Electronic, infrared, and resonance-Raman spectroscopic studies of the linear-chain, halogen-bridged, mixed-valence complexes [Pt(NH3)4][Pt(NH3)4X2]Y4(X = Br or I, Y = HSO4, ClO4, or BF4). The X-ray crystal structures of [Pt(NH3)4][Pt(NH3)4Br2][HSO4]4 and [Pt(NH3)4][Pt(NH3)4I2][HSO4]3[OH]·H2O

Abstract

The complexes [Pt(NH₃)₄][Pt(NH₃)₄Br₂]Y₄, Y = HSO₄, ClO₄, or BF₄, and [Pt(NH₃)₄][Pt(NH₃)₄I₂][HSO₄]₃[OH]·H₂O have been prepared by partial aerial oxidation of Magnus green in 50% H₂SO₄ in the presence of KBr or KI as appropriate. The complexes are deeply coloured and highly dichroic, as is typical of linear-chain, halogen-bridged species. The crystal structures of the two [HSO₄]^– salts have been refined within the space groups P2₁/c(Z= 1) for the bromide and Pmcn(Z=4) for the iodide to final R factors of 0.0373 and 0.0484, respectively. In the case of the bromide, the [Pt^II(NH₃)₄][Pt^IV(NH₃)₄Br₂] chains are longitudinally disordered so that all the Pt atom sites comprise ½Pt^II+½Pt^IV and the axial bromine atoms half-occupy two positions 0.559 Å apart; the key bond lengths are Pt^IV–Br 2.467 Å, and Pt^II–Br 3.022 Å. The Pt^IIN₄ and Pt^IVN₄ square planes are eclipsed when viewed down the chain axis. In the case of the iodide, the [Pt^II(NH₃)₄][Pt^IV(NH₃)₄I₂] chains are completely ordered, with the bridging iodine atoms occupying only one site; the Pt^IV–I and Pt^II–I distances are 2.686 and 3.148 Å, respectively. The Pt^IIN₄ and Pt^IVN₄ square planes are, in this case, staggered by 46° when viewed down the chain axis. The intense resonance-Raman spectra of the complexes, obtained by irradiating within the contours of their Pt^IVâ†� Pt^II intervalence bands, are dominated by progressions (at most to 14 members) in the axial, totally symmetric, chain-stretching mode, ν₁, ν_sym(X–Pt^IV–X), as shown by polarisation and intensity studies. The results are discussed with respect to related ones for other localised mixed-valence complexes.