Outer Coordination Sphere Research Articles

Adsorption of hydrated Fe(OH)<sup>2+</sup> on the kaolinite surface: A density functional theory study

The present study employed density functional theory (DFT) to analyze the adsorption configuration and mechanism of Fe(OH)<sup>2+</sup> on the kaolinite (001) surface. The findings demonstrated that Fe(OH)<sub>2</sub>(H<sub>2</sub>O)<sup>4+</sup> is the main type in which hydrated Fe(OH)<sup>2+</sup> can be found in aqueous solution. On the surface of kaolinite, Fe(OH)<sub>2</sub>(H<sub>2</sub>O)<sup>4+</sup> will be adsorbed. There are two forms of adsorption: outer-sphere and inner-sphere coordination (monodentate/bidentate) adsorption. Fe(OH)<sub>2</sub>(H<sub>2</sub>O)<sup>4+</sup> has a moderate propensity to adsorb on the alumina octahedral sheet of kaolinite when the outer-sphere coordination adsorption takes place. In cases of inner-sphere coordination adsorption, Fe exhibits a tendency to form monodentate adsorption compounds in conjunction with Ou atoms. Additionally, it prefers to create bidentate adsorption compounds through coordination with both Ot and Ou atoms. The adsorption mechanism analysis results show that the ionic property of Fe atom decreases after outer-sphere coordination adsorption. After inner-sphere coordination adsorption, some electrons of Fe atom are transferred to the surface O atom. The presence of electrons between the Fe and O atoms enhances the formation of bonds, hence enhancing the covalent nature of the Fe-O bond. Theoretical FT-IR (Fourier transform infrared spectroscopy) calculations show that the formation of Fe-O chemical bonds. Because of the lower adsorption energy and more chemical bonds, hydrate Fe(OH)<sup>2+</sup> is more likely to be bidentate adsorbed on the kaolinite surface.

Construction of an OCP-ATR-FTIR Spectroscopy Device to In Situ Monitor the Interfacial Reaction of Contaminants: Competitive Adsorption of Cr(VI) and Oxalate on Hematite.

The comprehensive understanding of contaminant interfacial behavior strongly depends on the in situ characterization technique, which is still a great challenge. In this study, we constructed a device integrated with open-circuit potentialand attenuated total reflectance Fourier transform infrared (OCP-ATR-FTIR) spectroscopy to simultaneously monitor the electrochemical and infrared spectral information on the interfacial reaction for the process analysis, taking the competitive adsorption of hexavalent chromium (Cr(VI)) and oxalate on hematite nanocubes (HNC) as an example. The synchronous OCP and infrared results revealed that Cr(VI) interacted with HNC via bidentate binuclear inner-sphere coordination, accompanied by electron transfer from HNC to Cr(VI), while oxalate was adsorbed on HNC through bidentate mononuclear side-on inner-sphere coordination with electron transfer from HNC to oxalate, and also outer-sphere coordination with negative charge accumulation. When oxalate was added to HNC with preadsorbed Cr(VI), oxalate would occupy the inner-sphere adsorption sites and thus cause the detaching of preadsorbed Cr(VI) from HNC. This study provides a promising in situ characterization technique for real-time interfacial reaction monitoring and also sheds light on the competitive adsorption mechanism of oxalate and Cr(VI) on the mineral surface.

Bio‐inspired Design of Bidirectional Oxygen Reduction and Oxygen Evolution Reaction Molecular Electrocatalysts

AbstractThe proper utilization of renewable energy sources has emerged as a major challenge in our pursuit of a sustainable and carbon‐neutral energy landscape. Small molecule activation is a key component for proper utilization of renewable energy resources, where O2/H2O redox couple is reckoned to be a potential game changer. In this regard, electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have become the prime interest of catalyst designers. Typically, these ORR and OER electrocatalysts are developed distinctly; however, very soon, the requirement of a bidirectional ORR/OER electrocatalyst becomes obvious for practical applicability and rapid energy transduction purposes. A bidirectional catalyst is defined as a catalyst capable of driving a redox reaction in opposing directions. This review has portrayed the development of enzyme structure‐inspired design of molecular bidirectional ORR/OER catalysts. The strategic incorporation of secondary and outer coordination sphere features has significantly enhanced the performance of these catalysts, which can be monitored via the key catalytic parameters. These bifunctional OER/ORR catalysts are vital for metal‐air battery and fuel cell applications and appropriately poised to lay the foundation for an efficient, economical, and eco‐friendly pathway for sustainable energy usage with the rational assembly of energy converting and storage devices.

