The complexes [MoX₄]²⁻ (M = Mo; X = O or S) exist as the monomeric tetrahedral species in aqueous alkaline solutions. Acidification of tetraoxomolybdate results in the condensation of the tetrahedral units via a series of polyoxomolybdates leading to the ultimate formation of the trioxide MoO₃. Heptamolybdate [Mo₇O₂₄]⁶⁻ is the first major polyanion of the acidification reaction. In contrast, acidification of tetrathiomolybdates leads to the formation of amorphous molybdenum trisulphide via a dinuclear Mo(V) complex. The formation of the dinuclear Mo(V) complex precludes the formation of any higher nuclearity Mo(VI)-S complexes in aqueous solution. Thus it is shown that the all-sulphur analogue of heptamolybdate [M0₇S₂₄]⁶⁻ does not exist in alkaline medium and also cannot be isolated from aqueous acidic medium

The organic diammonium salt N,N′-dibenzyl-N,N,N′,N′-tetramethylethylenediammonium dibromide dihydrate, (dbtmen)Br₂.2H₂O (1), was prepared by the reaction of N,N,N′,N′-tetramethyl-ethylenediamine (tmen) with benzyl bromide.1 crystallizes in the triclinic space group$$P\bar 1$$ with the following unit cell dimensions for C₂₀H₃₄Br₂N₂O₂ (M = 494.31):a = 8.6672(6) Å,b = 11.7046(8) Å,c = 11.7731(8) Å, α = 76.988(8)°, β = 88.978(8)°, γ= 76.198(8)γ,V= 1129.26(13) ų, Z=2. Three components, namely the (dbtmen)²⁺ dication, two bromide anions and two crystal water molecules constitute the structural arrangement of1. H₂O molecules are linked to bromide anions via O-H...Br hydrogen bonding interactions resulting in the formation of a four-membered O₂Br₂ cyclic dibromide. The O₂Br₂ units and the dications are arranged as alternating layers extending in the crystallographicbc plane. The arrangement of anions and cations may be viewed as a typical lamellar structure. The crystal water molecules can be removed by heating 1 at 140°C and the anhydrous dibromide thus formed can be fully rehydrated as evidenced by IR spectra and X-ray powder patterns.

The reaction of hydrated magnesium or calcium 4-nitrobenzoate (4-nba) generated in situ, with imidazole (Im) results in the formation of the complexes [Mg(H₂O)₂(Im)₂(4-nba)₂] 1 and [Ca(H₂O)₃(Im)(4-nba)₂]$\cdot$Im 2, which exhibit the same metal:4-nba:Im ratio but different degrees of hydration. Complex 1 crystallizes in the triclinic $P_{\bar{i}}$ space group and the Mg atom is located on an inversion centre, while 2 crystallizes in the monoclinic $P2_1/c$ space group and all atoms are located in general positions. In 1 the Im ligands, which are trans to each other, are coordinated to Mg, while 2 contains coordinated as well as free Im. The monodentate 4-nba ligands are disposed trans to each other in 1, while they adopt a cis orientation in 2 resulting in different supramolecular structures. Complex 1 exhibits two types of H-bonding interactions namely O-H𝛬O and N-H𝛬O, while in 2 three varieties of H-bond, viz. O-H𝛬O, N-H𝛬O and O-H𝛬N are observed. The Im ligand functions as a bifurcated H-bond donor in 1 while the O atom of the nitro group functions as a H-bond acceptor. In contrast, the nitro group in 2 is not involved in any H-bonding interactions. The free Im in 2 functions as a bifurcated acceptor and forms an extended chain linking adjacent complex molecules. The chains thus formed are further cross-linked with the aid of H-donor bonds from both the free as well as the coordinated Im. Both 1 and 2 exhibit 𝜋-𝜋 stacking interactions. Complex 1 is thermally more stable as compared to 2, and both complexes can be dehydrated to the corresponding anhydrous complexes by heating at 140 and 100°C respectively. At elevated temperatures, both the complexes can be pyrolysed to the corresponding oxide. The anhydrous complexes can be rehydrated to obtain the starting hydrated materials.

The reaction of benzyl chloride with tetramethylethylenediamine (tmen) results in the formation of the quaternary diammonium dichloride trihydrate (dbtmen)Cl₂.3H₂O 1 (dbtmen is N,N'-dibenzyl-N,N,N',N'-tetramethylethylenediammonium) in good yields. 1 crystallises in the monoclinic $P2_1/c$ space group and its structure consists of N,N'-dibenzyl-N,N,N',N'-tetramethylethylenediammonium dication, two chloride anions and three crystal water molecules all of which are located in general positions. The organic dication is H-bonded to the chloride anions and the crystal waters with the help of intra- and intermolecular C-H$\cdots$Cl and C-H$\cdots$O interactions, while the chloride anions are linked to the crystal waters via O-H$\cdots$Cl interactions. One of the crystal waters is linked through an intermolecular O-H$\cdots$O bond with another water resulting in the formation of a water dimer. The O-H$\cdots$Cl and O-H$\cdots$O interactions between the chloride anions and water molecules lead to the formation of a five-membered {O₃Cl₂} cyclic dichloride containing a water dimer. The five-membered rings are linked into a chain with the aid of a O-H$\cdots$Cl interaction. The organic cations are organised in zigzag fashion on either side of the chain and are further linked to the anionic water chain via weak C-H$\cdot$O and C-H$\cdots$Cl interactions, leading to the supramolecular organisation of the rings into a spiral-like of chain.

