HEMICAL BONDING

Water has physical hemical properties that are very different from those of other solvents [1] and its role in enhancing the reactivity and selectivity of some organic reactions is still a debated question. Recent experimental studies [3e, 9] and computer simulations [10] seem to indicate, at least with respect to the rate enhancement of aqueous Diels Alder reactions, that the main effects are due to the enforced hydrophobic interactions and hydrogen bond interactions. 

H20,CjH70H (Figure 13), are stere(x hemically related, and contain direct metal-metal bonds of lengths 2.386(1), 2.618(5), and 2.936(2) A, respectively. On the basis of a single-bond covalent radius for rhodium(iii). 

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction patterns of the active materials with those for the model compounds. How the 8-Ni(OH)2 in battery electrodes differs from the model compound is discussed in Section 5.3.I.3. In recent years, the arsenal of in situ techniques for electrode characterization has greatly increased. Most of the results confirm Bode s reaction scheme and essentially all the features of the proposed a/y cycle. For instance, recent atomic force microscopy (AFM) of o -Ni(OH)2 shows results consistent with a contraction of the interlayer distance fiom 8.05 to 7.2 A on charge [61-63]. These are the respective interlayer dimensions for the model a-Ni(OH)2 and y-NiOOH compounds. Electrochemical quartz crystal microbalance (ECQM) measurements also confirm the ingress of alkali metal cations into the lattice upon the conversion of a-Ni(OH)2 to y-NiOOH [45,64,65]. However, in situ Raman and surface-enhanced Raman spectroscopy (SERS) results on electrostretching modes that are consistent with a weakening of the O-H bond when compared with results for the model a- and 8-Ni(OH)2 compounds [66]. This has been ascribed to the delocalization of protons by intercalated water and Na ions. Similar effects have been seen in passive films on nickel in borate buffer electrolytes [67]. 

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