Table 1.
Chemical properties of selected divalent metal ions. The second ionization potential (eV) has proven to be a useful proxy for the polarizability of biologically relevant metal ions. The ionic radii are adapted from Irving and Williams [23] and all other values are adapted from Andreini et al. [24]. According to the hard and soft acids and bases (HSAB) classification, the metal ions (Lewis acids) are divided into hard (H), borderline (B), or hard-borderline (HB) types. The most common coordination geometries are abbreviated as follows: Td, tetrahedral; Oh, octahedral; D3h, trigonal bipyramidal; D4h, square planar; C4v, square pyramidal; IR, irregular.
| Ca | Mg | Mn | Fe | Co | Ni | Cu | Zn | |
|---|---|---|---|---|---|---|---|---|
| Ionic radius (Å) of the free metal ion | 0.99 | 0.66 | 0.78 | 0.76 | 0.74 | 0.73 | 0.72 | 0.72 |
| Second ionization potential (eV): | 11.87 | 15.04 | 15.64 | 16.18 | 17.06 | 18.15 | 20.29 | 17.96 |
| HSAB classification: | H | H | HB | B | B | B | B | B |
| Ligands: | O | O | N, O, S | N, S, O | N, O, S | S, N | S, N | N, S, O |
| Coordination numbers: | 6,7,8 | 6 | 4, 5, 6 | 4,5,6 | 4,5,6 | 4,5,6 | 4,5,6 | 4,5,6 |
| Dominating geometries: | Oh, IR, IR | Oh | Td, D3h, Oh | Td/D4h, C4v/D3H, Oh | Td, C4v/D3h, Oh | Oh, Td, D4h | Td/D4h, C4v/D3h, Oh | Td, C4v/D3h, Oh |