μs and 1 s. The enthalpy and entropy of activation, ΔH‡ and ΔS‡ can be obtained by observing the variation of rate constant with temperature. {| class="wikitable" style="text-align:center" |+ Kinetic parameters (at 25 °C) for water exchange: divalent ions, M2+ (aq) |- ! !!Be||Mg||V||Cr!!Mn!!Fe!!Co !!Ni!!Cu!!Zn||UO2 |- !Residence time (μs) | 0.001 ||2 ||0.00013 ||0.0032 ||0.0316 ||0.32 ||0.79 ||40 ||0.0005 ||0.032||1.3 |- !ΔH‡ (kJ mol−1) | || 43 ||69 ||13 ||34 ||32 ||33 ||43 ||23 || || |- !ΔS‡ (J deg−1mol−1) | || 8 ||21 ||-13 ||12 ||-13 ||-17 ||-22 ||25 || || |} Note the general increase in the residence time from vanadium to nickel, which mirrors the decrease in ion size with increasing atomic number, which is a general trend in the periodic table, though given a specific name only in the case of the lanthanide contraction. The effects of crystal field stabilization energy are superimposed on the periodic trend. {| class="wikitable" style="text-align:center" |+ Kinetic parameters (at 25 °C) for water exchange - trivalent ions, M3+ (aq) |- ! || Al|| Ti|| Cr|| Fe|| Ga|| Rh|| In|| La |- ! residence time (μs) | 6.3×106 ||16 ||2.0×1012 ||316 ||501 ||3.2×1013 ||50 ||0.050 |- !ΔH‡ (kJ mol−1) | 11 ||26 ||109 ||37 ||26 ||134 ||17 || |- !ΔS‡ (J deg−1mol−1) | 117 ||-63 ||0 ||-54 ||-92 ||59 || || |} Solvent exchange is generally slower for trivalent than for divalent ions, as the higher electrical charge on the cation makes for stronger M-OH2 bonds and, in consequence, higher activation energy for the dissociative reaction step, [M(H2O)n]3+ → [M(H2O)n-1]3+ + H2O. The values in the table show that this is due to both activation enthalpy and entropy factors. The ion [Al(H2O)6]3+ is relatively inert to substitution reactions because its electrons are effectively in a closed shell electronic configuration, [Ne]3s23p6, making dissociation an energy-expensive reaction. Cr3+, which has an octahedral structure and a d3 electronic configuration is also relatively inert, as are Rh3+ and Ir3+ which have a low-spin d6 configuration. Formation of complexes Metal aqua ions are often involved in the formation of complexes. The reaction may be written as pMx+(aq) + qLy− → [MpLq](px-qy)+ In reality this is a substitution reaction in which one or more water molecules from the first hydration shell of the metal ion are replaced by ligands, L. The complex is described as an inner-sphere complex. A complex such as [ML](p-q)+ may be described as a