Coordinate (dative) covalent bonding and the octet rule in Boron trifluoride:

A coordinate or dative covalent bond is a special type of covalent bond in which both shared electrons are contributed by only one of the two bonded atoms (the donor), while the other atom (the acceptor) contributes none but still gains a share of the electron pair. This typically happens when the acceptor atom has an incomplete octet and an empty orbital available to receive the pair, while the donor atom possesses a lone pair it can offer.

• In BF₃ (Boron trifluoride), boron forms three normal covalent bonds with three fluorine atoms, using all three of its valence electrons. This leaves boron with only 6 electrons around it in the Lewis structure — an incomplete octet — because boron has only three valence electrons to begin with and one empty 2p orbital remains after bonding.
• This electron deficiency makes BF₃ a strong Lewis acid (an electron-pair acceptor). When BF₃ reacts with a fluoride ion (F⁻), which carries a lone pair of electrons, boron's empty orbital accepts that entire lone pair, forming a new B–F coordinate covalent bond.
• The product is the tetrafluoroborate ion, BF₄⁻, in which boron is now surrounded by 4 bond pairs (8 electrons), completing its octet. Three of the B–F bonds are ordinary covalent bonds and one is a coordinate bond, though once formed all four B–F bonds in BF₄⁻ become indistinguishable in length and character.

This is the textbook example of an atom with an initially incomplete octet completing it by accepting, rather than mutually sharing, an electron pair. By contrast, in the formation of the hydronium ion (H₃O⁺) from water, the oxygen atom already has a complete octet (two bond pairs and two lone pairs) before it donates a lone pair to a proton; in the formation of ammonium (NH₄⁺) from ammonia, nitrogen likewise already has a complete octet (three bond pairs and one lone pair) before donating its lone pair to a proton; and carbon dioxide does not typically form a carbonate ion by simple coordinate bonding to complete an incomplete octet in the same sense — carbon in CO₂ already satisfies the octet rule through its double bonds. In all three of these alternative cases, the central or donor atom is not electron-deficient to begin with, which is precisely what distinguishes them from the unique boron case.