Probing Substrate Transport Effects on Enzymatic Hydrogen Catalysis: An Alternative Proton Transfer Pathway in Putatively Sensory [FeFe] Hydrogenase.

[FeFe] hydrogenases, metalloenzymes catalyzing proton/dihydrogen interconversion, have attracted intense attention due to their remarkable catalytic properties and (bio-)technological potential for a future hydrogen economy. In order to unravel the factors enabling their efficient catalysis, both their unique organometallic cofactors and protein structural features, i.e., "outer-coordination sphere" effects have been intensively studied. These structurally diverse enzymes are divided into distinct phylogenetic groups, denoted as Group A-D. Prototypical Group A hydrogenases display high turnover rates (104-105 s-1). Conversely, the sole characterized Group D representative, Thermoanaerobacter mathranii HydS (TamHydS), shows relatively low catalytic activity (specific activity 10-1 μmol H2 mg-1 min-1) and has been proposed to serve a H2-sensory function. The various groups of [FeFe] hydrogenase share the same catalytic cofactor, the H-cluster, and the structural factors causing the diverging reactivities of Group A and D remain to be elucidated. In the case of the highly active Group A enzymes, a well-defined proton transfer pathway (PTP) has been identified, which shuttles H+ between the enzyme surface and the active site. In Group D hydrogenases, this conserved pathway is absent. Here, we report on the identification of highly conserved amino acid residues in Group D hydrogenases that constitute a possible alternative PTP. We varied two proposed key amino acid residues of this pathway (E252 and E289, TamHydS numbering) via site-directed mutagenesis and analyzed the resulting variants via biochemical and spectroscopic methods. All variants displayed significantly decreased H2-evolution and -oxidation activities. Additionally, the variants showed two redox states that were not characterized previously. These findings provide initial evidence that these amino acid residues are central to the putative PTP of Group D [FeFe] hydrogenase. Since the identified residues are highly conserved in Group D exclusively, our results support the notion that the PTP is not universal for different phylogenetic groups in [FeFe] hydrogenases.

Cyclohexyl substituted diglycolamide ligands for highly efficient separation of strontium: Synthesis, extraction and crystallography studies

A series of novel diglycolamide (DGA) ligands containing cyclohexyl substituent on the amidic N atom were investigated for their solvent extraction and complexation ability toward Sr2+ in HNO3 solution. Compared with the most commonly used N,N,N′,N′-tetraoctyldiglycolamide (TODGA), the introduction of cyclohexyl substituent into the DGA skeleton can significantly enhance the extractability toward Sr2+ in the case of toluene as the diluent. And the more cyclohexyl substituents there are, the more significant the effect is. About 8−80 fold increase of the Sr2+ distribution value for LI−LVI ligands can be achieved than that for TODGA at 2.0 mol/L HNO3. The extractability of these cyclohexyl substituted DGA ligands follows the order of LI ≈ LIII > LII > LIV > LV > LVI > > TODGA. Meanwhile, for the DGA ligands with the same number of cyclohexyl substituents, LIII with moderate alkyl chain length (n-butyl) has higher extractability than LII with short chain (n-ethyl) and LIV with long chain (n-octyl). Slope analysis reveals the formation of 1:3 metal-ligand extracted species during the extraction process, which is also confirmed by the results obtained from ESI-MS, FT-IR and single-crystal X-ray diffraction analyses. Furthermore, the single crystal of DGA ligand with Sr(NO3)2 was obtained for the first time, and the analysis of its structure indicates that each DGA molecule participates in the complexation with the 9-fold coordinated Sr2+ through two amide carbonyl O atoms and one ether O atom, accompanied by two free NO3− in the outer coordination sphere. The extraction of Sr2+ by the novel DGA ligand is a spontaneous and exothermic process of increasing entropy, and follows neutral complexing extraction model. In addition, an extraction test of Sr2+ from simulated high level liquid waste was also carried out and demonstrates a highly efficient and selective separation of Sr2+ over the other co-existing ions.