The aqueous reaction of [Mg(H₂O)₆]Cl₂ with the in situ generated sodium salt of 4-nitrobenzoic acid (4-nbaH) and N-methylimidazole (N-MeIm) results in the formation of the dinuclear complex [Mg(H₂O)(N-MeIm)₂(4-nba)₂]₂ 1 (4-nba = 4-nitro benzoate), while the direct reaction of [Mg(H₂O)₆]Cl₂ with sodium 4-nba leads to the formation of the mononuclear complex, [Mg(H₂O)₆](4-nba)₂.2H2O 2. In the centrosymmetric dimer 1, each Mg atom is coordinated to an aquo ligand, two monodentate N-MeIm ligands and a monodentate 4-nba ligand. The second 4-nba ligand functions as a bridging bidentate ligand, linking the metal centers and completes the hexacoordination around each Mg(II). The dimeric molecules of 1 are linked into a one-dimensional chain along 𝑏 with the aid of intra- as well as intermolecular H-bonding interactions between the coordinated water and the free oxygen atom of the monodentate 4-nba ligand. In the mononuclear complex [Mg(H₂O)₆](4-nba)₂.2H₂O 2, the Mg(II) is located on an inversion center and its structure consists of an octahedral [Mg(H₂O)₆]²⁺ dication, a free uncoordinated 4-nba anion and a lattice water molecule. One of the H atoms attached of the lattice water is disordered over two positions. The hexaaquomagnesium(II) dication, the 4-nba anion and the lattice water molecule are linked by intra- and intermolecular H-bonding interactions resulting in the formation of alternating layers of [Mg(H₂O)₆]²⁺ dications and 4-nba anions in the crystallographic $bc$ plane. The lattice water molecules are situated between the cations, while the 4-nba anions are arranged antiparallel to each other along 𝑏.

The reaction of barium carbonate with 4-nitrobenzoic acid (4-nbaH) results in the formation of a Ba(II) coordination polymer, catena-poly[[(pentaaqua)(4-nitrobenzoato-O,O')barium(II)](𝜇-4-nitrobenzoato-O,O')] 1. The polymeric compound [[Ba(H₂O)₅(4-nba-O,O')](𝜇-4-nba-O,O')]$_n$ 1 was characterized by elemental analysis, IR and UV-Vis spectra, weight loss studies, X-ray powder diffraction and its structure determined. In 1 five water molecules are coordinated to the central metal and one of the 4-nba ligands is bonded to Ba(II) in a bidentate manner (4-nba-O,O') through the carboxylate O atoms. The [(pentaaqua)(4-nitrobenzoato-O,O')barium(II)] units are linked into an infinite one-dimensional chain along 𝑏-axis with the aid of the second 4-nba anion, which functions as a bridging bidentate (𝜇-4-nba-O,O') ligand. This results in nine coordination around each Ba(II) ion in the coordination polymer. A long Ba$\cdots$Ba distance of 6.750(1) Å is observed between adjacent Ba(II) ions in the chain and the oxygen atoms of the carboxylate group and the nitro functionalities of the 4-nba ligand are involved in several O-H$\cdots$O and C-H$\cdots$O interactions.

The aqueous reaction of barium carbonate with 2-nitrobenzoic acid (2-nbaH) results in the formation of a one-dimensional coordination polymer, catena-poly[[hexa(aqua)dibarium(II)]bis[($\mu_2$-2-nitrobenzoate-O,O,O-NO₂)($\mu_2$-2-nitrobenzoate-O,O,O')]] 1. On heating at 100°C compound 1 is dehydrated to anhydrous barium bis(2-nitrobenzoate) 2. The anhydrous compound can be re-hydrated to 1 on exposure to water vapour. Compounds 1 and 2 were characterized by elemental analysis, IR and UV-Vis spectra, DSC thermograms, weight loss studies and the structure of 1 was determined. 1 and 2 can be thermally decomposed to BaCO₃ by heating at 800°C. The polymer [[Ba(H₂O)₃]₂($\mu_2$-2-nba-O,O,O-NO₂)₂ ($\mu_2$-2-nba-O,O,O')₂]$_n$ 1 crystallizes in the centrosymmetric triclinic space group P$_{\bar{i}}$ and all atoms are located in general positions. The polymeric structure is based on a dimeric unit and consists of three water molecules coordinated to a central Ba(II) and two unique 2-nitrobenzoate (2-nba) anions, one of which ($\mu_2$-2-nba-O,O,O-NO₂) functions as a tridentate ligand and is linked to a Ba(II) through the oxygen atom of the -NO₂ group and forms a monoatomic $\mu_2$-carboxylate bridge between two symmetry related Ba(II) ions with a Ba$\cdots$Ba distance of 4.5726(14) Å. The second unique 2-nba anion ($\mu_2$-2-nba-O,O,O') also functions as a tridentate ligand with the carboxylate oxygen atoms linked to a Ba(II) ion in a bidentate fashion and one of the carboxylate oxygen atoms forming a monoatomic bridge between two symmetry related Ba(II) ions resulting in a Ba$\cdots$Ba separation of 4.5406(15) Å. These differing tridentate 2-nba ligands link {Ba(H₂O)₃} units into a one-dimensional polymeric chain extending along 𝑏 axis. In the infinite chain each nine coordinated Ba(II) is bonded to three water molecules and further linked to six oxygen atoms of four different 2-nitrobenzoate anions with alternating pairs of Ba(II) ions in the chain bridged by a pair of ($\mu_2$-2-nba-O,O,O-NO₂) and ($\mu_2$-2-nba-O,O,O') ligands resulting in alternating Ba$\cdots$Ba distances of 4.5406(15) and 4.5726(14) Å across the chain.

