Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: II. Growth kinetics

The experimental results of the effect of concentration of Mn(II) ions on the growth kinetics of different faces of ammonium oxalate monohydrate single crystals from aqueous solutions at a constant temperature and different predefined supersaturations are described and discussed. It was observed that: (1) at a given supersaturation σ, Mn(II) ions lead to a decrease in the growth rates of different faces of AO crystals, (2) the growth of a particular face of the crystals occurs above a critical supersaturation σ_d but there is also another supersaturation barrier σ* when the rate abruptly increases with σ, (3) the values of σ_d and σ* increase with increasing concentration of the impurity, and (4) the values of σ_d depend on the growth kinetics of a face but those of σ* are independent of face growth kinetics. The experimental R(σ) data for different Mn(II) concentrations c_i were analysed according to the model involving complex source of cooperating screw dislocations and concepts of instantaneous and time‐dependent impurity adsorption. It was found that: (1) for a given face the differential heat of adsorption Q_diff is higher during instantaneous impurity adsorption than that during time‐dependent adsorption, and (2) the values of Q_diff involved during instantaneous adsorption are related with face growth kinetics but those during time‐dependent adsorption are independent of face growth kinetics.     

On the origin of supersaturation barriers during the growth of single crystals from aqueous solutions containing impurities – a review

A generalised treatment of the appearance of supersaturation barriers σ_d, σ* and σ** during the growth of single crystals is outlined from the standpoint of well‐defined critical values of relative step velocities on a face. The final theoretical expressions are based on the premise that: (1) there are critical values of the relative step velocities associated with different average distances between adsorbed impurity particles during instantaneous, time‐dependent and time‐independent adsorption of the impurity on the growing surface, (2) the growth rate of a face is proporptional to velocity of steps on the growing face, and (3) Freundlich and Langmuir adsorption isotherms apply for different impurities. The theoretical expressions are then used to critically analyse the experimental data on supersaturation barriers observed during the growth of ammonium oxalate monohydrate and potassium dihydrogen phosphate single crystals from aqueous solutions containing different impurities. It was found that: (1) Langmuir adsorption isotherm is more practical for the analysis of the experimental data of the dependence of supersaturation barriers σ_d, σ* and σ** on the concentration c_i of an impurity, and (2) the ratios σ_d/σ* and σ*/σ** of successive supersaturation barriers for an impurity either increases or remains constant with an increase in impurity concentration c_i, and may be explained in terms of the mechanism of adsorption of impurity particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ study of growth and dissolution kinetics of ammonium oxalate monohydrate single crystals from aqueous solutions containing cationic impurities

The results of an in situ investigation of the effect of four different bi‐ and trivalent cations (Fe(III), Cu(II), Mn(II) and Cr(III)) on the displacement velocity of individual growth steps on the (110) face of ammonium oxalate monohydrate crystals as a function of supersaturation are described and discussed. It was observed that: (1) at a particular temperature of pure solutions and solutions containing impurities, the velocity v of movement of the [110] growth steps is always greater than that of the [111] steps, (2) fluctuations in the velocity of individual growth steps occur in all solutions containing similar concentrations of different impurities, (3) the value of kinetic coefficient β for growth steps decreases with an increase in the concentration c_i of Cu(II) impurity, but that for dissolution steps does not depend on c_i; moreover, the value of kinetic coefficient β for growth steps is higher than that of dissolution steps, and (4) in the presence of Mn(II) and Cr(III) impurities, the kinetic coefficient β for dissolution steps is several times greater than that for growth steps. The results are explained from the standpoint of Kubota‐Mullin model of adsorption of impurities at kinks in the steps and the stability of dominating complexes present in solutions. Analysis of the results revealed that: (1) the effectiveness of different impurities in inhibiting growth increases in the order: Fe(III), Cu(II), Mn(II), and Cr(III), and this behavior is directly connected with the stability and chemical constitution of dominating complexes in saturated solutions, (2) fluctuations in the velocity of growth steps is associated with the effectiveness of an impurity for adsorption; the stronger the adsorption of an impurity, the higher is the fluctuation in step velocity v, and (3) depending on the nature of the impurity, the kinetic coefficient for the dissolution steps can remain unchanged or can be higher than that of the growth steps. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: I. Growth habit and surface morphology

