Definition and Nature
What is a Dative Bond?: A dative bond is a special kind of covalent bond. Normally, in covalent bonds, two atoms share a pair of electrons, and each atom gives one electron to the shared pair. But in a dative bond, both electrons in the shared pair come from the same atom. It’s like one atom is offering a full gift of two electrons to make the bond.
Difference from Regular Covalent Bonds: In a regular covalent bond, both atoms participate equally—each brings one electron to share. But in a dative bond, one atom is more generous and provides both electrons. The other atom simply receives the shared pair. Even though this sounds one-sided, the bond works just like any other covalent bond.
Terminology: Dative bonds are also called coordinate covalent bonds. Even though one atom gives both electrons, it still forms a covalent bond, which means the electrons are shared between the atoms. This bond behaves like a regular covalent bond in all other ways.
Formation
Lone Pair Donation: Dative bonds happen when one atom already has a pair of unused electrons (called a lone pair), and another atom really needs electrons to complete its shell. The first atom donates that pair to form a bond with the second atom.
Donor and Acceptor Roles: The atom that gives away the lone pair is called the donor. The one that accepts the pair is called the acceptor. The acceptor usually has an empty space in its outer shell and wants to feel more stable.
Behaves Like Covalent Bond: Once the dative bond is formed, it is just as strong and useful as any regular covalent bond. You can’t even tell the difference by just looking at the bond—they look and act the same in a molecule.
Examples
Hydronium Ion (H₃O⁺): In this example, a water molecule (H₂O) uses its lone pair of electrons on oxygen to form a bond with a hydrogen ion (H⁺). This creates a new molecule called the hydronium ion, H₃O⁺.
Diagram Reference – H₃O⁺: In Rajah 5.28, you can see the oxygen atom in water giving its lone pair to a hydrogen ion. This makes a third bond between oxygen and hydrogen, and the whole molecule now has a positive charge.
Ammonium Ion (NH₄⁺): Here, ammonia (NH₃) uses the lone pair on its nitrogen atom to bond with a hydrogen ion (H⁺). This forms the ammonium ion (NH₄⁺) through a dative bond.
Diagram Reference – NH₄⁺: In Rajah 5.29, nitrogen donates a lone pair to a hydrogen ion, creating a fourth N-H bond. The result is the positively charged ammonium ion.
Lewis Acid-Base Interpretation
Lewis Base Role: In this type of bond, the atom or molecule that donates the lone pair (like water or ammonia) is called a Lewis base because it gives electrons.
Lewis Acid Role: The hydrogen ion (H⁺), which accepts the pair of electrons, is called a Lewis acid because it receives the electrons.
Bond Formation Benefit: The main reason this bond happens is to make the electron-deficient atom (the one missing electrons) more stable by helping it get a full shell of electrons.
Bond Equivalence and Charge Distribution
Bond Uniformity: After the dative bond is made, all the bonds in molecules like H₃O⁺ or NH₄⁺ look exactly the same—even the one made through dative bonding. You can’t tell them apart by just looking at the molecule.
Charge Delocalization: The positive charge from the hydrogen ion is not stuck to just one atom. Instead, it is spread out across the whole ion, making the whole structure more balanced and stable.
Summary
Key Concept: A dative bond is a bond where one atom does all the giving. It donates both electrons to form a bond using a lone pair, while the other atom just accepts.
Formation Context: This kind of bond usually forms when one atom has extra electrons it isn’t using, and another atom really needs more electrons to be stable.
Important Examples: Two common examples where this happens are the hydronium ion (H₃O⁺) and the ammonium ion (NH₄⁺). Even though they form using a dative bond, they behave just like normal molecules afterward.
Chemical Bonding Map: Dative bonds are a key part of the big picture in chemical bonding. They help explain how atoms can be flexible and creative in sharing electrons to form stable compounds.