Final answer:
The carbonate ion features three identical carbon-oxygen bonds due to resonance structures, while the formate ion has carbon-oxygen bonds with lengths influenced by resonance but to a lesser extent than carbonate.
Step-by-step explanation:
When comparing the carbon-oxygen bond lengths in various ions, it's important to consider the concept of resonance structures. For the formate ion (HCOO-), usually found in reactions with methanol, we witness a blend of a single and double carbon-oxygen bond, which gives an average bond length shorter than a single bond, but longer than a double bond. In contrast, the carbonate ion (CO32-) has three resonance structures that contribute equally to the final structure, resulting in all carbon-oxygen bonds having the same length, which is an average of single and double bond lengths.
Experiments have shown that in the carbonate ion, all three C-O bonds are identical due to resonance, which is the process where the double bond is delocalized over the three oxygen atoms, making the bond lengths equal. For the formate ion in methanol, we can expect a single carbon-oxygen bond length to be influenced by resonance as well, though to a different extent since it has only two oxygen atoms.