= Solution
The nitrogen-centered <highest occupied molecular orbital> of <ammonia> supplies a <lone pair>; planar <borane> has an empty out-of-plane $p$-derived <lowest unoccupied molecular orbital>. Approach along this acceptor axis gives constructive <orbital overlap> and donation of the nitrogen pair into the boron-centered acceptor. The resulting \b[N-to-B <coordinate covalent bond> forms the <ammonia borane> adduct].
The occupied <molecular orbital> formed by this interaction is bonding, while the corresponding antibonding combination remains empty. <Borane> pyramidalizes as coordination increases from three to four. <Ammonia> can use its existing outward-pointing lone pair; its inversion to the planar transition geometry is not a prerequisite. The <Lewis base> and <Lewis acid> descriptions summarize the donor and acceptor roles, while the frontier <molecular orbitals> explain their directional interaction.
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