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 -C 3 H 7 ) 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 -C 3 H 7 ) 4 ][Pt(CN) 4 ] and [Pt(CN- iso -C 3 H 7 ) 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 -C 3 H 7 ) 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 -C 3 H 7 ) 4 ][Pt(CN) 4 ] from water gives monoclinic crystals of a hexadecahydrate [Pt(CN- iso -C 3 H 7 ) 4 ][Pt(CN) 4 ]·16H 2 O. 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.