Inner-sphere surface complex

– No water molecule is present between the ion or molecule and the surface functional group to which it is bound, and the bonding is covalent or ionic. Inner-sphere complexes can be monodentate (metal is bonded to only one oxygen) or bidentate (metal is bonded to two oxygens) and mononuclear or binuclear. Inner-sphere complexation can increase, reduce, neutralize, or reverse the charge on the
sorptive regardless of the original charge. Adsorption of ions via inner-sphere complexation can occur on a surface regardless of the original charge. Inner-sphere complexation is usually slower than
outer-sphere complexation and is often not reversible.