Metal Complexes Research Articles

Amino-functionalised mesoporous Silica Nanoparticles for the delivery of Isoniazid and its Metal Complexes

The effective delivery of poor water-soluble anti-tubercular drugs such as isoniazid to their target sites is a major factor that has made it difficult for tuberculosis to be eradicated. Isoniazid derivatives and nano-particulate drug delivery systems are explored as one way to address this limitation. This study investigated the use of amino-functionalised mesoporous silica nanoparticles (MCM-41) as nano-carriers for the delivery of isoniazid-based metal complexes, Cu-INH and Fe-INH. The Sol-gel method was used to synthesise the nano-carriers, which produced a series of well-ordered nano-carriers. Their physicochemical properties were modified through surface functionalisation with amino groups via post-grafting synthesis. The isoniazid-based metal complexes were incorporated into the nano-carriers using rotary evaporation. The nano-carriers were characterised using X-ray diffraction (XRD), Zeta potential, Fourier Transmission Infra-red (FT-IR), Brunaeur Emmett Teller (BET), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Thermogravimetric analysis (TGA). These characterisation techniques confirmed the successful surface functionalisation of the nano-carriers with amino groups and their encapsulation with INH, Cu-INH and Fe-INH. The amino-functionalised nano-carriers were observed to have the highest drug loading capacities and entrapment efficiencies within the range of 7.85%-19.31% and 39.26%-96.53%, respectively. The release rates of INH, Cu-INH, and Fe-INH from the nano-carriers were also studied in Phosphate Buffer Solution (PBS) with pH of 7.4 and 5.4 using ultraviolet-visible spectroscopy. The cumulative release rates of INH, Cu-INH, and Fe-INH from their nano-carriers depended on their release media's textural properties and environmental conditions. Amino functionalised nano-carriers had high release rates, with A-MCM-41+Cu-INH having the highest of 37.09% in PBS at 5.4 after 24 hours. The cumulative data obtained was fitted into Zero order, First order, Hixson Crowell, Higuchi, Korsmeyer-Peppas and Weibull models. The Hixson Crowell, Korsmeyer-Peppas, and Weibull models fitted well with correlation coefficients (R2) of 0.9813, 0.9769 and 0.9802, respectively, for MCM-41+INH in PBS of pH 5.4.

Synthesis and Investigation of New Pyridine Metal Complexes as Possible Antibacterial Agents Against Digestive System harmful Bacteria causing Human Malnutrition

Synthesis and Investigation of New Pyridine Metal Complexes as Possible Antibacterial Agents Against Digestive System harmful Bacteria causing Human Malnutrition

Relativistic Effects in Ligand Field Theory (I): Optical Properties of d1 Atoms in Oh' Symmetry.

Ligand field theory, which explains the splitting of degenerate nd atomic orbitals due to static electric fields from point-charge ligands, is rederived using Dirac orbitals instead of Schrödinger orbitals, specifically using the nd3/2 and nd5/2 spinors. This formalism is, to some extent, equivalent to incorporating the spin-orbit interaction either in the nd atomic orbitals or in the ligand field orbitals (e.g., the t2g and eg orbitals arising from Oh symmetry). The spin-orbit interaction is of fundamental importance in the description of the magnetic and optical properties of the 4d and 5d transition metal complexes. Algebraic equations for the relativistic energy levels of d1 octahedral complexes as functions of the spin-orbit coupling constant ξnd and the ligand field parameters Dq and Dp are derived. It is demonstrated that these parameters allow a direct link between the ligand field theory and ab initio relativistic calculations, consistent with the emerging ab initio ligand field theory. The spin-orbit coupling constant and ligand field parameters of ReF6 obtained from optical absorption spectra are carefuly in the light of the new theory.

Structural and Potential Biological Properties of SBDTC and SMDTC Containing Metal Complexes: A Brief Review

S-benzyldithiocarbazate (SBDTC) and S-methyldithiocarbazate (SMDTC) are working as a preliminary reactant for the synthesis of several Schiff base ligands. SBDTC and SMDTC can act as NS, SNNS, SNNNS, and NOS chelating agents and have biological and chemical potential for synthesizing many transition and inner transition metal complexes. The metal complexes prepared from SBDTC, SMDTC, and their derivatives have shown promising biological activities like antibacterial, cytotoxicity, antioxidant, antitumor, and anticancer activity. Last few decades, a lot of works have been reported on SBDTC and SMDTC containing Schiff base metal complexes. In this review, it is cosidered to show the various synthesis procedures, and structural and biological activities of SBDTC and SMDTC-containing metal complexes.

