TABLE 2.

Ionic radii, coordination geometry, and pK_a of water molecules coordinated to Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Ca²⁺

Td, tetrahedral; Sq, square planar; TBP, trigonal bipyramidal; Oct, octahedral; PBP, pentagonal bipyramid; HBP, hexagonal bipyramidal.

	Metal ion
	Mg2+	Mn2+	Co2+	Ni2+	Zn2+	Cd2+	Ca2+
Ionic radius (Å)	0.86	0.81	0.89	0.83	0.88	0.95	1.1
Coordination	Oct	Oct	Oct	Oct	Oct	Oct	Oct
Geometry	Td	Td	Td	Td	Tda	Td	PBP
	Sq	Sq	Sq	TBP	TBP	HBP
	TBP	TBP
pKa of the water molecule	11.4	11.5	10.0	10.6	7.0	9.0	12.8

^a Although Zn²⁺ can form octahedral complexes, the majority of Zn²⁺ complexes are tetrahedral.