Manifestations of Weak O-H center dot center dot center dot F Hydrogen Bonding in M(H2O)(n)(B12F12) Salt Hydrates: Unusually Sharp Fourier Transform Infrared nu(OH) Bands and Latent Porosity (M = Mg-Ba, Co, Ni, Zn)

Eight M(H2O) n(Z) salt hydrates were characterized by single-crystal X-ray diffraction (Z(2-) = B12F122-): M = Ca, Sr, n = 7; M = Mg, Co, Ni, Zn, n = 6; M = Ba, n = 4, 5. Weak O-H center dot center dot center dot F hydrogen bonding between the M(H2O)(n)(2+) cations and Z(2-) resulted in room-temperature Fourier transform infrared (FTIR) spectra having sharp nu(OH) bands, with full widths at half max of 10-30 cm(-1), which are much more narrow than nu(OH) bands in room temperature FTIR spectra of most salt hydrates. Clearly resolved nu(asy)m(OH/OD) and nu(sym)(OH/OD) bands with Delta nu(OH) as small as 17 cm(-1) and Delta nu(OD) as small as 11 cm(-1) were observed (Delta nu(OX) = nu(asym)(OX) - nu(sym)(OX)). The isomorphic hexahydrates (R<(3))over bar> have two fac-(H2O)(3) sets of H2O ligands and nearly octahedral coordination spheres. They exhibited four resolvable nu(OH) bands, one nu(asym)(OH)/nu(sym)(OH) pair for H2O ligands with longer O(H)center dot center dot center dot F distances and one nu(asym)(OH)/nu(sym)(OH) pair for H2O ligands with shorter O(H)center dot center dot center dot F distances. The nu(OH) bands for the three H2O molecules with shorter, slightly stronger O(H)center dot center dot center dot F hydrogen bonds were broader, more intense, and red-shifted by ca. 25 cm(-1) relative to the bands for the three other H2O molecules, the first time that such small differences in relatively weak O(H)center dot center dot center dot F hydrogen bonds in the same crystalline hexahydrate have resulted in observable IR spectroscopic differences at room temperature. For the first time room temperature nu(OH) values for salt hexahydrates showed the monotonic progression Mg2+ > Co2+ > Ni2+ > Zn2+, essentially the same progression as the p Ka values for these metal ions in aqueous solution. A further manifestation of the weak O-H center dot center dot center dot F hydrogen bonding in these hydrates is the latent porosity exhibited by Ba(H2O)(5,8)(Z), Sr(H2O) (n,m)(Z), and Ca(H2O)(4,6)(Z). Finally, the H2O/D2O exchange reaction Co(D2O)(6)(Z) -> Co(H2O)(6)(Z) was ca. 50% complete in 1 h at 50 degrees C in N-2/17 Torr H2O(g).