Axial Water Molecules Research Articles

Correlation between local structure and molar ratio of Au (III) complexes in aqueous solution: An XAS investigation

The local and geometrical structure around gold (III) e.g., Au 3+ ions in aqueous solution with different OH −/Cl − molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCl n (OH) 4− n ] − solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl − ligands by OH − ligands. A square–planar geometry for [AuCl n (OH) 4− n ] − with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH −/Cl − molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.

Catena-Poly[zinc(II)-μ-aqua-κ2O:O-bis(μ-quinoline-4-carboxylato-κ2O:O′)

The asymmetric unit of the title complex, [Zn(C10H6NO2)2(H2O)]n, consists of one quinoline-4-carboxyl­ate anion, half of a Zn2+ cation and half of a coordinated water mol­ecule. The cation and the water O atom have crystallographically imposed inversion and twofold rotation symmetry, respectively. The metal centre displays an elongated ZnO6 octa­hedral coordination geometry provided by the O atoms of four anions at the equatorial plane and two axial water mol­ecules. Each anion and water mol­ecule act as bridges between ZnII cations, forming a polymeric chain parallel to [001]. The chains are further linked into a three-dimensional framework through O—H⋯N hydrogen bonds.

Diaquabis[5-(2-pyridyl)tetrazolato-κ2N1,N5]iron(II)

The title complex, [Fe(C6H4N5)2(H2O)2], was synthesized by the reaction of ferrous sulfate with 5-(2-pyrid­yl)-2H-tetra­zole (HL). The FeII atom, located on a crystallographic center of inversion, is coordinated by four N-atom donors from two planar trans-related deprotonated L ligands and two O atoms from two axial water mol­ecules in a distorted octa­hedral geometry. The FeII mononuclear units are further connected by inter­molecular O—H⋯N and C—H⋯O hydrogen-bonding inter­actions, forming a three-dimensional framework.

Synthesis and Crystal Structures of Two Metal Urea Nitrates

Cu(urea)2(H2O)3(NO3).(urea)(NO3) (1) and Zn(urea)4(H2O)2·2(NO3) (2) have been obtained from the corresponding metal nitrates and urea in warm aqueous solvent. These are unusual examples of complexes of transition metals coordinated only by urea and water. 1 crystallizes in the monoclinic space group P21/m with unit cell parameters a = 9.489(3), b = 13.059(3), c = 7.103(2) A and β = 100.28(3)°. The copper ion occupies a 2e position on a mirror plane and displays four short Cu–O bonds (2 × Cu–OH2 and 2 × Cu–O=C(NH2)2) and two much longer contacts (Cu–OH2 and Cu–ONO2) consistent with a Jahn–Teller distortion. Between the cations are located unbound urea and nitrate. 2 crystallizes in the monoclinic space group P21/n with a = 6.4239(11), b = 17.690(4), c = 7.5877(13) A and β = 91.138(14)°. The zinc ion resides on the 2a position, an inversion centre, and displays pseudo–octahedral coordination geometry. It is coordinated by four equatorial urea and two axial water molecules. The four molecules of urea are nearly planar and form N–H···O intramolecular hydrogen bonds. Unbound charge-balancing nitrate is also present. In each compound there is extensive intermolecular hydrogen bonding between the water, urea, and nitrate present. Pyrolysis of the compounds under 10% hydrogen in nitrogen at 300 °C was found to yield metallic copper in the case of 1 and zinc oxide for 2. The structures of two new metal urea nitrates have been determined at low temperature

Insight into the copper coordination environment in the prion protein through density functional theory calculations of EPR parameters

Density functional theory (DFT) calculations of Cu(II) electron paramagnetic resonance (EPR) parameters for the octarepeat unit of the prion protein were conducted. Model complexes were constructed and optimized using the crystal structure of the octarepeat unit of the prion protein. Copper g and A tensors and nitrogen hyperfine and quadrupole coupling constants were calculated using DFT. Solvent effects were incorporated using the conductor-like screening model as well as through the inclusion of explicit water molecules. Calculations using the model with an additional axial water molecule added to the coordination sphere of the Cu(II) metal center give the best qualitative agreement for the copper g and A tensors. The S-band experimental EPR spectra were interpreted in light of the DFT calculations of the directly coordinated nitrogen hyperfine coupling constants which indicate that the three directly coordinated nitrogen atoms in the octarepeat unit are not equivalent. These results demonstrate that DFT calculations of EPR parameters can provide important insight with respect to the structural interpretation of experimental EPR data.