The effect of concentration of Mn(II) ions on the growth habit and the surface micromorphology of different as‐grown faces of ammonium oxalate monohydrate (AO) single crystals grown from aqueous solutions was studied at a constant temperature of 30 °C and predefined supersaturations up to 20%. It was observed that the growth habit and the surface morphology of the crystals strongly depend on the supersaturation used for growth and the impurity concentration in the solution. The experimental results were analysed in terms of connected nets determined from different projections of the structure of AO crystals. Analysis of the observations revealed that: (1) the directions of connected nets corresponding to basic growth units composed of single (NH₄)₂C₂O₄ · H₂O molecules are in excellent agreement with the low‐index crystallographic directions of the orientations of growth layers, (2) all faces appearing in the growth morphology of AO crystals are F faces, and (3) the {001} face growing from pure aqueous solutions is essentially a kinetically rough face but the presence of Mn(II) impurity leads to their appearance in the morphology due to increase in the strength of bonds of the connected nets composing the surface graph.     

Cobalt(II) and nickel(II) complexes ofN-protected amino acids. Crystal and molecular structure of bis (N-acetylglycinato)tetraaquocobalt(II)

Binary complexes of formula [M(II)(acgly)₂(H₂O)₄] (M(II)=Co(II), Ni(II), acgly=N-acetylglycinate ion) were synthesized, and for the Co(II) complex the crystal and molecular structure was determined. The crystals are monoclinic, space groupP2₁/c,a=4.838(1),b=10.785(2),c=14.340(6) Å,=96.96(2)°,Z=2. The structure was solved by the heavy-atom method and refined through full-matrix least-squares calculations toR=0.0394 for 1069 observed reflections. The coordination around the cobalt atom is slightly elongated octahedral arising from one carboxylate oxygen of each of the two centrosymmetrically related monodentateN-acetylglycinate anions and four water molecules.     

Transition metal(II) complexes of a biological buffer. Structural and spectroscopic study on Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) complexes of N-[2-hydroxy-1,1-bis-(hydroxymethyl)ethyl]glycine

The structural and spectroscopic properties of complexes of N-[2-hydroxy-1,1-bis-(hydroxymethyl)ethyl]glycine (thmmg) with Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) were investigated. The complex [Zn(thmmg)₂] crystallizes in the space groupP2₁/n, witha=8.796(2),b=9.050(3),c=9.926(2) Å,=97.35(1)° andZ=2; the copper(II) homolog is isomorphous. The complex [Ni(thmmg)₂]·6H₂O crystallizes in the space groupP2₁/n witha=8.432(8),b=8.495(2),c=15.967(1) Å,=95.57(2)° andZ=2; the cobalt(II) homolog is isomorphous. Ni(II) and Zn(II) complexes show an elongated octahedral coordination arising from two symmetry related amino acid molecules acting as a tridentate chelate through the amino nitrogen, one carboxylate oxygen, and one hydroxyl oxygen; the water molecules in the Ni(II) complex are not involved in metal coordination. The same ligand bonding mode may be suggested also for the Cu(II) and Co(II) complexes, while for Cd(II) only the nitrogen atom and the carboxylate oxygens seem to be involved in metal coordination.     

In situ measurement of growth kinetics of {100} KDP crystal faces in the presence of polyphosphate impurities

The face growth rate and critical supersaturation of {100} face were in situ measured using the laser‐polarization‐interference technique in the presence of potassium pyrophosphate, trimetric sodium phosphate and sodium hexametaphosphate impurities. The polyphosphate impurities inhibit the growth rate of prismatic faces. The face growth rate as a function of supersaturation at different impurity concentrations, as well as critical supersaturation as a function of impurity concentrations, was found in good agreement with a two‐dimensional nucleation model in the pure system and Kubota and Mullin's model in the presence of impurities. The average distance L between active sites available for impurity adsorption as well as the edge free energy was calculated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of growth kinetics of ammonium oxalate monohydrate crystals from aqueous solutions

Experimental results of the dependence of linear growth rates of ammonium oxalate monohydrate [(NH₄)₂C₂O₄ · H₂O; AO] single crystals on solution supersaturation are presented. The AO crystals were grown by constant-temperature, constant-supersaturation method at 30 and 40 °C in the supersaturation range of 1–9%. It was observed that the supersaturation dependence of growth rates follows the parabolic growth law. Analysis of the supersaturation dependence of linear growth rates of AO crystals showed (1) that growth models involving surface diffusion and direct incorporation of growth units give kinetic parameters similar to those reported for other compounds grown from solutions, and (2) that the the BCF model of cooperating screw dislocations is also applicable. An inverse relationship between the estimated values of the length, L, of the line containing the dislocations and growth rate, R, and a direct relationship between L and interplanar distance, d_hkl, of the face {hkl} were found. Both these relationships are associated with the process of generation of screw dislocations in the growing layer.     