Crafting Fast and Efficient H2Evolution Electrocatalysts with Tactical Inclusion of Nucleobases

In order to achieve a hydrogen-driven energy infrastructure that is carbon-neutral, it is imperative that there be a synthetic catalyst that is developed from a non-noble metal. The unique interplay between the abundant transition metal containing an active site and the surrounding protein-based outer coordination sphere (OCS) is the essence of the remarkable H2 production displayed by hydrogenase enzymes. Here, we report a series of biomimetic cobalt complexes [Co(dimethylglyoxime)2(Nnucleobase derivative)Cl] crafted by strategic incorporation of a nucleobase and its derivatives (adenine, adenosine, adenosine monophosphate and hypoxanthine) around a common template. The nucleoside- and nucleotide-appended complexes electrocatalyze H2 evolution from neutral aqueous solutions at a rapid rate (turnover frequencies of ∼13 000 and ∼12 000 s–1, respectively) while operating at an overpotential of <400 mV. The intricate proton exchange network created between the different fractions of nucleobase derivatives is one of the prime reasons behind such fast and energy-efficient catalysis.

Synthesis, Spectral Characterization and Potential Fluorescent Properties of Three Lanthanide(III) Ions Complexes with Nalidixic Acid.

Three new solid lanthanide complexes with nalidixic acid (HNal) with the stoichiometry [Ln(Nal)3]·5·.5H2O (Ln=Tb, Dy and Ho) were synthesized applying the green synthesis method from the aqueous solutions without the organic solvent addition and fully characterized by the elemental analysis, XRF, complexometric titration, gravimetric analysis, molar conductivity and solubility measurements, powder X-Ray diffraction, UV-Vis and infrared (FT-IR) spectroscopies. Moreover, the luminescent properties of the Tb(III), Dy(III), and Ho(III) complexes in the solid state and in the solutions were investigated. On the basis of the detailed spectral analysis, it was concluded that the nalidixate ligands bind to the lanthanide ions by the bidentate carboxylate and carbonyl groups while water molecules belong to the outer coordination sphere. At the excitation of UV light, the complexes exhibited characteristic emission of central lanthanide ions, the intensity of which depends significantly on the excitation wavelength and/or the solvent. Thus, the application of nalidixic acid (apart from biological activity) for the synthesis of luminescent lanthanide complexes was confirmed which can find potential applications in the field of photonic devices and/or bioimaging agents.

Delayed Doublet Emission in a Cerium(III) Complex.

Doublet emission from open-shell molecules has demonstrated its research and application value in recent years. However, understandings of the photoluminescence mechanism of open-shell molecules are far less than that of closed-shell molecules, leading to challenges in molecular design of efficient doublet emission systems. Here we report a cerium(III) 4-(9H-carbozol-9-yl)phenyl-tris(pyrazolyl)borate complex Ce(CzPhTp)3 with a new luminescence mechanism of delayed doublet emission, which also represents the first example with metal-centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)3 is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The photoluminescence mechanism discovered may provide a new way for the design of efficient doublet emission and bring insights into rational molecular design and energy level regulation in open-shell molecules.

Delayed Doublet Emission in a Cerium(III) Complex

AbstractDoublet emission from open‐shell molecules has demonstrated its research and application value in recent years. However, understandings of the photoluminescence mechanism of open‐shell molecules are far less than that of closed‐shell molecules, leading to challenges in molecular design of efficient doublet emission systems. Here we report a cerium(III) 4‐(9H‐carbozol‐9‐yl)phenyl‐tris(pyrazolyl)borate complex Ce(CzPhTp)3 with a new luminescence mechanism of delayed doublet emission, which also represents the first example with metal‐centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)3 is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The photoluminescence mechanism discovered may provide a new way for the design of efficient doublet emission and bring insights into rational molecular design and energy level regulation in open‐shell molecules.

Expanding the Horizon of Bio-Inspired Catalyst Design with Tactical Incorporation of Drug Molecules.

