The Co-ordinate Bond: A Shared Pair from a Single Source
The Basics of Chemical Bonding
Before diving into co-ordinate bonds, let's quickly review the two primary types of chemical bonds: ionic and covalent.
- Ionic Bonding: This occurs when atoms transfer electrons. A metal atom loses one or more electrons to a non-metal atom. The metal becomes a positively charged ion (cation), and the non-metal becomes a negatively charged ion (anion). These oppositely charged ions are then strongly attracted to each other. Table salt, or sodium chloride (NaCl), is a classic example.
- Covalent Bonding: This happens between non-metal atoms. Instead of transferring electrons, these atoms share pairs of electrons. Each atom contributes one electron to the shared pair. A simple example is a hydrogen molecule (H$_2$), where two hydrogen atoms share their single electrons.
The co-ordinate bond is a special member of the covalent bonding family. It follows all the rules of covalent bonding, with one key difference: the origin of the shared electrons.
What is a Co-ordinate Bond?
A co-ordinate (dative covalent) bond is a covalent bond in which both electrons of the shared pair are donated by the same atom.
For this to happen, two conditions must be met:
- One atom must have a lone pair of electrons. A lone pair is a pair of electrons in the outer shell of an atom that is not involved in bonding.
- Another atom must have an empty orbital to accept that lone pair. This atom is often electron-deficient.
The atom that donates the lone pair is called the donor (or Lewis base[1]). The atom that accepts the lone pair is called the acceptor (or Lewis acid[1]). Once formed, a co-ordinate bond is identical in every way (strength, length) to any other covalent bond. The "dative" label simply tells us the bond's history—where the electrons came from.
Classic Examples of Co-ordinate Bonding
Let's look at some of the most important examples you will encounter in your studies.
1. The Formation of the Ammonium Ion (NH$_4^+$)
Ammonia (NH$_3$) has a nitrogen atom with a lone pair. A hydrogen ion (H$^+$) is just a proton—it has no electrons and an empty 1s orbital. The nitrogen atom in ammonia donates its lone pair to the hydrogen ion, forming a co-ordinate bond and creating the ammonium ion (NH$_4^+$).
Reaction: NH$_3$ + H$^+$ → NH$_4^+$
In the resulting NH$_4^+$ ion, all four N-H bonds are identical. It is impossible to tell which one was formed by the co-ordinate bond.
2. The Formation of the Hydronium Ion (H$_3$O$^+$)
This is very similar to the ammonium ion. A water molecule (H$_2$O) has an oxygen atom with two lone pairs. A hydrogen ion (H$^+$) can attach to one of these lone pairs, forming a co-ordinate bond and creating the hydronium ion (H$_3$O$^+$).
Reaction: H$_2$O + H$^+$ → H$_3$O$^+$
3. The Bond in Carbon Monoxide (CO)
Carbon monoxide is a more complex example. Both carbon and oxygen have lone pairs, but in the triple bond between C and O, two bonds are normal covalent bonds, and one is a co-ordinate bond where the oxygen atom donates a lone pair to the carbon atom. This helps carbon achieve a stable octet.
4. Complex Ions: [Al(H$_2$O)$_6$]$^{3+}$
In many metal ions dissolved in water, co-ordinate bonds are essential. Take the aluminum ion (Al$^{3+}$). It is small and highly charged, meaning it has a strong affinity for lone pairs. Six water molecules can each donate a lone pair from their oxygen atoms to the Al$^{3+}$ ion, forming six co-ordinate bonds and creating the complex ion [Al(H$_2$O)$_6$]$^{3+}$.
Properties and Behavior of Co-ordinate Bonds
Once a co-ordinate bond is formed, it is generally very stable and behaves just like any other covalent bond. However, in some cases, when the compound containing the co-ordinate bond is dissolved in water or reacts with other substances, the bond can break in a specific way.
This process is called hydrolysis. When a complex ion like [Al(H$_2$O)$_6$]$^{3+}$ is in water, the highly charged central ion can pull electron density so strongly from the co-ordinate bonds to the water molecules that one of the O-H bonds in a water molecule breaks, releasing an H$^+$ ion and making the solution acidic. This is why solutions of aluminum salts are acidic.
| Feature | Standard Covalent Bond | Co-ordinate (Dative) Bond |
|---|---|---|
| Electron Source | One electron from each atom | Both electrons from a single atom (the donor) |
| Prerequisites | Atoms need unpaired electrons | Donor needs a lone pair; Acceptor needs an empty orbital |
| Representation | A single line (e.g., H-H) | An arrow (e.g., H$_3$N → BH$_3$) |
| Bond Properties | Has specific length and strength | Identical length and strength to a standard covalent bond once formed |
| Example | H$_2$, O$_2$, CH$_4$ | NH$_4^+$, H$_3$O$^+$, CO |
Common Mistakes and Important Questions
Q: Is a co-ordinate bond stronger or weaker than a normal covalent bond?
A: Once formed, a co-ordinate bond is identical in strength and length to a standard covalent bond between the same two atoms. The "dative" label only describes how the bond was formed, not its properties.
Q: Can I tell which bond is the co-ordinate bond in a molecule like NH$_4^+$?
A: No. After the ammonium ion is formed, all four N-H bonds are exactly the same. The positive charge is spread evenly (delocalized) over the entire ion. The co-ordinate bond loses its identity once it is made.
Q: Are co-ordinate bonds only found in ions?
A: No, they are found in both neutral molecules and ions. Carbon monoxide (CO) is a neutral molecule that contains a co-ordinate bond, while the ammonium ion (NH$_4^+$) is a charged species.
The co-ordinate bond is a fascinating and essential concept in chemistry that builds upon the foundation of standard covalent bonding. It explains the formation of many common and important ions and molecules, from the ammonium in fertilizers to the hydronium ion that defines acidity. Remember, it is defined by its unique formation mechanism—a lone pair donor and an empty orbital acceptor—but once formed, it is simply another strong, stable covalent bond. Mastering this concept opens the door to understanding more complex chemical structures like coordination complexes and organometallic compounds.
Footnote
[1] Lewis Acid/Base: A Lewis acid is a species that can accept a pair of electrons (the acceptor in a co-ordinate bond). A Lewis base is a species that can donate a pair of electrons (the donor in a co-ordinate bond). This is a broader definition than the standard acid/base theory.