In Mechanisms of Inorganic Reactions by Fred Basolo and Ralph G. Pearson, a passage discusses the history of coordination chemistry. Ammoniates had been assumed to be analogous to hydrates. They assess the comparision as,
This analogy is essentially correct for in hydrates, except for interstitial and anion water, the water is coordinated to the central metal ion. [emphasis added]
What is the meaning of anion water, as used in this context?