The development of potent H2 production catalysts is a key aspect in our journey toward the establishment of a sustainable carbon-neutral power infrastructure. Hydrogenase enzymes provide the blueprint for designing efficient catalysts by the rational combination of central metal core and protein scaffold-based outer coordination sphere (OCS). Traditionally, a biomimetic catalyst is crafted by including natural amino acids as OCS features around a synthetic metal motif to functionally imitate the metalloenzyme activity. Here, we have pursued an unconventional approach and implanted two distinct drug molecules (isoniazid and nicotine hydrazide) at the axial position of a cobalt core to create a new genre of synthetic catalysts. The resultant cobalt complexes are active for both electrocatalytic and photocatalytic H2 production in near-neutral water, where they significantly enhance the catalytic performance of the unfunctionalized parent cobalt complex. The drug molecules showcased a dual effect as they influence the catalytic HER by improving the surrounding proton relay along and exerting subtle electronic effects. The isoniazid-ligated catalyst C1 outperformed the nicotine hydrazide-bound complex C2, as it produced H2 from water (pH 6.0) at a rate of 3960 s-1 while exhibiting Faradaic efficiency of about 90 %. This strategy opens up newer avenues of bio-inspired catalyst design beyond amino acid-based OCS features.

Cyanide replaces substrate in obligate-ordered addition of nitric oxide to the non-heme mononuclear iron AvMDO active site.

Thiol dioxygenases are a subset of non-heme mononuclear iron oxygenases that catalyze the O2-dependent oxidation of thiol-bearing substrates to yield sulfinic acid products. Cysteine dioxygenase (CDO) and 3-mercaptopropionic acid (3MPA) dioxygenase (MDO) are the most extensively characterized members of this enzyme family. As with many non-heme mononuclear iron oxidase/oxygenases, CDO and MDO exhibit an obligate-ordered addition of organic substrate before dioxygen. As this substrate-gated O2-reactivity extends to the oxygen-surrogate, nitric oxide (NO), EPR spectroscopy has long been used to interrogate the [substrate:NO:enzyme] ternary complex. In principle, these studies can be extrapolated to provide information about transient iron-oxo intermediates produced during catalytic turnover with dioxygen. In this work, we demonstrate that cyanide mimics the native thiol-substrate in ordered-addition experiments with MDO cloned from Azotobacter vinelandii (AvMDO). Following treatment of the catalytically active Fe(II)-AvMDO with excess cyanide, addition of NO yields a low-spin (S = 1/2) (CN/NO)-Fe-complex. Continuous wave and pulsed X-band EPR characterization of this complex produced in wild-type and H157N variant AvMDO reveal multiple nuclear hyperfine features diagnostic of interactions within the first- and outer-coordination sphere of the enzymatic Fe-site. Spectroscopically validated computational models indicate simultaneous coordination of two cyanide ligands replaces the bidentate (thiol and carboxylate) coordination of 3MPA allowing for NO-binding at the catalytically relevant O2-binding site. This promiscuous substrate-gated reactivity of AvMDO with NO provides an instructive counterpoint to the high substrate-specificity exhibited by mammalian CDO for L-cysteine.

Indolyl Phosphine Nickel(II) Fluorido Complexes: Synthesis and Intermediates in Suzuki–Miyaura Cross‐Coupling Reactions

AbstractThe C−F bond activation of pentafluoropyridine and 2,3,5,6‐tetrafluoropyridine at [Ni(cod)2] (cod=1,5‐cyclooctadiene) in the presence of the phosphine PPh2(Ind) (Ind=3‐methyl‐2‐indolyl) led to the formation of the nickel(II) fluorido bis(phosphine) complexes trans‐[Ni(F)(2‐C5NF4){PPh2(Ind)}2] and trans‐[Ni(F)(2‐C5HNF3){PPh2(Ind)}2]. The complexes are characterized by the presence of intramolecular hydrogen bonds between the NH group of the phosphine ligands and the fluorido ligand. Stochiometric model reactions of nickel(II) fluorido complexes with PhB(OH)2 revealed that the former can be considered as intermediates in Suzuki–Miyaura cross coupling reactions. Catalytic experiments were attempted using 10 mol‐% of trans‐[Ni(F)(2‐C5NF4){PPh2(Ind)}2] as catalyst and the activities of the PPh2(Ind) complex were compared to the ones of an analogous nickel(II) fluorido complex, bearing PPh3 instead of PPh2(Ind) as ligands. The latter exhibited a somewhat lower catalytic activity suggesting a slight influence of the H‐bonds in the outer coordination sphere.

