Effects of Pt···Pt Bonding, Anions, Solvate Molecules, and Hydrogen Bonding on the Self-Association of Chugaev's Cation, a Platinum Complex with a Chelating Carbene Ligand

Five salts, [(C4H9N4)PtII(CNCH3)2](BPh4)·CH3OH, [(C4H9N4)PtII(CNCH3)2](PF6)·CH2Cl2, [(C4H9N4)PtII(CNCH3)2]Cl·4H2O,
[(C4H9N4)PtII(CNCH3)2]Br·3.5H2O, and [(C4H9N4)PtII(CNCH3)2]Cl·0.1H2O, have been crystallized and examined by
single crystal X-ray diffraction. While the internal structure of the cation is similar in all salts, the interactions
between cations vary in the different salts. Yellow [(C4H9N4)PtII(CNCH3)2](BPh4)·CH3OH and red [(C4H9N4)PtII(CNCH3)2](PF6) form face-to-face dimers with Pt···Pt separations of 3.6617(6) and 3.340(2) Å, respectively. In the
latter, hydrogen bonding of the chelating ligand to adjacent anions facilitates the close approach of pairs of cations.
The salts [(C4H9N4)PtII(CNCH3)2]Cl·4H2O, [(C4H9N4)PtII(CNCH3)2]Br·3.5H2O, and [(C4H9N4)PtII(CNCH3)2]Cl·0.1H2O
form columnar structures with Pt···Pt separations that range from 3.2514(5) to 3.5643(6) Å. The water molecules
and anions surround these columns and form bridges between neighboring columns. The electronic spectra of
aqueous solutions of [(C4H9N4)PtII(CNCH3)2]Cl·4H2O show spectral changes upon increasing concentrations of the
platinum complex that are indicative of the formation of a dimer in solution with an equilibrium constant for dimerization
of 23(1).