Influence of complex formation upon inclusion of Mn(II), Co(II), Ni(II), and Cu(II) in ZnC2O4·2H2O

The inclusion of 3d‐impurities Mn(II), Co(II), Ni(II) and Cu(II) in a crystalline precipitate of ZnC₂O₄·2H₂O is investigated. This study is a part of the systematic one deal with the mechanism of inclusion of 3d‐ions in sparingly soluble oxalate systems. The experiments are carried out in bi‐ end multi‐component systems at two different mediums – one with deficiency of oxalate ions, another with excess. The insertion of 3d‐ions upon mass crystallization of ZnC₂O₄·2H₂O does not proceed by a simple ionic substitution. The results show that the inserted amount of impurity depends on some physicochemical characteristics of the neutral monooxalato complexes [MnC₂O₄]^o, [CoC₂O₄]^o, [NiC₂O₄]^o and [CuC₂O₄]^o. Good agreement between included impurity and the concentration of its complex in the solution is established. The stability constant of monooxalato complex affects the impurity inclusion. This effect depends on the medium nature. In the deficiency of oxalate ions the factor determining the inclusion is thermodynamic one – stability of monooxalato complexes. In the excess of oxalate ions inserted amount depends on kinetic factor – the formation rate of these complexes. In the term of that the insertion of Mn(II) is definitely different in the two mediums while that of the Ni (II) does not depend on the medium. The copper shows deviation from overall dependence in the two mediums due to the Jahn‐Teller distortion. Its double decreasing insertion in the excess of oxalate ions is related with stabilization of [Cu(C₂O₄)2]^2‐. The conclusions presume that by varying the background medium and taking in view the ions present in the solution, the amount of inserted impurities can be predicted and controlled. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of cationic impurities on solubility and crystal growth processes of ammonium oxalate monohydrate: Role of formation of metal‐oxalate complexes

The effect of different bi‐ and trivalent cationic impurities on the solubility of ammonium oxalate and the composition and distribution of chemical complexes formed in saturated ammonium oxalate aqueous solutions as a function of impurity concentration are investigated. The knowledge of the composition and stability of complexes formed in saturated aqueous solutions is then employed to explain the appearance of dead zones of supersaturation for growth and the difference in the effective segregation coefficient of the impurities. Analysis of the experimental results revealed that: (1) at a constant temperature, the dependence of concentration of complex species formed in saturated solutions on the concentration of different impurities can be described by an equation similar to that of the concentration dependence of density of solutions, (2) the dominant metal‐containing species present in saturated solutions are negatively‐charged, most stable oxalato complexes like Cu(C₂O₄)₂²⁻, Mn(C₂O₄)₃⁴⁻, Zn(C₂O₄)₃⁴⁻, Cr(C₂O₄)₃³⁻ and Fe(C₂O₄)₃³⁻, (3) in the investigated range of impurity concentration, the solubility of ammonium oxalates increases linearly with the concentration of all impurities and the increase is associated with the stability of dominant complexes, (4) appearance of dead supersaturation zones in the presence of impurities is associated with instantaneous adsorption of all growth sites by dominant oxalato complexes in relatively short adsorption time, and (5) the segregation coefficient of an impurity cation M of charge z + increases with a decrease in the solubility product constant K_sp for the hydrolysis products of reactions of the type: M^{z +} ↔ M₁^{(z −1)+} + H⁺ (where the cation M has z + charge, and H⁺ is hydrogen ion). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth kinetics of zinc(tris)thiourea sulphate (ZTS) crystals

Growth kinetics of zinc (tris) thiourea sulphate (ZTS) crystals investigated as a function of supersaturation is reported in this communication. Crystal growth rates were investigated normal to the (100), (010) and (001) faces under growth conditions employed for bulk crystal growth. The growth rates normal to (010) and (100) were found to follow the continuous growth model (R_G = Cσ) with respect to the supersaturation whereas the growth rates normal to (001) was found to satisfy birth and spread (B+S) model (R_G = Aσ^5/6 exp(‐B/σ)). The growth rates observed normal to the studied face are in agreement with the growth mechanism predicted from the estimated α (Jackson) factor. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and Characterisation of Gel-grown Lead(II)Chloride and Lead(II)Bromide Single Crystals