Electrode Integration of Synthetic Hydrogenase as Bioinspired and Noble Metal-Free Cathodes for Hydrogen Evolution

Diiron complexes mimicking the H-cluster of [FeFe]-hydrogenases have been extensively studied as (electro-)catalysts for proton reduction under homogeneous conditions. The incorporation of such complexes as “active sites” within macromolecular scaffolds such as organic polymers is receiving increasing attention as this strategy allows controlling the environment, that is, the outer coordination sphere, around the molecular catalytic center, to tune its performance as well as its stability. Here, we report on the synthesis and characterization of a library of metallo-copolymers featuring a bioinspired diiron active site and internal proton relays based on a previous report [Brezinski et al. Angew. Chem. Int. Ed. 2018, 57, 11898–11902]. The polymers are further functionalized with various amounts of pyrene groups for efficient noncovalent anchoring onto multi-walled carbon nanotubes (MWNTs), enabling the preparation of molecularly engineered electrode materials. The addition of pyrene anchors resulted in improved activity and stability, with a pyrene loading of about ∼8% corresponding to an optimized balance between polymer hydrophilicity and surface affinity. The best material displayed an average turnover frequency (TOFH2) of 4.3 ± 0.6 s–1 and a conservative turnover number for H2 production (TONH2) of 3.1 ± 0.4 × 105 after 20 h of continuous bulk electrolysis in aqueous conditions at 0.39 V overpotential. Interestingly, comparing such activities with an analogous diiron site deprived from polymeric scaffold revealed that latter could only show TONH2 of ∼4 ± 2 × 103 and TOFH2 of 0.06 ± 0.02 s–1 in 20 h under the same conditions. Post operando analysis of the modified electrodes suggests that electrode inactivation occurs via leaching of the diiron core from MWNT. In addition, a life cycle assessment was carried out to evaluate the performance of the engineered electrode materials not only from a technical perspective but also from an environmental point of view.

Cobalt complexes with α-amino acid ligands catalyze the incorporation of CO2 into cyclic carbonates.

Arguably one of the largest research areas involving carbon dioxide (CO2) fixation is the coupling of CO2 to epoxides to form cyclic carbonates and polycarbonates. In this sense, there is an ever-increasing demand for the development of higher-performing catalytic systems that could counterbalance sustainability and energy efficiency in the production of cyclic carbonates. The use of abundant first-row transition metals combined with naturally occurring amino acids may be an ideal catalytic platform to fulfill this demand. Nevertheless, detailed information on the interactions between metal centers and natural products as catalysts in this transformation is lacking. Here a series of Co(III) amino acid catalysts operating in a binary system showed outstanding performance for the coupling reaction of epoxides and CO2. Nine new complexes of the type trans(N)-[Co(aa)2(bipy)]Cl (aa: ala, asp, lys, met, phe, pro, ser, tyr, and val) were used to explore the structure-activity relationship influenced by the complex outer coordination sphere, and its effect on the catalytic activity in the coupling reaction of CO2 and epoxides.

Outer coordination sphere influences on cofactor maturation and substrate oxidation by cytochrome P460.

Product selectivity of ammonia oxidation by ammonia-oxidizing bacteria (AOB) is tightly controlled by metalloenzymes. Hydroxylamine oxidoreductase (HAO) is responsible for the oxidation of hydroxylamine (NH2OH) to nitric oxide (NO). The non-metabolic enzyme cytochrome (cyt) P460 also oxidizes NH2OH, but instead produces nitrous oxide (N2O). While both enzymes use a heme P460 cofactor, they selectively oxidize NH2OH to different products. Previously reported structures of Nitrosomonas sp. AL212 cyt P460 show that a capping phenylalanine residue rotates upon ligand binding, suggesting that this Phe may influence substrate and/or product binding. Here, we show via substitutions of the capping Phe in Nitrosomonas europaea cyt P460 that the bulky phenyl side-chain promotes the heme-lysine cross-link forming reaction operative in maturing the cofactor. Additionally, the Phe side-chain plays an important role in modulating product selectivity between N2O and NO during NH2OH oxidation under aerobic conditions. A picture emerges where the sterics and electrostatics of the side-chain in this capping position control the kinetics of N2O formation and NO binding affinity. This demonstrates how the outer coordination sphere of cyt P460 is tuned not only for selective NH2OH oxidation, but also for the autocatalytic cross-link forming reaction that imbues activity to an otherwise inactive protein.

