Photochemical studies of the alkylammonium molybdates. Part 7. Octahedral sites for multi-electron reduction of [Mo8O26(MoO4)2]8 –

Abstract

Single-crystal e.s.r. spectra of u.v.-irradiated octakis(methylammonium) decamolybdate(VI) dihydrate, [NH₃Me]₈[Mo₈O₂₆(MoO₄)₂]·2H₂O, show the formation of two distinct localized Mo^VO₅(OH) sites, resulting from a hydrogen-bonding proton transfer from [NH₃Me]⁺ to a bridging oxygen atom in the anion. Analysis of the e.s.r. parameters g, ^95,97Mo hyperfine structure, and ¹H superhyperfine structure indicates the direct participation of the H 1_s orbital in the semi-occupied molecular orbital with a spin population of 0.96–0.97 in MO 4d orbitals. From the direction of the maximum principal value of the ¹H superhyperfine tensor, which lies close to the MO^V⋯ H⁺(O) direction, two paramagnetic sites in the anion are determined and discussed in terms of the probability of the charge-transfer complex formation. One site has an arrangement of a MoO₄ group and two terminal oxygen atoms (cis-dioxo-group) about the central molybdenum atom and another is composed of one terminal and five bridging oxygen atoms. It is suggested that [Mo₈O₂₆(MoO₄)₂]^{8 –} suffers from the simultaneous multi-electron (up to four-electron) reduction photochemically.