Table 2. Calculated ionic radii for the alkaline earth metal ions ^a using the mean bond distances in the structure surveys, Table S2 (ESI), using r₀ = 1.34 Å, ref. 47, as the radius of coordinated oxygen; the number of crystal structures used to obtain these ionic radii are listed within subscript square brackets with the radii proposed by Shannon, ref. 34, given in italics.

Ionic radius/Å
Ion	CN = 6		CN = 7		CN = 8		CN = 9
Mg2+	0.724[368]	0.720			n/a	0.89
Ca2+	0.973[41]	1.00	1.064[22]	1.06	1.130[13]	1.12	1.17[0] b	1.18
Sr2+	1.128[7]	1.18	1.224[2]	1.21	1.274[13]	1.26	1.332[2]	1.31
Ba2+	1.292[4]	1.35	1.38[0] c	1.38	1.430[3]	1.42	1.489[7]	1.47

^aThe radius of the four-coordinated beryllium(ii) ion is r_Be(II),CN4 = 0.273_[13] and 0.27 Å; the radii of the four- and five-coordinated magnesium ions are r_Mg(II),CN4 = 0.567_[3] and 0.57 Å, r_Mg(II),CN5 = 0.664_[2] and 0.66 Å; the radius of the ten-coordinated barium(ii) ion is r_Ba(II),CN10 = 1.511_[1] and 1.52 Å; no hydrate or solvate data is available for the radium(ii) ion, but Shannon lists r_Ra(II),CN8 = 1.48 Å.

^bEstimated radius derived from a second-degree polynomial with a slope (r_{Ca(II),CN9(est)} = –0.0125 CN² + 0.2535 CN – 0.098) comparable to that of the strontium (6 ≤ CN ≤ 9) and barium ions (CN = 6, 8, 9).

^cEstimated radius derived from a second-degree polynomial with a slope (r_{Ba(II),CN7(est)} = –0.0097 CN² + 0.2043 CN + 0.414) comparable to that of the calcium (6 ≤ CN ≤ 8) and strontium (6 ≤ CN ≤ 9) ions.