Carbonate is a pseudohalide ligand.  With a saturated pi-system, it has no pi-acceptor properties. With multiple electronegative elements, it is not strongly basic.  The latter is consistent with the pK’s of carbonic acid: pK = 6.77 and pK = 9.93.
To a single metal ion, carbonate is observed to bind in both unidentate (κ-) and bidentate (κ-) fashions.  In the covalent bond classification method, κ-carbonate is anX ligand and κ-carbonate is an X ligand. With two metals, the number of bonding modes increases because carbonate often serves as a bridging ligand.  It can span metal-metal bonds as in [Ru(CO)Cl], where again it functions as an (X) ligand.  More commonly all three oxygen centers bind, as illustrated by .  In such cases, carbonate is an LX ligand, providing 3e to each metal. More complicated motifs have been characterized by X-ray crystallography including {(VO)(μ-OH)(CO)}.