Structural investigations of vapochromic behavior. X-ray single-crystal and powder diffraction studies of [Pt(Cn-iso-C₃H₇)₄][M(CN)₄] for M = Pt or Pd

We have synthesized [Pt(CN-iso-C₃H₇)₄][M(CN)₄] (M = Pt, Pd) and studied their reversible hydration and sorption properties with UV-vis, FT- IR spectroscopy, and X-ray diffraction. Powder diffraction studies show that anhydrous [Pt(CN-iso-C₃H₇)₄][Pt(CN)₄] and [Pt(CN-iso-C₃H₇)₄][Pd(CN)₄] crystallize in a tetragonal space group with nearly identical lattice constants. Gravimetric studies reveal that variable guest-host stoichiometries occur when solid [Pt(CN-iso-C₃H₇)₄][Pt(CN)₄] sorbs the guest at room temperature from the gas phase [water, 12.1 (1) molecules per formula unit, chloroform 6.0(1), methanol 8.0(1), and trifluoroethanol 4.1(1)]; these sorption processes are reversible. The unit cell distances in the tetragonal ab-plane expand dramatically when the solvent guests are sorbed, but changes along the c-axis (the M-M direction) are minimal. Crystallization of [Pt(CN-iso-C₃H₇)₄][Pt(CN)₄] from water gives monoclinic crystals of a hexadecahydrate [Pt(CN-iso-C₃H₇)₄][Pt(CN)₄] · 16H₂O. This salt consists of alternating cation/anion chains along b with an average Pt-Pt distance of b/2 = 3.1521(1) Å. The sixteen water molecules per formula weight interlace neighboring chains via H-bonding with each other and the CN^- ions of the Pt(CN)₄^2- units. The shifts in the UV-vis and IR spectra that occur when solvent guests are sorbed by the double complex salts are discussed in terms of the lattice expansions that are observed. A mechanism for the lattice expansions that accompany the sorption of guest molecules is proposed.