Bis(4-carboxylpyrazol-1-yl)acetic acid (H3bcpza) (2), obtained in a one-pot synthesis, contains carboxyl groups that differ in their pKa values. The ligand exhibits heteroscorpionate κ3-N,N,O-coordination whereupon the peripheral carboxyl groups are not involved in metal binding. The corresponding carbonyl complexes [Mn(H2bcpza)(CO)3] (4), [Re(H2bcpza)(CO)3] (5) and [Ru(H2bcpza)Cl(CO)2] (6a/6b) are partially soluble in water but readily soluble in PBS buffer.