Highly Fluorescent π-Conjugated Azomethines and Divalent Metal Complexes as Antibacterial and Antibiofilm Nominees.

π-Conjugated azomethine ligands differing in the naphthalene or phenylmethane-centered core structure and their divalent cobalt, nickel, copper, and zinc metal complexes were prepared and well-characterized by spectral analyses in solid state. Magnetic natures of the complexes were determined by magnetic susceptibility measurements in solid-state. Their remarkable photophysical characteristics were recorded by Uv-vis and Fluorescence spectroscopic techniques. At their excitation wavelenght of 265 nm, all molecules exhibited triple fluorescence emission bands with promising intensities above 673 nm in near infra-red region. Antibacterial and antibiofilm activities of the π-conjugated azomethines are promising for potential applications in medical and healthcare settings. Hence, the antibacterial/antibiofilm activity of the π-conjugated azomethine ligands and their metal complexes against some clinically important bacteria namely Staphylococcus aureus (MSSA), Methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus epidermidis, Escherichia coli, Pseudomonas aeruginosa and Proteus mirabilis was investigated, and the obtained results have shown that the ligands and complexes had a remarkable antibacterial effect, especially on Proteus mirabilis. Metal complexes have been found to have a significant inhibitory effect on biofilm formation by MRSA, MSSA, and P. mirabilis compared to ligands. The copper (II) complex of ligand-2 showed the highest inhibition percentage, significantly reducing biofilm formation for MRSA and MSSA. Furthermore, cobalt (II) complexes of the ligands selectively inhibited the growth of the opportunistic pathogen P. mirabilis biofilms, indicating that metal complexes might be a good choice for future antibiofilm studies.

Targeted synthesis, structural elucidation, density functional theory, and molecular docking studies of transition metal(II)/(III) complexes of levofloxacin‐based Schiff base for promising their antioxidant and antibacterial applications

Levofloxacin and 4‐aminoantipyrine undergo a condensation process to synthesize a novel Schiff base ligand (L). Metal complexes containing Co(II), Cu(II), Cd(II), Mn(II), Zn(II), Cr(III), Ni(II), and Fe(III) metal ions are prepared using this Schiff base ligand. The new ligand and its complexes are characterized by elemental analyses, Fourier transform infrared, mass spectrometry, ultraviolet–visible, magnetic moment, 1H‐NMR, molar conductivity, and powder X‐ray diffraction, and further thermogravimetric analysis (TG, DTA) verifies their thermal stability. The metal to ligand ratio is 1:1, according to the analytical results. Spectroscopic methods reveal that metal complexes are expected to have an octahedral geometry and demonstrate a neutral tridentate ligand behavior. It also becomes apparent from the molar conductivity values that all metal complexes are electrolytic in nature except zinc(II) and cadmium(II) complexes which are non‐electrolytes. Moreover, the bioactivity of the compounds are investigated. The antibacterial activity is investigated in relation to various Gram‐positive and Gram‐negative bacteria. The results give good indication for higher activity of the prepared compounds than the standard antibiotic drug. Also, the antioxidant activity of the newly prepared ligand and its complexes has been investigated via 2,2‐diphenyl‐1‐picrylhydrazyle (DPPH) free radical scavenging assay. The findings show that Schiff base complexes exhibit good antioxidant activity. Finally, theoretical density functional theory using density functional theory/B3LYP method and molecular docking studies with two receptors of 3T88 and 3Q8U are calculated for the investigated compounds.

Towards Construction of the “Periodic Table” of 1-Methylbenzotriazole

Metal complexes of benzotriazole-type ligands continue to attract the intense interest of many inorganic chemistry groups around the world for a variety of reasons, including their aesthetically beautiful structures, physical properties and applications. 1-methylbenzotriazole (Mebta) is the N-substituted archetype of the parent 1H-benzotriazole. The first attempt to build a “periodic table” of Mebta, which includes its complexes with several metal ions, is described in this work. This, at first glance, trivial ligand has led to interesting results in terms of the chemistry, structures and properties of its metal complexes. This work reviews the to-date published coordination chemistry of Mebta with Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(I), Cu(II), Zn(II), Pd(II), Au(I) and {UVIO2}2+, with emphasis on their preparations, reactivity, structures and properties. Unpublished results from our group comprising other Co(II), Ni(II), Cu(II) and Zn(II) complexes, as well as Cd(II), Hg(II), Ag(I), In(III) and Sn(IV) ones are briefly reported. Mebta can also provide access to 1D and 3D heterometallic thiocyanato-bridged Co(II)/Hg(II) and Ni(II)/Hg(II) compounds. In almost all cases, Mebta behaves as a monodentate ligand with the nitrogen of position 3 of the azole ring as the donor atom. However, there are two copper complexes in which this molecule adopts a bidentate bridging coordination behavior. Our efforts to complete the “periodic table” of Mebta are continued.

