Transition-metal coordination mechanisms ligand field stabilization

Since the aquo ligand is usually very labile (see Section 3.1), the reaction proceeds through a dissociative SN mechanism with the formation of a reduced-coordination intermediate. The rupture of the M-OH2 bond occurs before the formation of the M-OH-M bond. However, in the case of transition metal ions highly stabilized in octahedral symmetry by the crystal field (Cr, Ni ), the lability of coordination water is very low and the condensation must proceed- via an associative mechanism with temporary formation of the [H302] bridging ligand. 

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