Table 5.

Ionic radii, coordination geometry, and pKa of water molecules coordinated to (Mg²⁺, Co²⁺, Mn²⁺, Cd²⁺, Fe²⁺, Ni²⁺, Zn²⁺, and Ca²⁺).

Metal ion
	Mg2+	Co2+	Mn2+	Cd2+	Fe2+	Ni2+	Zn2+	Ca2+
Ionic radius (Å)	0.86	0.89	0.81	0.95	0.92	0.83	0.88	1.1
Coordination geometrya	Oct	Oct	Oct	Oct	Oct	Oct	Oct	Oct
	Td	Td	Td	Td	Td	Td	Tda	Td
	Sq	Sq		HBP		TBP	TBP	HBP
	TBP	TBP
pKa of the water molecule	11.4	9.4	10.1	9.8	8.4	10.6	9.6	12.8

Td represents Tetrahedral; Sq represents Square planar; TBP represents Trigonal bipyramidal; Oct represents Octahedral; PBP represents pentagonal bipyramid; and HBP represents hexagonal bipyramidal.

^aEven though Zn²⁺ can form octahedral complexes, the majority of Zn²⁺ complexes are tetrahedral.