A double zwitterionic Schiff base was synthesized using the amino acid L-histidine and o-vanillin (2-hydroxy-3methoxy benzaldehyde). Both the phenol and carboxyl groups are deprotonated, and the imine nitrogen atom and histidineimidazole ring are protonated to give the double zwitterion with an intramolecular (imine)N-H þ 3 3 3 -O(phenol) hydrogen bond (ketoamine form). Such a ketoamine form in a double zwitterion is assumed in the catalytic cycle of enzymatic transformations of amino acids with the cofactor (vitamin B6) pyridoxal-5 0 -phosphate (PLP). A high-resolution, lowtemperature, single-crystal X-ray diffraction data set on N-o-vanillylidene-L-histidine (also named 3-methoxysalicylidene-Lhistidine or N-(2-oxy-3-methoxy-benzylidene)-L-histidine, OVHIS) is used in the analysis of molecular electrostatic properties and intermolecular interactions. All oxygen atoms in the molecule (four in total) are mutually almost coplanar and located (externally) on the same side of molecule. These four O atoms carry significant negative charge and form a large area of strong negative electrostatic potential (appropriate for bonding to a metal atom). The protonated and, thus, positively charged imidazole ring is situated on the opposite side of the molecule from the area of the O atoms. Consequently, the OVHIS molecule is highly polarized and has a very high molecular dipole moment of 42.4 D in the solid state (calculated from experimental X-ray data). Two strong intermolecular charge-assisted N-H þ 3 3 3 -O hydrogen bonds (with H 3 3 3 O distances of 1.61 A ˚) together with other D-H 3 3 3 O interactions (D = N, C) contribute to a large molecular dipole enhancement which occurs upon crystallization. The topologies of the bonding within the molecule as well as its hydrogen bonds have been investigated according to Bader's quantum theory of atoms in molecules (QTAIM). 1 H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) was used to confirm the zwitterionic structure in the solid state.

The title compound, [Ni(C(28)H(30)N(3)O(2)PS)], crystallizes with two independent molecules in the asymmetric unit. The Ni(II) atoms are in significantly deformed square-planar environments formed by an ONNP donor set from a thiosemicarbazide-based tetradentate ligand. The Ni(II) atom and the ONN donor atoms are nearly coplanar, while the P atom deviates from their mean planes by 0.278 (4) and 0.202 (4) Angstrom for the two independent molecules. The P-containing chelate rings are remarkably non-planar, adopting a boat conformation, which is unusual for chelate rings in transition metal complexes with thiosemicarbazide-based tetradentate ligands. The orientation of the ethoxy group bonded to this chelate ring is caused by an intramolecular C-H...pi interaction with the opposing phenyl ring. There are no hydrogen bonds; instead, numerous intermolecular C-H...pi interactions dominate in the crystal packing.

The experimental geometry obtained from single-crystal X-ray diffraction data for [Mn(CO) 4 (PH 2) 2 C:] + [1] is compared with the results of theoretical calculations made in the singlet ground state at the ab initio level by using HF and DFT methods, following similar procedures than those used for other organometallic complexes [2,3]. In addition to the structural computations, in order to theoretically quantify the highly electrophilic character showed experimentally by the carbene, further calculations were carried out involving the doublet ground state resulting from the addition of one electron to the cation. Electron affinities (EA) were found to be between 6.24 eV and 6.97 eV for the Mn complex, which confirmed the expectations. The effect of the ligands on EAs has been analyzed by replacing P-bonded H atoms by NH 2 ligands, whereas the effect of the metal fragment has been studied by replacing Mn + by Cr and also comparing with our previous results with Ru 2+ complexes, from which it seems clear that the effect of the positive charge on the metal dominates over the effect of changing the ligands to more-accepting ones.

The asymmetric unit of the title compound, (C12H18N3O4)2[ZnCl4]·CH3OH, consists of two Girard reagent-based cations, a tetra­chlorido­zincate anion and a mol­ecule of methanol as solvate. These components are inter­connected in the crystal structure by an extensive network of O—H⋯O, N—H⋯O, C—H⋯O, O—H⋯N, O—H⋯Cl, N—H⋯Cl and C—H⋯Cl hydrogen bonds. The shortest inter­molecular inter­action is realized between the cation and anion [H⋯Cl = 2.29 (5) Å; O—H⋯Cl = 167 (3)°]. C—H⋯O inter­actions also play a important role in the inter­connection of the cations.

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