Complexes of group 2 metals with ligands other than water

     In this section we consider complexes formed in aqueous solution in which the metal centre is coordinated by O- and N-donor ligands to give cationic species. Two important ligands are [EDTA]^4-  and [P₃O₁₀]^5-. Both form water-soluble complexes with Mg²⁺‏ and the heavier metal ions, and are sequestering agents used in water-softening to remove Mg²⁺and Ca²⁺‏ ions. Macrocyclic ligands, including crown ethers and cryptands  form stable complexes with Mg²⁺‏, Ca²⁺‏, Sr²⁺‏ and Ba²⁺‏. In an analogous manner to that noted for group 1 cations, selectivity corresponding to matching of cation and ligand-cavity sizes is observed. Thus, values of the stability constants for complexation with cryptand-222 (cavity radius 140 pm) in water follow the sequence Ba²⁺> Sr²⁺‏ » Ca²⁺> Mg²⁺‏.   An important class of macrocyclic ligands are the porphyrins and the parent compound is shown in Figure 1.1a; deprotonation of the twoNHgroups of a porphyrin gives a dianionic porphyrinato ligand. Chlorophylls, the pigments in green plants involved in photosynthesis, are porphyrinato derivatives containing Mg²⁺‏ coordinated within a square planar array of the four N-donor atoms. The structure of chlorophyll a is shown in Figure 1.1b; the extensive conjugation in the ring system means that the molecule absorbs light in the visible region (λ _max 660 nm) and this initiates a complicated series of reactions involving other systems containing Mn or Fe. Note that it is the ligand (not Mg²⁺)that is involved in these redox reactions.