Single crystals of pure and doped lead(II)chloride and lead(II)bromide were grown by gel technique employing a modified two-stage chemical reaction. Methods to minimise the predomination of needle morphology during the growth of these crystals have been investigated and the results are discussed. The grown crystals were characterised by optical transmission spectrum. Undoped and monovalent cation (K⁺, Na⁺, Cu⁺, Ag⁺ and Hg⁺) doped crystals of PbCl₂ and PbBr₂ were subjected to d.c. electrical conductivity studies. Using the log σT versus T⁻¹ plot, the activation energies for the migration of anion vacancies in lead(II)halides are calculated. They are found to be less for the doped crystals than those of undoped ones.     

Crystal and molecular structure of tetrakis(N,N′-diallylthiourea)nickel(II) iodide

Summary The crystal structure of tetrakis(N,N-diallylthiourea)nickel(II) iodide [Ni(C₇H₁₂N₂S)₄I₂] has been determined by a three-dimensional X-ray analysis. FinalR, after anisotropic least-squares refinement, is 8·8%. The crystals are tetragonal (P4/n):a = 11·24(1),c = 15·43(1) Å,Z = 2. Ni(II) is on a 4-fold axis; the coordination around it is flattened pyramidal and involves four sulphur atoms from four diallylthiourea molecules (Ni-S = 2·221 Å). Ni(II) is out of the plane through the sulphur atoms by 0·40 Å. Two I^- ions lie on opposite sides with respect to the nickel atom along the 4-fold axis, at distances Ni...I(1) = 3·74 Å, Ni...I(2) = 6·64 Å. The orientation of the allylthiourea molecules is determined mainly by a hydrogen bond formed by one nitrogen (N(1)) with the iodine which is nearer to the nickel.The authors are indebted to Prof. C. Furlani who kindly supplied the crystals of the compound.     

Crystal and molecular structure of a cobalt(II) complex with bis(benzimidazol-2-ylmethyl)-methylamine (L)

The reaction product of Co(II) chloride and the title ligand L, formulated as CoLCl₂·CH₃OH, was prepared and characterized by means of structural and spectroscopic measurements. The violet crystals are orthorhombic, (space groupP2₁2₁2₁) witha=8.093(2),b=14.883(3),c=16.831(3) Å, andZ=4. The structure consists of discrete molecules with pseudo-, noncrystallographic twofold symmetry in which the Co atom is coordinated in trigonal bipyramidal geometry by three nitrogen and two chlorine atoms. The ligand L is coordinated to the Co atom in afac mode and two chlorine atoms are incis-positions. The structure was confirmed by IR-spectra.     

Effect of impurities on the metastable zone width of solute–solvent systems

The novel approach to interpret the metastable zone width obtained by the polythermal method using the classical theory of three-dimensional nucleation proposed recently [K. Sangwal, Cryst. Growth Des. 9 (2009) 942] is extended to describe the metastable zone width of solute–solvent systems in the presence of impurities. It is considered that impurity particles present in the solution can change the nucleation rate J by affecting both the kinetic factor A and the term B related with the solute–solvent interfacial energy γ. An expression relating metastable zone width, as defined by the maximum supercooling ΔT_max of a solution saturated at temperature T₀, with cooling rate R is proposed in the form: (T₀/ΔT_max)²=F(1−Z ln R), where F and Z are constants. The above relation can also be applied to describe the experimental data on maximum supercooling ΔT_max obtained at a given constant R as a function of impurity concentration c_i by the polythermal method and on maximum supersaturation σ_max as a function of impurity concentration c_i by the isothermal method. Experimental data on ΔT_max obtained as a function of cooling rate R for solutions containing various concentrations c_i of different impurities and as a function of concentration c_i of impurities at constant R by the polythermal method and on σ_max as a function of impurity concentration c_i by the isothermal method are analyzed satisfactorily using the above approach. The experimental data are also analyzed using the expression of the self-consistent Nývlt-like approach [K. Sangwal, Cryst. Res. Technol. 44 (2009) 231]: ln(ΔT_max/T₀)=Φ+β ln R, where Φ and β are constants. It was found that the trends of the dependences of Φ and β on impurity concentration c_i are similar to those observed in the trends of the dependences of constants F and Z on c_i predicted by the approach based on the classical nucleation theory.     