Vibrational spectroscopic study of the hydrated platinum(II), palladium(II) and cis‐diammineplatinum(II) ions in acidic aqueous solutions

AbstractMid infrared, far‐infrared and Raman spectroscopic studies, combined with force field analyses, were performed for the hydrated platinum(II) and palladium(II) ions and cis‐diammineplatinum(II) complex in acidic aqueous solutions. Simplified Density Functional Theory (DFT) calculations were made for the equatorial plane of the platinum complexes. Careful subtraction of solvent spectra allowed a number of ‘internal’ modes of coordinated H2O and NH3 to be determined as weak residual bands. The [Pt(OH2)6]2+ and [cis‐Pt(NH3)2(OH2)4]2+ complexes were found to be six‐coordinated with four ligands strongly bound in an equatorial plane. The assignments of the vibrational modes in the equatorial plane could be performed on the basis of the experimental observations and by comparison with metal–ligand vibrations of square‐planar complexes, aided by normal coordinate calculations. For the weakly coordinated axial aqua ligands, the low wavenumber range and the polarizibility properties allowed the assignments of the bands at about 365 and 325 cm−1 to the stretching modes of one short and one longer Pt— O* bound to axial aqua ligands, respectively. A similar picture with even less strongly bound axial water molecules emerges from Raman spectroscopy data for the hydrated palladium(II) ion, [Pd(OH2)6]2+. The results are consistent with a description of the [Pt(OH2)6]2+ and [Pd(OH2)6]2+ aqua ions in C4v symmetry, and with the [cis‐Pt(NH3)2(OH2)4]2+ complex in the Cs point group, and also in qualitative agreement with the structures devised from previous extended X‐ray absorption fine structure (EXAFS) measurements. Copyright © 2008 John Wiley & Sons, Ltd.

Diaqua[5-(2-pyridyl)tetrazolato-κ2N1,N5]manganese(II)

The title compound, [Mn(C6H4N5)2(H2O)2], was synthesized by the hydro­thermal reaction of Mn(NO3)2 with picolino­nitrile in the presence of NaN3. The Mn atom lies on an inversion centre. The distorted octa­hedral Mn environment contains two planar trans-related N,N′-chelating 5-(2-pyrid­yl)­tetra­zolate ligands in the equatorial plane and two axial water mol­ecules. O—H⋯N hydrogen bonds generate an infinite three-dimensional network.

An Oxidized Tryptophan Facilitates Copper Binding in Methylococcus capsulatus-secreted Protein MopE

Proteins can coordinate metal ions with endogenous nitrogen and oxygen ligands through backbone amino and carbonyl groups, but the amino acid side chains coordinating metals do not include tryptophan. Here we show for the first time the involvement of the tryptophan metabolite kynurenine in a protein metal-binding site. The crystal structure to 1.35 angstroms of MopE* from the methane-oxidizing Methylococcus capsulatus (Bath) provided detailed information about its structure and mononuclear copper-binding site. MopE* contains a novel protein fold of which only one-third of the structure displays similarities to other known folds. The geometry around the copper ion is distorted tetrahedral with one oxygen ligand from a water molecule, two histidine imidazoles (His-132 and His-203), and at the fourth distorted tetrahedral position, the N1 atom of the kynurenine, an oxidation product of Trp-130. Trp-130 was not oxidized to kynurenine in MopE* heterologously expressed in Escherichia coli, nor did this protein bind copper. Our findings indicate that the modification of tryptophan to kynurenine and its involvement in copper binding is an innate property of M. capsulatus MopE*.

