Rates of Water Exchange in 2,2′-Bipyridine and 1,10-Phenanthroline Complexes of Co^II and Mn^II

The rates and activation parameters of water exchange at pH 3.0 have been determined using variable temperature 17O NMR spectroscopy for four Co^II complexes and one Mn^II complex: [Co(bpy)(H₂O)₄]²⁺, [Co(bpy)₂ (H₂O)₂]²⁺, [Co(phen)-(H₂O)₄]²⁺, [Co(phen)₂ (H₂O)₂]²⁺, and [Mn(bpy)(H₂O)₄]²⁺ (bpy = 2,2′-bipyridyl and phen = 1,10-phenanthroline). Substitution of aquo ligands with 1,10-phenanthroline or 2,2′-bipyridyl leads to an increase in the rate of exchange in the manganese complexes, from k²⁹⁸ (1.8±0.1)×107s-1 for [Mn(H₂O)₆]²⁺ to (7.2±0.3)×107 s^-1 for [Mn(phen)₂ (H₂O)₂]²⁺, whereas the trends are more complex for the cobalt complexes. We have used the new data in conjunction with literature data for similar complexes to analyse the effect of M-OH₂ distance and degree of substitution.