are indicated using an arrow instead of simple solid line.
Examples of Coordinate Covalent Bonds
A classic example of a coordinate covalent bond forms when gaseous ammonia (NH3) and hydrogen chloride (HCl) react and form ammonium ions (NH4+) and chloride ions and eventually ammonium chloride (NH4Cl). This is the “white smoke” chemistry demonstration.
NH3(g) + HCl(g) → NH4Cl(s)
The hydrogen chloride molecule transfers a hydrogen atom (proton) to the lone pair of the ammonia molecule, forming the ammonium ion. In the ammonium ion, a coordinate covalent bond connects the fourth hydrogen to the molecule. Once the chemical bond forms, there is no way of telling the ordinary covalent and coordinate covalent bonds apart. Meanwhile, the departure of the hydrogen from hydrogen chloride leaves a chloride anion.
Another example of a coordinate covalent bond forms in the reaction between ammonia and boron trifluoride (BF3) that forms ammonia boron trifluoride (NH3·BF3). Boron trifluoride is an interesting compound because it does not assume a noble gas structure around the boron atom. Boron only has three pairs of bonding electrons, even though there is space for four pairs. So, although BF3 is stable, it is electron deficient. The lone pair from nitrogen forms a dative bond with beryllium, giving it the octet structure.
As another example, consider carbon monoxide (CO). This molecule contains two covalent bonds formed by equally sharing electrons from carbon and oxygen and one coordinate bond using a lone pair supplied by the oxygen atom.
In contrast, carbon dioxide (CO2) does not have a dative bond. In each carbon dioxide molecule there are four carbon-oxygen bonds. Each bond contains an electron from each atom. The bonds are covalent, but not coordinate.
Other examples of compounds containing coordinate or dative bonds include:
- Hydronium ion (H3O+): Oxygen is the donor of an electron pair while hydrogen is the acceptor.
- Aluminum chloride (Al2Cl6): Each aluminum atom needs two electrons to complete its valence shell. A chlorine atom has a long pair. An Al atom forms a coordinate bond with the Cl atom on the adjacent AlCl3 group. So, aluminum chloride is essentially a covalent dimer molecule and not an ionic compound.
Coordinate Bond vs Covalent Bond
There are differences between a coordinate bond and a covalent bond formed when each atom contributes an electron.
- A coordinate bond is more polar.
- The coordinate bond is rigid and directional.
- Coordinate bonds are longer than regular covalent bonds.
- Coordinate bonds are weaker than regular covalent bonds.
- Coordination does not necessarily involve a two-electron bond between two atoms.
- A pure covalent bond can occur between two atoms of the same element (e.g., H2). A coordinate bond always involves two dissimilar atoms.
- Coordinate compounds tend to be sparingly soluble in water.
- IUPAC (1997) “Dipolar bond”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.D01752
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN: 978-0-08-037941-8.
- Haaland, Arne (1989). “Covalent versus Dative Bonds to Main Group Metals, a Useful Distinction”. Angewandte Chemie International Edition in English. 28 (8): 992–1007. doi:10.1002/anie.198909921
- Himmel, Daniel; Krossing, Ingo; Schnepf, Andreas (2014). “Dative Bonds in Main-Group Compounds: A Case for Fewer Arrows!”. Angewandte Chemie International Edition. 53 (2): 370–374. doi:10.1002/anie.201300461
- Khare, E.; Holten-Andersen, N.; Buehler, M.J. (2021). “Transition-metal coordinate bonds for bioinspired macromolecules with tunable mechanical properties”. Nat Rev Mater. 6: 421–436. doi:10.1038/s41578-020-00270-z