Coordination of Schiff bases derived from pyrrole-2-carboxaldehyde and triamines to copper(II) and nickel(II) ions. Crystal structures of [Cu(C15H20N5)]NO3 and [Ni(C15H20N5)]ClO4

The Schiffbases 1,9-bis(2-pyrrolyl)-2,5,8-triazanona-1,8-diene (H₂L¹), 1,10-bis(2-pyrrolyl)-2,5,9-triazaundeca-l,9-diene (H₂L²), and 1,11-bis(2-pyrrolyl)-6-methyl-2,6,10-triazaundeca-l,10-diene (H₂L⁴) react with copper(II) nitrate or nickel(II) perchlorate in the presence of triethylamine to give new complexes [Cu(HL^1,2,4)]NO₃ and [Ni(HL^1,2,4)]ClO₄. The crystal structures of [Cu(HL²)]NO₃ and [Ni(HL²)]CIO₄ have been determined from single crystal diffractometer data and refined to finalR factors of 5.09 and 5.3%, respectively. Crystallographic data: [Cu(HL²)]NO₃: monoclinic,P2₁/c,a=10.036(2),b=14.500(2),c=13.317(2) Å,=108.14(1)°,Z=4, andd _c =11.427 Mg m^–3; [Ni(HL²)]ClO₄: monoclinic,P2₁/n,a=10.578(3),b=13.953(3),c=12.394(4) Å,=93.78(2)°,Z=4, andd _c =1.549 Mg m^–3. In both the structures the potentially pentadentate ligand (HL²)^– acts as a tetradentate one leaving one pyrrole group uncoordinated. Interesting is the metal dependent sequence of the three chelate rings. While the Ni (II) ion coordinates the (HL²)^– ligand to form a 5-5-6-membered ring system, the ring arrangement in the copper complex is of the 5-6-5 type.     

Synthesis,crystal and molecular structure of trans‐bis(2‐pyridinepropanol)bis(saccharinato)nickel(II)

Trans‐bis(2‐pyridinepropanol)bis(saccharinato)nickel(II), [Ni(sac)₂(pypr)₂], where sac and pypr are the saccharinate anion and the 2‐pyridinepropanol molecule, respectively, crystallizes in the triclinic space group P (No. 2) with a = 8.1981(8), b = 9.9680(10), c = 10.4956(10) Å, α = 90.740(3)° β = 108.142(3)°, γ = 111.025(3)°, Z = 1, V = 1.537 ų. The structure of the nickel(II) complex consists of neutral molecules in which the nickel(II) ion sits on a center of symmetry and is octahedrally coordinated by two sac ligands, and two neutral pypr ligands. The pypr acts as a bidentate N‐ and O‐donor ligand forming a seven‐membered chelate ring, while sac behaves as a monodentate O‐donor ligand via the carbonyl O atom. The Ni‐N bond distance is 2.1016(15) Å, whereas the Ni‐O_pypr and Ni‐O_sac bond distances are 2.1280(12) and 2.0792(11) Å, respectively. The individual molecules are held together with a strong hydrogen bond between the hydroxyl O atom of pypr and N atom of sac and the C‐H…O type weak hydrogen bonds between some ring hydrogen atoms of pypr and sac, and the carbonyl and sulfonyl O atoms of sac in the neighbouring molecules. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-Assembly of Two Isomorphous Thiocyanate Complexes of Co(II) and Ni(II) with 3-Hydroxypicolinamide

Abstract  

The synthesis, thermal and spectral characterization, and crystal structure of isomorphous thiocyanate cobalt(II) and nickel(II) complexes with 3-hydroxypicolinamide (3-OHpia), [M(C₆H₆N₂O₂)₂(NCS)₂]·2H₂O, are reported. The metal(II) ions are chelated by two cis-oriented 3-OHpia and two thiocyanate ligands in distorted octahedral geometry. The distortion within the coordination sphere is mainly imposed by formation of the chelate rings. The compounds crystallize in monoclinic space group P2/c with two symmetrically independent molecules and a = 14.4945(2) ?, b = 8.5906(1) ?, c = 16.3865(3) ?, β = 105.987(2)°, Z = 4 (1) and a = 14.4927(5) ?, b = 8.5912(3) ?, c = 16.2712(6) ?, β = 105.740(4)°, Z = 4 (2). Commonly observed supramolecular amide synthons are not robust enough to accommodate thiocyanate ions and H₂O molecules. But instead, neutral complexes are linked through hydrogen bonds leading to two different hydrogen bonding ribbon motifs involving amide moieties and H₂O molecules [C(8)R ₂²(12) along c axis] and amide moieties and thiocyanate ions [C(8)R ₂²(16) along b axis] for symmetrically related molecules labelled as 1 [Co1 (1) and Ni1 (2)] and 2 [Co2 (1) and Ni2 (2)], respectively.     

