The yellow (CH3NH3)4PbI6•2H2O(P21/n, a = 10.421(3) Å, b = 11.334(2) Å, c = 10.668(2) Å, β = 91.73(2)°, Z = 2) contains isolated PbI64− octahedra, CH3NH3+ cations of two types and H2O molecules. The cations and the water molecules are hydrogen-bonded to form [Formula: see text] units arranged in centrosymmtric [Formula: see text] pairs; the centres of these composite units and the Pb atoms form a distorted NaCl-type lattice. The Pb—I bond lengths in the PbI64− anions are compared with those in [Pb(II)I6]ε complexes containing shared I atoms and the effect of the sharing on the bond lengths is discussed. A scheme is proposed for the extensive three-dimensional hydrogen bonding in the structure.

The aim of this investigation is to provide a classification and examples of N—H …π (and also O—H …π) bonds to the aromatic π systems in organic ammonium tetraphenylborates that would serve as reference for X—H …π(arene) bonds in general. To this end the crystal structures of the tetraphenylborates of the following cations have been determined: Me3NH+, Et3NH+, quinuclidinium, DabcoH+, Et(iso-Pr)2NH+ (monohydrate), (Ph3B)NH[—(CH2)2—]2NHMe+ (Me2CO solvate), Me2NH2+ (MeCN and Et2CO solvates), Et2NH2+, (iso-Pr)2NH2+, azoniacycloheptane, guanidinium (monohydrate), MeNH3+, EtNH3+, and 1-adamantammonium (monohydrate). These structures contain a variety of normal, bifurcated, and trifurcated N—H …π bonds as well as normal O—H …π bonds to the phenyl groups of the anion. The X—H …π bonds will form whenever opportunity arises, even though the result may be unfavourable bonding geometry. Branched bonds and orientational disorder represent compromise solutions in situations where the H(X) hydrogens are presented with competing phenyl acceptors or where the general organization of the crystal structure offers unfavourable bonding conditions to these hydrogens. The distributions of the distances from X or H(X) to the centre of the phenyl-ring skeleton are analyzed in detail, as are also those of the mean X … C and H(X)… C distances to the ring carbons.

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The recent work of Buckingham and Clark concerning the mechanism of acid-catalyzed hydrolysis of Co(III)-chelated carbonato complexes has emphasized the importance of preprotonation of the carbonate ligand prior to opening of the chelate ring.* 1•2 Their determinations of acidity constants for a number of bicarbonate chelates (Ka = 0.42-2.3 M) formed on protonation of the corresponding carbonate species in acidic solution show previous workers have substantially overestimated the acidity of the protonated carbonyl oxygen.3 As several Co-( ) carbonate chelates are known to be remarkably stable toward hydrolysis under acidic conditions,2•4 the isolation of the chelating bicarbonate species from solutions of moderate acidity should be possible. Structurally characterized monodentate bicarbonate complexes are rare,5 and are generally prepared by nucleophilic attack of metal-coordinated hydroxide on CO2 in nonaqueous solvents. To our knowledge, there is only one structurally characterized chelated bicarbonate complex, [Rh(H)2(P(i-Pr)3)202C0H], which was formed from reaction of a rhodium hydride complex with wet CO2 in hexane as solvent.6 Herein we report the synthesis and characterization of the chelated bicarbonate complex [Co(tepa)02COH] (€104)2* 3H2O (1) (tepa = tris(2-(2-pyridyl)ethyl)amine7 ) formed by protonation of the parent carbonate complex [Co(tepa)02-C0]C104 (2) in acidic aqueous solution.