31P MAS-NMR spectra and spin-lattice relaxation time T1 in solid were measured on A4[Pt2I(P2O5H2) 4]·nH2O (A: Cs, Rb, K, Na and Li ). Continuous spectrum changes in both line and splitting widths were observed with changing the cation size and the number of crystalline water. Complexes with A: Cs (n=0) and Rb (n=2) afforded two kinds of 31P resonance lines with chemical shifts close to those in the Pt3+ and Pt2+ monomer complexes indicating a mixed valence structure. On the other hand, complexes with A: Na (n=2) and Li (n=4) yielded a broad single line suggesting the formation of paramagnetic Pt sites with an averaged valence of Pt2.5+. An analogous continuous change was observed in 31P T1 in consistent with the above analysis. The observed spectra and T1 depending on the cation size and the H2O number suggest valence fluctuation due to spin jumps in and between Pt-Pt units sensitively influenced by crystal packings and hydrogen bonding.
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