Acetylene \(\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)\) can be produced from the reaction of calcium carbide \(\left(\mathrm{CaC}_{2}\right)\) with water. Use both the localized electron and molecular orbital models to describe the bonding in the acetylide anion \(\left(\mathrm{C}_{2}^{2-}\right)\)

The localized electron model is a bonding model based on Lewis structures and the concept of electron pairs localized between two atoms. This model allows us to represent the molecular connectivity and emphasize the electron pairs around the atoms.

Using the localized electron model for the acetylide anion (C₂²⁻), we consider the Lewis structure of two C atoms. Carbon has four valence electrons, so a total of 8 electrons if we use the two C atoms. The acetylide anion has a 2- charge, meaning that there are two extra electrons making it a total of 10 electrons. Based on these 10 electrons, we can form a triple bond between the two carbon atoms, which satisfies the octet rule for each carbon atom. The remaining two electrons are localized as a lone pair on the negatively charged C atom. The Lewis structure for the acetylide anion (C₂²⁻) can be represented as: ``` C C ≡ : ``` Here, the triple bond represents the sharing of six electrons between the two carbon atoms with two electrons representing a lone pair on the rightmost carbon atom (notated with a colon).

The molecular orbital model is a bonding model based on the principles of quantum mechanics that describe the distribution and energy of electrons in molecules as a combination of atomic orbitals. The molecular orbitals are a linear combination of atomic orbitals (LCAO) which can accommodate an electron pair.

We will consider the molecular orbital energies of the two carbon atoms in their ground state: 1s², 2s², 2p². The LCAO for the C₂²⁻ can be described by combining the atomic orbitals of the two carbon atoms. To analyze bonding, focus on the valence orbitals of the carbon atoms. The C₂²⁻ ion involves a linear arrangement of two carbon atoms and σ and π components. Starting with the planar σ orbitals, the 2s and one 2p orbital of each carbon atom can overlap to form σ and σ* orbitals. For the π orbitals, the remaining 2p orbitals of the carbon atoms can overlap side by side, resulting in two π and two π* orbitals. Lastly, considering the 2- negative charge of the acetylide anion, add two additional electrons to the π orbitals to obtain the correct electron configuration. C₂²⁻ Molecular Orbital Diagram: ``` π* ______ ______ π ______ ____]--> σ* _____-------> σ ______------- 2s 2p ``` The arrows indicate the filling of electrons (4 in the π orbitals and 2 in the σ orbitals) for both carbon atoms. The final configuration for the π orbitals has four electrons, while the σ component has a bonding and antibonding electron pair. In conclusion, we have described the bonding in the acetylide anion (C₂²⁻) using the localized electron model and the molecular orbital model. The localized electron model allowed us to represent the molecular connectivity and emphasize electron pairs, by featuring a triple bond and a lone pair on the rightmost carbon atom. The molecular orbital model described the distribution and energy of electrons as a combination σ and π orbitals, derived from the linear combination of atomic orbitals on the carbon atoms.