Question

Which of the following contains a coordinate covalent bond? (Hint: How many covalent bonds would you expect the central atom (underlined) to form?) (a) \(\mathrm{H}_{2} \underline{\mathrm{O}}\) (b) \(\mathrm{BF}_{4}^{-}\) (c) \(\mathrm{H}_{3} \mathrm{O}^{+}\)

Short answer

\(\mathrm{BF}_{4}^{-}\) and \(\mathrm{H}_{3} \mathrm{O}^{+}\) contain a coordinate covalent bond.

Step-by-step solution

Understanding Coordinate Covalent Bonds

A coordinate covalent bond involves one atom donating both electrons to form a bond, unlike a regular covalent bond where each atom contributes one electron. This happens typically when one atom has a lone pair that it can share with an electron-deficient atom.

Analyzing Water: \(\mathrm{H}_{2} \underline{\mathrm{O}}\)

In water, the oxygen atom, which is the central atom, forms two covalent bonds with two hydrogen atoms. Oxygen also has two lone pairs, but all the bonds are normal covalent bonds where each participating atom donates one electron. Therefore, \(\mathrm{H}_{2} \mathrm{O}\) does not contain a coordinate covalent bond.

Analyzing Tetrafluoroborate Ion: \(\mathrm{BF}_{4}^{-}\)

In \(\mathrm{BF}_{4}^{-}\), the boron is the central atom bonded to four fluorine atoms. Boron typically forms three bonds, and in this structure, the fourth bond to one of the fluorine atoms is formed by a coordinate bond, where the fluorine donates an extra lone pair to complete the boron's octet. Thus, \(\mathrm{BF}_{4}^{-}\) contains a coordinate covalent bond.

Analyzing Hydronium Ion: \(\mathrm{H}_{3} \mathrm{O}^{+}\)

In the hydronium ion, the oxygen atom forms bonds with three hydrogen atoms. Oxygen normally forms two bonds, but the third hydrogen forms a bond through a coordinate covalent bond using one of oxygen's lone pairs. Hence, \(\mathrm{H}_{3} \mathrm{O}^{+}\) contains a coordinate covalent bond.

Key concepts

Covalent Bonds

Covalent bonds are the glue that holds molecules together, involving the sharing of electrons between atoms. This electron sharing can be equal or unequal, depending on the atoms involved. It usually occurs between non-metal atoms. In a standard covalent bond, each atom contributes an electron to form the bond.

• Electrons circulate in pairs around the atoms involved.
• These pairs form a stable balance of attractive and repulsive forces between atoms.

The classic example is the hydrogen molecule, \(H_2\), where each hydrogen atom shares one electron to form a pair. This bond is crucial in forming stable molecules and compounds.

Lone Pairs

Lone pairs are valence electron pairs that are not shared with another atom. Instead, they belong entirely to one atom. These pairs play a crucial role in determining the shape and reactivity of molecules. In terms of structure, lone pairs are found in the outermost electron shell of an atom.

• They help in maintaining the geometry of the molecule.
• They can participate in forming coordinate covalent bonds by donating to electron-deficient atoms.

For instance, in a water molecule \(H_2O\), oxygen has two lone pairs that influence the molecule's bent shape and give it its unique properties.

Electron-Deficient Atom

An electron-deficient atom is one that does not have a complete octet in its outer shell. These atoms are often found in molecules where there is potential for forming coordinate covalent bonds. Boron is a common example, like in \(BF_3\), which doesn't achieve a full octet naturally.

• These atoms seek additional electrons to achieve greater stability.
• Such atoms tend to accept lone pairs from other atoms.

When an electron-deficient atom receives a lone pair from another atom, it forms a coordinate covalent bond, completing its octet and achieving a more stable molecular structure.

Tetrafluoroborate Ion

The tetrafluoroborate ion, \(BF_4^{-}\), is an example of a molecule containing a coordinate covalent bond. Here, boron forms bonds with four fluorine atoms despite typically forming only three.

• Boron is the central atom, naturally electron-deficient.
• The fourth bond is formed by one fluorine atom donating a lone pair to boron.

This donation completes boron's octet, making the \(BF_4^{-}\) ion stable. Such interactions are crucial for understanding molecules with expanded octets or requiring additional electron sharing.

Hydronium Ion

The hydronium ion \(H_3O^+\) showcases another instance of a coordinate covalent bond formation. It forms when water, \(H_2O\), accepts a proton \(H^+\) to become \(H_3O^+\).

• Oxygen in water initially forms two covalent bonds and possesses two lone pairs.
• In \(H_3O^+\), one lone pair forms a coordinate covalent bond with the hydrogen ion.

This additional bond allows oxygen to engage in bonding with three hydrogens, despite its natural tendency to bond with two. Such formations are essential for understanding the behavior of acids and bases in solution.