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?

To understand how a ligand can be both sigma donating and pi donating, let's first understand what these terms mean:

1. Sigma donating: A ligand that donates electron density to a metal center through a sigma bond is known as a sigma donor. In simple terms, it donates electrons to the metal center by forming a covalent bond, where the electron pair is shared between the ligand and the metal.

2. Pi donating: A ligand that donates electron density to a metal center through a pi bond is known as a pi donor. In this case, the electron density is supplied by the pi orbitals of the ligand, which overlap with the vacant orbitals of the metal center.

Now, to find a ligand that exhibits both sigma donating and pi donating properties, we can look at carbon monoxide (CO) as an example. CO has a lone pair of electrons on the carbon atom, which can act as a sigma donor and form a covalent bond with the metal center. Additionally, the pi antibonding molecular orbital of CO can overlap with the vacant metal d-orbitals, making it a pi donor as well.

So, in the case of CO, it can act as both a sigma donor and a pi donor towards a metal center. Whether CO acts as a Lewis acid or a Lewis base depends on the specific reaction and the context in which it is used. In general, CO can behave as a Lewis base by donating its electron pair to a vacant metal orbital, but it can also behave as a Lewis acid by accepting electron pairs from other ligands.