For each of the following pairs, identify the molecule or ion that is more likely to act as a ligand in a metal complex: (a) carbonic acid \(\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)\) or carbonate \(\left(\mathrm{CO}_{3}^{2-}\right),(\mathbf{b})\) water $\left(\mathrm{H}_{2} \mathrm{O}\right)\( or hydronium ion \)\left(\mathrm{H}_{3} \mathrm{O}^{+}\right),(\mathbf{c})\( phosphine \)\left(\mathrm{PH}_{3}\right)$ or phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\).

In carbonic acid (H2CO3), there are two O-H bonds and one C=O bond. Both oxygen atoms are electron-rich with lone pairs, but they are involved in O-H bonding, which makes them less likely to act as ligands. For the carbonate ion (CO3^2-), the three oxygen atoms are doubly bonded to the carbon atom, and each one has lone pairs available for bonding. As an anionic species, the carbonate ion is also more likely to interact with metal cations to form metal complexes. Therefore, the carbonate ion (CO3^2-) is more likely to act as a ligand in a metal complex than carbonic acid (H2CO3).

In water (H2O), there are two O-H bonds, resulting in two lone pairs available on the oxygen atom, which can potentially bond to metal centers. In the hydronium ion (H3O^+), there are three O-H bonds, leaving no lone pairs available on the oxygen atom. Additionally, the positive charge on the hydronium ion makes it less likely to act as a ligand. Therefore, water (H2O) is more likely to act as a ligand in a metal complex than the hydronium ion (H3O^+).

In phosphine (PH3), there is a lone pair on the phosphorus atom, available for bonding to metal centers. In phosphoric acid (H3PO4), there are three O-H bonds and one P=O bond. The oxygen atoms are involved in O-H and P=O bonding, leaving no available lone pairs for interaction with metal centers. Moreover, potential steric hindrance around phosphorus introduces challenges for neighboring metal atoms to bond with it. Consequently, phosphine (PH3) is more likely to act as a ligand in a metal complex than phosphoric acid (H3PO4). To summarize the results: - Pair (a): The carbonate ion (CO3^2-) is more likely to act as a ligand in a metal complex. - Pair (b): Water (H2O) is more likely to act as a ligand in a metal complex. - Pair (c): Phosphine (PH3) is more likely to act as a ligand in a metal complex.