Structure of the Hydrated Platinum(II) Ion and the cis-Diammineplatinum(II) Complex in Acidic Aqueous Solution: An EXAFS Study

Careful analysis of Pt L3-edge extended X-ray absorption fine structure (EXAFS) spectra shows that the hydrated platinum(II) ion in acidic (HClO 4) aqueous solution binds four water molecules with the Pt-O bond distance 2.01(2) A and one (or two) in the axial position at 2.39(2) A. The weak axial water coordination is in accordance with the unexpectedly small activation volume previously reported for water exchange in an interchange mechanism with associative character. The hydrated cis-diammineplatinum(II) complex has a similar coordination environment with two ammine and two aqua ligands strongly bound with Pt-O/N bond distances of 2.01(2) A and, in addition, one (or two) axial water molecule at 2.37(2) A. This result provides a new basis for theoretical computational studies aiming to connect the function of the anticancer drug cis-platin to its ligand exchange reactions, where usually four-coordinated square planar platinum(II) species are considered as the reactant and product. (195)Pt NMR spectroscopy has been used to characterize the Pt(II) complexes.

Structural effects of Cu(ii)-coordination in the octapeptide region of the human prion protein

The copper-binding ability of the prion protein is thought to be central to its function. The structural effects of copper coordination in the octapeptide region of the human prion protein have been investigated by molecular dynamics simulations. Simulations were performed with the apo state, in order to investigate the behavior of the region without copper ions, as well as with the octapeptide region in the presence of copper ions. While the structure of the apo state is greatly influenced by the interaction between the rings in the histidine, tryptophan and proline residues, the region shows evidence of highly ordered coordination sites in the presence of copper ions. The position of the tryptophan indole ring is stabilized by cation-pi interactions. Two stable orientations of the indole ring with respect to the equatorial coordination plane of copper were observed, which showed that the indole ring can reside on both sides of the coordination plane. The interaction with the indole ring was found to occur without a mediating axial water molecule.

Self‐Assembly of Dimeric MnIII–Schiff‐Base Complexes Tuned by Perchlorate Anions

AbstractThe synthesis and structural and spectroscopic characterisation of 12 manganese(III)–Schiff‐base complexes of the formula [MnLn(H2O/CH3OH)2](ClO4)(mH2O) (m = 0, 1) are described. The multidentate H2Ln and H4Ln Schiff‐base ligands were obtained by condensation of different diamines (1,2‐diaminoethane, 1,2‐diamino‐2‐methylethane, 1,2‐diamino‐2,2‐dimethylethane, 1,3‐diamino‐2,2‐dimethylpropane) and 2,3‐dihydroxybenzaldehyde, 3‐methoxy‐2‐hydroxybenzaldehyde or 3‐ethoxy‐2‐hydroxybenzaldehyde. The crystal structures of H2L3 and H4L10 ligands were solved by X‐ray crystallography, revealing their ability to bind metal centres. Recrystallisation of complexes 2 and 8 from methanol yielded single crystals of [MnL2(H2O)2](ClO4) and [MnL8(H2O)2](ClO4). Their X‐ray characterisation shows a tetragonally elongated octahedral geometry for the manganese coordination sphere, formed by the chelation of the N2O2 donor set of the inner compartment of the Schiff base to the equatorial plane of the octahedron and by the binding of two water molecules in the axial positions. The octahedron entities are linked in pairs by μ‐aquo bridges between neighbouring axial water molecules and also by π–π stacking interactions, establishing dimeric structures. Perchlorate anions are accommodated in the cavities of the framework and form hydrogen bonds with the aqua ligands, leading to infinite spirals of the complexes generated by a screw axis along a. Moreover, the peroxidase activity of the crystallographically solved complexes 2 and 8, studied in the presence of the water‐soluble trap ABTS, agrees with the predicted rate according to the activity/rhombicity correlation.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Aqua(phthalocyaninato-κ4N)magnesium(II) 3-chloropyridine disolvate

The structure of the title compound, [Mg(C(32)H(16)N(8))(H(2)O)].2C(5)H(4)ClN, comprises MgPcH(2)O [Pc is phthalocyaninate(2-)] and 3-chloropyridine solvent molecules interacting via O-H...N hydrogen bonds and pi-pi interactions. The central Mg atom is (4+1)-coordinated by four equatorial isoindole N atoms of the macrocycle and by the O atom of an axial water molecule. The MgPcH(2)O molecule is not planar, the Mg atom being displaced by 0.496 (2) A from the isoindole N(4) plane towards the water O atom. MgPcH(2)O molecules related by a twofold screw axis interact via O-H...N(azamethine) hydrogen bonds, forming a polymeric chain along the b axis, while those related by inversion centres form pi-pi interacting dimers.

