Conclusions. In present study, the aqueous solutions containing F -with same component and external condition, but prepared by both different paths, path 1 and path 2, exhibit a different NMR and UV-Vis signal. A detailed analysis finds that the path 1 solutions initially containing certain amount of M 2+ -F -CIPs whereas the path 2 solutions initially containing abundant M 2+ -F -outer-sphere complexes is responsible for the signal difference. Ab initio calculations show that CIPs are thermodynamically slightly more stable than the outer-sphere complexes but the transformation between CIPs and outer-sphere complexes is kinetically difficult due to the high energy barrier.