Explain how a ligand can be both sigma donating and pi donating and give an example of a ligand that does both. Is this ligand a Lewis acid or a Lewis base?
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A ligand is a molecule or ion that forms a coordination complex with a central metal atom or ion. It can donate electrons to the metal atom/ion through different types of bonding interactions. When a ligand donates electron density to a metal through the overlap of its sigma (σ) molecular orbital with the metal's d-orbitals, it is considered as sigma (σ) donating. On the other hand, when a ligand donates electron density to the metal through the overlap of its pi (π) molecular orbitals with the metal's d-orbitals, it is called pi (π) donating.
An example of a ligand that can be both sigma and pi-donating is carbon monoxide (CO). Carbon monoxide has both sigma and pi bonds between the carbon and oxygen atoms, and it can donate electron density to a metal via both types of interactions.
Now, to determine whether CO acts as a Lewis acid or Lewis base, we need to look at its behavior in the complex. In general, a Lewis acid is a species that accepts electron pairs, whereas a Lewis base is a species that donates electron pairs. Since CO is donating electron density to the metal, it acts as a Lewis base in this context.
To summarize, carbon monoxide (CO) can be both sigma and pi-donating ligand, and in a coordination complex, it acts as a Lewis base.