CRYSTAL-CHEMISTRY OF TETRARADIAL SPECIES .6. EMBARRAS-DE-RICHESSES - LITHIUM CATION COORDINATED BY 8 O-H...PHENYL BONDS

Lithium tetraphenylborate tetrahydrate, [Li(OH2)(4)]BPh(4) (LiTBw; tetragonal, I4(1)/a, a = 27.566(2) Angstrom, c = 12.228(2) Angstrom, Z = 16) is remarkable in that the four H2O molecules coordinating the Li+ ion all form O-H...pi hydrogen bonds to the phenyl groups of the anion. LiTBw thus appears to be the first reported example of such an exhaustive O-H...pi coordination and can be described as a 3-dimensional, completely H-bonded polymer in which all the H2O hydrogens are bonded to phenyl groups and all the phenyl groups are involved in O-H...phenyl bonds. Six of the O-H...phenyl bonds are essentially normal and two are highly bent, possibly bifurcated. The existence of the H-bonds has been corroborated from variable-temperature FT-ir spectra of weakly deuterated LiTBw. The O-Li-O angles in the LiO4 coordination tetrahedron (of symmetry C-1) exhibit large departures from the tetrahedral angle, two of the angles bisected by a quasi-S-4 axis being 118.3 degrees and 122.5 degrees, respectively. An ab initio (6-31G*) investigation of the Li(OH2)(4)(+) and Be(OH2)(4)(2+) cations has shown that such large O-M-O angles are to be expected even in the free ions and are thus not necessarily the result of packing effects. A detailed comparison with several Li(OH2)(4)(+) and Be(OH2)(4)(2+) salts provides a rationale for the observed M(OH2)(4)(n+) (M = Li, Be) geometries.