A coordinate covalent bond could be formed between?

Analyze the question A coordinate (dative) covalent bond forms when one atom supplies both electrons of the shared pair. So we need one species with a lone pair (donor) and another with an empty orbital (acceptor).

Concept

• Donor: has a lone pair, e.g. NH₃, H₂O, Cl⁻, PCl₃ (P has a lone pair).
• Acceptor: has an electron-deficient centre, e.g. BCl₃, AlCl₃ (only 6 electrons around the central atom), H⁺ (empty 1s orbital).

Check each option

• A. NH₃ and PCl₃: Both are donors (each has a lone pair). Neither has an empty orbital to accept, so no coordinate bond forms.
• B. BCl₃ and AlCl₃: Both are electron-deficient acceptors. Neither has a lone pair on the central atom to donate, so no coordinate bond forms between them.
• C. BCl₃ and NH₃: NH₃ has a lone pair on nitrogen, and BCl₃ has an empty p-orbital on boron. The nitrogen donates its lone pair to boron, forming H₃N→BCl₃, a classic coordinate covalent bond.
• D. H⁺ and AlCl₃: H⁺ is an acceptor and AlCl₃ is also an acceptor. Neither can donate a pair of electrons from the central atom, so no coordinate bond forms this way.

Tip: Look for a donor + acceptor pair. Whenever you see NH₃ (or H₂O) paired with BCl₃, AlCl₃ or H⁺, think coordinate bond. If both species are donors or both are acceptors, no dative bond forms. Common examples: NH₄⁺ (NH₃ + H⁺), H₃O⁺ (H₂O + H⁺), and H₃N→BF₃.