M^II solution Chemistry

The aqua ions M²⁺ are more acidic than those in similar periods in group 2. Zinc (such as Be) is amphoteric, dissolving at high pH to form [Zn(OH)₄]^2-. Other than these ions are not amphoteric as they have slight tendency to complex with the hard ion OH^-, but Hg²⁺ is extremely strong protolyzed and readily precipitates like HgO unless complexing ligands are exist.

All the ions can create strong complexes, the entire formation constants for tetrahedral [ML₄] species (example [HgCl₄]^{2 -}) with a selection of ligands L being displayed in Table 1. There is an increasing trend to 'soft' class b behavior in the order Zn<Cd<<Hg. Complexes with Hg²⁺ are between the strongest known with some element. Additionally to the [ML₄] complexes, mercury can create linear [HgL₂] and occasionally [HgL₃]. Like in the solid compounds, these tendencies indicate a pronounced tendency to covalent bonding; on grounds of size alone, the large Hg²⁺ ion could maintain a coordination number of six or even eight.