Answer
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In the localized electron model, the acetylide anion (C2^2-) can be described as having a triple bond between the two carbon atoms. Each carbon atom donates two valence electrons, and they form a σ bond and two π bonds between them. The localized model emphasizes the overlap of sp hybrid orbitals from each carbon atom to form the σ bond and the overlap of two p orbitals from each carbon atom to form the two π bonds.
In the molecular orbital model, the acetylide anion is described as having three molecular orbitals: one σ MO and two π MOs.
The σ MO is a bonding orbital, and the two π MOs are degenerate and accommodate the remaining four electrons of the anion. The molecular orbital model provides a more accurate description of the electron distribution across the entire molecule, showing the delocalization of electrons in π bonding.