Synthesis,spectroscopic characterization of thiosemicarbazone complexes of nickel(II) and X-ray structures of anisaldehyde thiosemicarbazone (ATSZH) and bis(tolualdehyde thiosemicarbazonato)nickel(II)

Complexes of nickel(II) with anisaldehyde (ATSZH), tolualdehyde (TTSZH), and vanillin (VTSZH) thiosemicarbazones have been synthesized and characterized by means of UV-Visible, I.R., Raman,¹H and¹³C NMR spectroscopy. The thiosemicarbazones have been found to exist in the thione form.¹H and¹³C NMR as well as electronic spectral data support a square planar structure for Ni(ATSZ)₂ and Ni(TTSZ)₂ complexes.¹H and¹³C NMR spectra indicate that the structure of Ni(VTSZ)₂ complex differs from the other two nickel(II) complexes and it may possibly consist of mixtures of isomers. The X-ray structures of Ni(TTSZ)₂ and ATSZH were determined: the nickel complex is monoclinic witha=26.412(6),b=12.135(3),c=6.888(1)Å,=95.41(5)°,Z=4,R=0.0647, space groupC2/c. The structure consists of discrete Ni(TTSZ)₂ molecules in which the metal, located on a symmetry centre, is N,S-chelated by two ligands forming two five-term chelate rings. The structure of ATSZH is also monoclinic:a=7.859(1),b=13.481(3),c=10.028(3) Å,=106.32(4)°,Z=4,R=0.0412, space groupP2₁/c. The molecules, connected by NO hydrogen bonds, reveal a considerable tilting of the aromatic ring in respect to the thiourea moiety if compared with the nickel structure.     

Nickel(II) and cobalt(II) nitrate and chloride networks with 2-aminopyrimidine

The coordination chemistry of 2-aminopyrimidine (PymNH₂) with nickel(II) and cobalt(II) nitrate and chloride is reported, including seven new X-ray crystal structures. Two [Ni(NO₃)₂(PymNH₂)₂(OH₂)] isomers were found (A: C2/c, a=13.3006(5), b=7.9727(3), c=28.5453(11), β=101.758(2), V=2963.48(19), Z=8 and B·1/2 acetone: P2₁/c, a=7.66060(10), b=10.6792(2), c=20.6790(3), β=100.2970(10), 1664.48(5), Z=4). In both cases one nitrate is monodentate and the other is chelating and the PymNH₂ ligands coordinate through ring nitrogen atoms. Hydrogen bonding results in double sheet structure for isomer A, and a three dimensional channeled network for isomer B. [Co(NO₃)₂(PymNH₂)₂(OH₂)] (C2/c, a=13.3507(2), b=7.99520(10), c=28.6734(3), β=102.3540(10), V=2989.77(7), Z=8) is isostructural to Ni isomer A. [CoCl₂(PymNH₂)] (Cmcm, a=3.6139(2), b=14.3170(7), c=12.9986(7), V=672.55(6), Z=4) is a sheet coordination network, consisting of corner-sharing chains of Co₂(μ-Cl)₂ bridged by PymNH₂ through ring nitrogen atoms; [CoCl₂(PymNH₂)₂] (C2/c, a=11.2774(6), b=6.5947(4), c=16.5687(9), β=92.269(3), V=1231.27(12), Z=4) is a tetrahedral molecule knit into a ribbon structures through pairs of hydrogen bonds. Isostructural trans-[NiCl₂(PymNH₂)₄] (C2/c, a=7.67760(10), b=18.7224(3), c=15.0418(2), β=99.6740(10), V=2131.41(5), Z=4) and trans-[CoCl₂(PymNH₂)₄] (C2/c, a=7.69120(10), b=18.5957(2), c=15.1091(2), β=99.5280(10), V=2131.14(5), Z=4) are simple octahedral molecules, with hydrogen-bonding producing sheet structures.