A seven-coordinate Fe III compound: [Fe{O(CH 2CO 2) 2}(H 2O) 2(NO 3)]. Preparation, structure and magnetic properties

A seven-coordinate Fe III complex, [Fe(oda)(H 2O) 2(NO 3)], was obtained after dissolving Fe(NO 3) 3 · 9H 2O in an aqueous solution of oxydiacetic acid (H 2oda) at room temperature. In the solid state, the Fe III center adopts a pentagonal bipyramid geometry with an {FeO 7} core formed by a tridentate oda 2− and a bidentate NO 3 − in the equatorial plane, and two axial water molecules. Magnetic measurements and EPR spectra revealed the presence of S = 5/2 Fe III centers with rhombic zero field splitting parameters ( D = 0.81 cm −1, E/ D = 0.33 ). Weak antiferromagnetic interactions with J ≈ −0.06 cm −1 operating between neighboring Fe ions connected through Fe–O–C–O⋯H–O–Fe paths are estimated using the molecular field approximation.

Aqua(phthalocyaninato)magnesiumn-propylamine disolvate

The crystals of the title compound, [Mg(C(32)H(16)N(8))(H(2)O)].2C(3)H(9)N, are built up from MgPc(H(2)O) [Pc is phthalocyaninate(2-)] and n-propylamine molecules that interact via O-H...N hydrogen bonds. The MgPc(H(2)O) molecule is non-planar. The central Mg atom is coordinated by the four equatorial isoindole N atoms of the Pc ring system and by the O atom of an axial water molecule. The Mg atom is displaced by 0.509 (1) A from the N(4) plane towards the water O atom. MgPc(H(2)O).2(n-propylamine) molecules related by the inversion centre are linked by N-H...O hydrogen bonds to form a dimeric aggregate.

Synthesis, spectral and magnetic properties and crystal structures of copper(II) pyridinecarboxylates as potential antimicrobial agents

Synthesis and characterization of six new complexes [Cu{2,6-(MeO) 2nic} 2(H 2O)] 2 ( 1), [Cu{2,6-(MeO) 2nic} 2(H 2O)] 2 · 3DMF ( 2), where 2,6-(MeO) 2nic is 2,6-dimethoxynicotinate and DMF is N, N-dimethylformamide, [Cu(3-pyacr) 2(H 2O) 2] n ( 3), where 3-pyacr is trans-3-(3-pyridyl)acrylate, [Cu(en) 2(H 2O) 2]X 2, where X is 2,6-(MeO) 2nic ( 4) or 3-pyacr ( 5) and en is ethylenediamine, and [Cu(3-pyacr) 2(dien)(μ-H 2O) 0.5] 2 · 7H 2O ( 6), where dien is diethylenetriamine are reported. The characterizations were based on elemental analysis, infrared, electronic and EPR spectra, and magnetic measurements over a temperature range of 1.8–300 K. Crystal structures of complexes 2, 4 and 6 have been determined by X-ray single crystal structure analysis. The available evidence supports dimeric structure of the acetate type for 1 and 2. Crystal structure of polymeric complex 3 has been determined from X-ray powder diffraction data. The 3-pyacr anions in pairs form bridges between two octahedrally surrounded copper(II) atoms in such a way that one 3-pyacr is coordinated to the first Cu II by an oxygen atom of its carboxyl group and to the second Cu II by the nitrogen atom of its pyridine ring, while the other is coordinated to the same two Cu II atoms in a similar way, but the other way round. Environment about the copper(II) atom for 4 and 5 is a square bipyramid (4+2). In complex 6 both Cu II central atoms are bridged only by an axial water molecule forming a dimeric structure with the considerably long separation of Cu II atoms of 5.194 Å and the angle Cu1–O3–Cu1a of 150.79°. Moreover, results of the quantitative determination of antimicrobial activity of the complexes as well as above organic ligands alone are discussed.

