= Lewis acid
{c}
A <Lewis acid> accepts an <electron pair> to form a <coordinate covalent bond>. A <Lewis base> supplies that pair. This definition covers reactions without proton transfer: $\mathrm{BF_3+NH_3\longrightarrow F_3B\leftarrow NH_3}$ pairs the empty acceptor orbital of boron with the nitrogen lone pair.
Metal cations commonly act as <Lewis acids> by coordinating ligands. Coordination to a carbonyl oxygen can increase the electrophilicity of the carbonyl carbon, explaining many catalytic effects. <proton transfer> concerns proton donation; Lewis acidity instead describes electron-pair acceptance, so the two classifications answer different questions.
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