Peripheral nucleic bases boost H2 production by synthetic molecular catalysts in acidic water.

The strategic inclusion of nucleic bases adenine, cytosine, and thymine, in the form of outer coordination sphere, positively impacts the electro- and photocatalytic H2 production by cobaloxime cores. These cobaloxime derivatives showcased their optimal H2 production in acidic media due to specific protonation of adenine and cytosine below pH 5.0.

Realistic modelling of hydrogen bonding of 2-cyclohexenone with H2O and H3BO3 in the outer coordination sphere of a chiral diene/Rh(I) complex by ab initio molecular dynamics.

A full-DFT Born-Openheimer MD (BOMD) study of the potential hydrogen bonding of 2-cyclohexenone π-complexed to Rh(I) in explicit 1,4-dioxane is presented. The complex is a key intermediate in the academically and industrially important asymmetric Rh-catalysed 1,4-addition of arylboronic acids to α,β-unsaturated ketones with the directing ligand phbod, a chiral bicyclic 1,4-diene. The ketone O atom (Ok) behaves as a single H-bond acceptor persistently throughout most of the simulation time while the donor is mobile and liable to exchange. Well-tempered metadynamics show that H-bonding with a (H2O)3 cluster is favorable by free energy but kinetically labile while with just H3BO3 is unfavorable but kinetically much more persistent. When both (H2O)3 cluster and H3BO3 are within H-bond distance from Ok, the non-H-bonded and the various H-bonded species are close in energy, implying the free energy surface is complex and quite flat. The most stable species features a H-bond with a water acceptor but not with H3BO3. The non-H-bonded state is 0.7 kcal mol-1 higher in free energy. Model static DFT studies reveal that H-bonding with both (H2O)3 cluster and H3BO3 is favorable by enthalpy, but unfavorable by free energy when the entropy term is added.

Improving the synthetic H2 production catalyst design strategy with the neurotransmitter dopamine.

The strategic incorporation of the neurotransmitter dopamine around a cobaloxime core resulted in excellent electrocatalytic (rate 8400 s-1) and photocatalytic H2 production under neutral aqueous conditions. The influence of the synthetic outer coordination sphere features continues even with a phenylene-diimino-dioxime motif-coordinated cobalt core.

Single atom catalysts supported on metallic C5N monolayers for oxygen reduction/evolution reactions with more active sites than loaded metal atoms

Single atom catalysts (SACs) supported by carbonaceous materials present extraordinary activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which however suffer from low loadings of transition metal (TM) atom. We here proposed a strategy of designing SACs toward ORR/OER containing higher number of active sites than that of loaded TM atoms, which include not only the metal centers but also the non-metal atoms activated by the doping of TM atoms. By incorporating TM atoms of Fe, Co and Ni into our previously predicted metallic monolayers of C5N, we investigated their ORR/OER activities and found that orthorhombic-C5N supported Co atoms coordinated with three C atoms and one N atom can serve as superior bifunctional electrocatalysts for both ORR and OER with overpotentials of 0.38 and 0.3 V, respectively. Our density functional theory calculations also demonstrate that the structural configurations of the first, second and even outer coordination sphere of metal centers can simultaneously affect the catalytic performance of SACs. This study provides a novel idea to address the challenge faced by the SACs toward ORR/OER caused by the low loadings of TM atoms and obscure structure-property relationship.

Di-chlorido-(4,4'-dimethyl-2,2'-bi-pyridine-κ2 N,N')zinc(II) aceto-nitrile monosolvate.

In the title complex, [ZnCl2(C12H12N2)]·CH3CN, the zinc(II) atom is fourfold coordinated by two chloride ligands and a bidentate 4,4'-dimethyl-2,2'-bi-pyridine ligand in a distorted tetra-hedral shape with a mol-ecule of aceto-nitrile sitting in the outer coordination sphere of the complex. π-π stacking inter-actions between the pyridyl rings in adjacent mol-ecules contribute to the alignment of the complexes in columns parallel to the a axis.