A new Cu(ii) [12]metallocrown-4 pentanuclear complex based on a Cu(ii)-malonomonohydroxamic acid unit

The first example of a Cu(II) [12]-MC-4 hydroxamic metallacrown compound containing a carboxyl group as a supporting donor function is described. The solution equilibria of malonomonohydroxamic acid (MACZ, H2L) with Cu(II) are investigated in aqueous solution using a combination of potentiometry, UV-vis absorption spectrophotometry, EPR spectroscopy and ESI mass spectrometry. Among the four complexes fitting the best speciation model ([CuL], [Cu5L4H−4]2−, [CuL2]2− and [CuL2H−1]3−), a pentameric metallacrown molecule of composition Cu : L = 5 : 4 predominates in solution over the pH 4 to 11 range, and the corresponding complex was isolated in the solid state. The crystallization of the complex [Cu5L4H−4]2− in the presence of [Cu(en)2(H2O)2]2+ cations resulted in the isolation of [Cu(en)2(H2O)2]n[Cu(en)2(H2O)(μ-H2O){Cu5(L4H−4)(H2O)3}2]n·20nH2O (1), whose crystal structure has been determined by X-ray analysis. The structure of 1 consists of centrosymmetric complex cations [Cu(en)2(H2O)2]2+, infinite complex anionic chains [Cu(en)2(H2O)(μ-H2O){Cu5(L4H−4)(H2O)3}2]n2n− and solvate water molecules. Within the complex anionic chains, the decanuclear double-decked bis([12]-MC-4) complex anions {Cu5(L4H−4)(H2O)3}24− are united by the [Cu(en)2(H2O)2]2+ complex cations due to the bridging function of the axial water molecule O(5). The magnetic behaviour of 1, studied in the temperature range 1.8–300 K, suggests the presence of both antiferromagnetic and ferromagnetic contributions to the observed magnetic susceptibility, resulting in a ground state of S = 2 per formula unit.

Stability of Different Zinc(II)−Diamine Complexes in Aqueous Solution with Respect to Structure and Dynamics: A QM/MM MD Study

In the context of our detailed study of the chemical behavior of aquo- and ammine-Zn(II) complexes, ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations were performed at the Hartree-Fock (HF) level for the zinc(II)-diamine complexes in aqueous solution. The initial structures of cis and trans isomers of the tetraaquodiamminezinc(II) complex were found to transform into the triaquodiamminezinc(II) complex by releasing one water ligand after approximately 6 and approximately 22 ps of simulation time, respectively. The structural and dynamical properties of these three zinc complexes, i.e., cis-[Zn(NH3)2(H2O)4]2+, trans-[Zn(NH3)2(H2O)4]2+, and [Zn(NH3)2(H2O)3]2+, were analyzed in terms of radial distribution functions (RDF), coordination number distributions (CND), angular distribution functions (ADF), tilt and theta angle distributions, ligands' mean residence times (MRTs), and ion-ligand stretching frequencies. One considerably elongated Zn-O bond of 2.43 A was observed in the case of the cis isomer for one of the water ligands located in the trans position to an ammonia ligand. In the trans isomer the average Zn-O bond length was observed to be 2.23 A, while in the triaquodiamminezinc(II) complex two distinct Zn-O bonds, namely 2.12 A for the ligands in the trigonal plane and 2.26 A for axial water molecules, were observed. As both of the octahedral isomers are transformed into the pentacoordinated structure within the picosecond range, they might be regarded as "metastable species or intermediates", while the triaquodiamminezinc(II) complex is the most stable species of the zinc(II)-diamine complex in aqueous solution.

Synthesis, crystal structure and magnetism of a single-molecule magnet, [Mn 16O 16(OMe) 6(OAc) 16(MeOH) 3(H 2O) 3] · 6H 2O, and of a mixed bridge 1D chain, [Mn(μ-OMe)(μ-OAc) 2] n

The syntheses, structures and magnetic characterisations of the hexadecanuclear manganese(III/IV) carboxylate cluster [Mn 16O 16(OMe) 6(OAc) 16(MeOH) 3(H 2O) 3] · 6H 2O ( 1 · 6H 2O) and a linear chain Mn III complex [Mn(μ-OMe)(μ-OAc) 2] n ( 2) are reported. AC magnetic susceptibility measurements confirm 1 to be a single-molecule magnet with relaxation parameters of τ 0 = 3.00 × 10 −9 s and energy barrier, Δ E, of 52.2 cm −1. Complexes 1 · 6H 2O and 2 were obtained by reaction of n-Bu 4NMnO 4 with Mn(NO 3) 2 · 4H 2O in a mixture of methanol and acetic acid. Complex 1 · 6H 2O crystallises in the triclinic space group P 1 ¯ and the cluster structure consists of six Mn IV and 10 Mn III ions held together by 14 μ 3-O 2−, two μ 2-O 2−, four μ 2-OMe − and two μ 2-OAc − groups to give a roughly elliptical planar [Mn 16O 16(OMe) 4(OAc) 2] 16+ core. Peripheral ligation consists of the remaining 14 μ 2-OAc −, two μ 2-OMe − groups and three axial water and methanol molecules. Complex 2 crystallises in the orthorhombic space group Pbcn and consists of infinite linear 1D chains of manganese(III) atoms bridged by two μ-OAc − ligands and one μ-OMe −. The magnetic data on 1 · 6H 2O and 2 are compared with those of other recently reported Mn 16 and Mn(III) μ-carboxylate chain analogues.