The reaction of the sodium salt of 4-nitrobenzoic acid (4-nbaH) with [Ni(H₂O)₆]Cl₂ or [Ni(en)₃]Cl₂.2H₂O (en is ethylenediamine) results in the formation of the known octahedral compound [Ni(H₂O)₄($\eta^1$-4-nba)₂]$.2$H₂O (4-nba = 4-nitrobenzoate) 1 or the title compound di(aqua)bis(ethylenediamine) nickel(II) bis(4-nitrobenzoate) 2 respectively. Compounds 1 and 2 were characterized by elemental analysis, infrared spectra, DSC thermograms, weight loss studies and the structure of 2 was determined. Both 1 and 2 can be thermally decomposed to green NiO. The title compound [Ni(H₂O)₂(en)₂](4-nba)₂ 2 crystallizes in the centrosymmetric monoclinic space group $P2_1/c$ with the Ni(II) situated on an inversion center. The crystal structure of 2 consists of a hexacoordinated Ni(II) complex cation and an uncoordinated 4-nba anion. In the octahedral complex cation, the central metal is linked to two symmetry related bidentate en ligands and two water molecules. In the crystal structure, the cations and anions are linked by three varieties of hydrogen bonding interactions. A comparative study of seven nickel 4-nitrobenzoate compounds is described.

Aqueous reaction of ammonium thiosulfate with piperazine (pip) results in the formation of the title compound (pipH₂)[S₂O₃]$\cdot$H₂O 1 (pipH₂ = piperazinediium) in good yield. 1 was characterized by elemental analysis, IR, Raman andNMRspectra, X-ray powder pattern and its structure was determined. On heating at 100°C, 1 transforms to anhydrous piperazinediium thiosulfate 2, which can be rehydrated to the monohydrate on exposure tomoisture. The structure of 1 consists of two crystallographically independent piperazinediium (pipH₂)²⁺ cations located on inversion centers, a thiosulfate anion and a lattice water. The organic cations, thiosulfate anion and lattice water are linked by six varieties of hydrogen bond namely O-H$\cdots$O, O-H$\cdots$S, N-H$\cdots$O, N-H$\cdots$S, C-H$\cdots$O and C-H$\cdots$S, leading to the formation of alternating layers of (pipH₂)²⁺ cations and water linked thiosulfate chains. A comparative study of several compounds charge balanced by the piperazinediium cation is described.

A new coordination polymer namely [[Ca₆(H-gly)₁₂(H₂O)₁₈]Cl₁₂·6H₂O]$_n$ (1) (H-gly = glycine) has been isolated from the calcium chloride-glycine-water system and structurally characterized. Each Ca(II) in 1 is eight-coordinated and is bonded to eight oxygen atoms three of which are from terminal water molecules and five oxygen atoms from four symmetry related zwitterionic glycine ligands. The H-gly ligands exhibit two different binding modes viz. a monodentate carboxylate ligation and a $\mu_3$-tetradentate bridging carboxylate binding mode, which results in the formation of a one-dimensional coordination polymer. In the infinite chain the Ca(II) atoms are organized in a zigzag fashion. A comparative study reveals a rich and diverse structural chemistry of calcium halide-glycine compounds.

The synthesis, crystal structure, redox characteristics and photochemistry of a new heptamolyb-date supported magnesium-aqua coordination complex viz. (hmtH)₂ [{Mg(H₂O)₅}₂ {Mo₇O₂₄}]·3H₂O 1 (hmt = hexamethylenetetramine) is reported. The cyclic voltammogram reveals quasireversible redox behaviour while the degradation of rhodamine B (RhB) by 1 under sunlight irradiation indicates its photocatalytic activity. In the crystal structure of 1, two [Mg(H₂O)₅]²⁺ units are linked by a 𝜇₂-bridging bidentate hepta-molybdate ligand resulting in a heptamolybdate supported magnesium-aqua coordinated dianionic complex [{Mg(H₂O)₅}₂ {Mo₇O₂₄}]²⁻ charge balanced by two (hmtH)⁺ cations. The cations, anions and the lattice water molecules are linked by several hydrogen bonding interactions.