Draw the Lewis structure of the 1:1 adduct that forms in the Lewis acid-base reaction between antimony tribromide (SbBr3) and bromide ion (Br-). Explicitly draw all H atoms. Include all valence lone pairs in your answer. Do not include charges in your answer. They will not be considered in the grading. In cases where there is more than one answer, just draw one.
The Correct Answer and Explanation is:
Lewis Structure of the 1:1 Adduct of SbBr₃ and Br⁻:
Here is the correct Lewis structure:
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Br: Br—Sb—Br
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Br
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- The Sb (antimony) atom is at the center, bonded to four bromine atoms.
- Three of the Br atoms are from the original SbBr₃ molecule.
- One Br⁻ acts as a Lewis base, donating a lone pair to Sb.
- All Br atoms have three lone pairs of electrons shown as pairs of dots (: or ..).
Explanation
The reaction involves a Lewis acid-base interaction. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor.
Antimony tribromide (SbBr₃) has a trigonal pyramidal geometry and an incomplete octet (only six valence electrons around Sb in bonding). This makes it electron-deficient and allows it to act as a Lewis acid. The central Sb atom has an empty 5p orbital that can accept an electron pair.
The bromide ion (Br⁻) has a full octet and carries a lone pair on its valence shell, making it a strong Lewis base.
When Br⁻ donates its lone pair to SbBr₃, it forms a coordinate covalent (dative) bond. The result is a tetrahedral adduct, where Sb is now surrounded by four Br atoms — the original three plus the donated Br⁻. This gives antimony a complete octet (8 electrons: 4 bonding pairs = 8 electrons).
In the Lewis structure:
- Each Br has three lone pairs.
- The added Br⁻ binds to Sb using one of its lone pairs.
- The structure doesn’t include formal charges because they aren’t required here.
This adduct is stabilized because the central Sb atom achieves a stable octet configuration through the donation from the bromide ion. This behavior is typical of Group 15 halides, which often form such adducts with halide ions or other Lewis bases.