Synthesis and characterization of mesostructured tungsten nitride by using tungstic acid as the precursor

Mesostructured tungsten nitride was firstly prepared from tungstic acid via the temperature programmed reaction with ammonia. The N2 adsorption isotherm of as-synthesized tungsten nitride was of type IV with a type H-3 hysteresis loop. BJH pore size distribution was of bimodal distribution (2.5 and 3.5 nm), among which the latter was the main channel of tungsten nitride. The surface area of as-synthesized tungsten nitride was up to 89 m2 g−1. XRD pattern showed that the crystal phase of the product was β-W2N. The effect of synthesis parameters on the surface area of tungsten nitride was investigated extensively. The nitridation mechanism was investigated by in-situ XRD and N2 adsorption analysis. It was found that H2WO4 was initially transformed into WO3 by eliminating the axial water molecules, and WO3 retained the layered and porous structure of H2WO4. Below 773 K, WO3 was just partially reduced to W20O58 and W20O40. Above 773 K, β-W2N phase could be detected. It indicated that during nitridation, WO3 was gradually reduced and then the homogeneous substitution of oxygen vacancies in the reduced oxides with nitrogen atoms occurred. Based on the experimental results, a reduction-nitridation mechanism was firstly proposed.

Control of magnetic spin states by various mixed anionic ligands in trinickel complexes: synthesis, crystal structures and physical properties

This study provides an opportunity to control the magnetic spin of nickel atoms using various mixed anionic ligands. A series of linear trinickel complexes supported by two kinds of ligands, oligo-alpha-pyridylamido and sulfonyl amido/amido, were synthesized and their structures were determined by X-ray diffraction. The three nickel atoms of [Ni(3)(Lpts)(2)(dpa)(2)] (dpa(-) = dipyridylamido, Lpts(2-) = N,N'-bis(p-toluenesulfonyl)pyridyldiamido) display short Ni-N ( approximately 1.90 Angstrom) bond distances, which are consistent with a low spin state of Ni(II) ions, and exhibit spin states of (0, 0, 0) for the three Ni(II) ions. One of the terminal Ni(II) ions of [Ni(3)(Lms)(2)(dpa)(2)(H(2)O)] (Lms(2-) = N,N'-bis(4-methylsulfonyl)-pyridyldiamido) and [Ni(3)(Lpts)(2)(pepteaH(2))] (pepteaH(2)(2-) = pentapyridyldiamidodiamine) bonded with an axial ligand exhibits a square pyramidal (NiN(4)X) geometry with long Ni-N bond distances ( approximately 2.10 Angstrom) which are consistent with a high spin Ni(II) configuration. The spin states of these trinickel complexes are (1, 0, 0). Complexes interchanged by the removal or addition of an axial water molecule. The structural features of are comparable with those of . Both the terminal Ni(II) ions in [Ni(3)(LAc)(2)(dpa)(2)] (Lac(2-) = N,N'-biacetyl-pyridyldiamido) are in square pyramidal geometry and exhibit high spin. The spin states of the nickel ions in are (1, 0, 1), and the two terminal nickel ions exhibit antiferromagnetic interactions. The molecular structure of [Ni(3)(Lpts)(2)(dpa)(2)](BF(4)), which was obtained by the one-electron oxidation is similar to those of the neutral analogue , except for the presence of a counter anion to compensate for the positive charge on the Ni(3) core. All of the Ni-Ni bond lengths of are slightly shorter (ca. 0.05 Angstrom) than those in the neutral analogues. This is attributed to the formation of partial Ni-Ni bonding.