An example is boron trifluoride (BF3), whose boron atom has a vacant orbital that can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia (NH3).
ɐn ɛɡzˈæmpəl ɪz bˈoːɹɑːn tɹˈaɪflʊɹɹˌaɪd bˌiːˈɛf θɹˈiː, hˌuːz bˈoːɹɑːn ˈæɾəm hɐz ɐ vˈeɪkənt ˈɔːɹbɪɾəl ðæt kæn fˈɔːɹm ɐ kˈoʊvələnt bˈɑːnd baɪ ʃˈɛɹɪŋ ɐ lˈoʊn pˈɛɹ ʌv ᵻlˈɛktɹɑːnz ˌɔn ɐn ˈæɾəm ɪn ɐ bˈeɪs, fɔːɹ ɛɡzˈæmpəl ðə nˈaɪtɹədʒən ˈæɾəm ɪn æmˈoʊniə ˌɛnˈeɪtʃ θɹˈiː.