Electronic spectra and crystal structures of [NH4]6[XMo9O32]·6H2O (X = NiIV or MnIV)

Abstract

The crystal structures of the isomorphous compounds [NH₄]₆[XMo₉O₃₂]·6H₂O, (X = Ni or Mn) which crystallize in the rhombohedral space group R32, Z= 1 with a= 10.084(2)Å, α= 104.31(2)° for X = Mn and a= 10.087(4)Å, α= 104.35(3)° for X = Ni have been determined. The stereochemistry of the oxyanions is similar to that described previously for the manganese(IV) compound, with the Mn–O bond length [1.897(3)Å] being slightly longer than the Ni–O distance [1.872(2)Å], and both metal ions conforming to a crystallographic 32 ligand co-ordination geometry. The low-temperature polarized electronic spectra have been recorded, that of the manganese(IV) compound suggesting a ligand-field splitting parameter Δ= 22 500 cm^–1, with the ⁴T₂ excited state exhibiting only a small trigonal splitting. The Racah interelectron repulsion parameter B is substantially reduced from the free-ion value, implying considerable covalence in the Mn^IV–O bonding. Although the spin-forbidden transitions cannot be fitted accurately using a single Racah parameter C, it is apparent that this undergoes only a modest reduction from the free-ion value. The electronic spectrum of the nickel(IV) compound is poorly resolved and suggests a ligand-field splitting parameter Δ≈ 21 000 cm^–1.