Explain the following observations: in general Cu(II) has a higher stability constant (K1) for complex formation than the equivalent Ni(II) complexes in aqueous solution (starting material is the hexa-aqua complex) but [Cu(OH2)6] 2+ has a faster rate of water exchange (k ≈ 1 × 109 s −1 ) than [Ni(OH2)6] 2+ (k ≈ 3 × 104 s −1 ).