Triazole-Based Schiff Base Metal Complexes: Synthesis, Characterization, Computational Studies and Biomedical Activity Against Cancer, H. pylori, and COVID-19

Triazole-Based Schiff Base Metal Complexes: Synthesis, Characterization, Computational Studies and Biomedical Activity Against Cancer, H. pylori, and COVID-19

The Importance of Mesoporous Materials (Silica, Alumina, and Zeolite) as Solid Supports for Metal Complex Catalysts in Organic Transformations

The Importance of Mesoporous Materials (Silica, Alumina, and Zeolite) as Solid Supports for Metal Complex Catalysts in Organic Transformations

Diastereoselective Synthesis of Sulfoxide-Functionalized N-Heterocyclic Carbene Ruthenium Complexes: An Experimental and Computational Study.

The synthesis of sulfoxide-functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6-p-cymene)(NHC-SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X-ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2-C,S) coordination mode of the NHC-SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β-alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC-functionalised S-Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β-ol-selectivities.

Accelerated Iron Evolution in Quaternary Red Soils through Anthropogenic Land Use Activities

Iron in soil exists in various valence states and is prone to changes with alterations in soil environmental conditions. Its migration and transformation are crucial for soil formation and understanding soil evolution. This study focuses on Quaternary red soils found in woodland, sparse forest grassland, grassland, and cultivated land located in the semi-humid region of the middle temperate zone. For comparison, buried Quaternary red soil was also examined. A soil reconstruction model was used to quantitatively calculate the variation of different forms of iron in order to analyze various forms of iron composition, migration, and transformation within the soil profile, as well as the evolutionary traits of Quaternary red soils influenced by diverse land use activities. This study found that after exposure and use, iron from the topsoil of buried Quaternary red soil migrated to the subsoil, altering the iron distribution. Free iron and crystalline oxides decreased in the topsoil but increased in specific subsoil layers, with woodland and grassland showing the most significant changes. Silicate-bound iron pooled in the soil weathered to form free iron under different land uses, and poorly crystalline iron oxides transformed into crystalline oxides, with grassland exhibiting the highest transformation intensity. Conversion processes predominated over iron migration in the Quaternary red soils. The evolution of Quaternary red soils can be divided into three stages, marked by changes in iron composition and crystallization due to anthropogenic land use activities. Initially, during 140−94 ka BP, iron composition was stable. Then, between 94–24 ka BP, plant decomposition formed iron–metal complexes, releasing and crystallizing poorly crystalline iron oxides. Finally, from 24 ka BP to the present, anthropogenic activities intensified, increasing the formation and conversion rates of these oxides. This study quantifies iron migration and transformation in Quaternary red soils, providing insights for sustainable soil management, especially in regions where human activities have accelerated iron evolution. Based on these findings, the following policy recommendations are proposed: implement sustainable land use practices, encourage land management strategies that preserve natural vegetation, promote research on soil management techniques, develop and implement regulatory policies, and support educational programs to maintain the health and stability of Quaternary red soils, particularly in regions prone to accelerated iron evolution due to anthropogenic activities.

Factors Controlling Cage Escape Yields of Closed- and Open-Shell Metal Complexes in Bimolecular Photoinduced Electron Transfer.

The cage escape yield, i.e., the separation of the geminate radical pair formed immediately after bimolecular excited-state electron transfer, was studied in 11 solvents using six Fe(III), Ru(II), and Ir(III) photosensitizers and tri-p-tolylamine as the electron donor. Among all complexes, the largest cage escape yields (0.67-1) were recorded for the Ir(III) photosensitizer, showing the highest potential as a photocatalyst in photoredox catalysis. These yields dropped to values around 0.65 for both Ru(II) photosensitizers and to values around 0.38 for the Os(II) photosensitizer. Interestingly, for both open-shell Fe(III) complexes, the yields were small (<0.1) in solvents with dielectric constant greater than 20 but were shown to reach values up to 0.58 in solvents with low dielectric constants. The results presented herein on closed-shell photosensitizers suggest that the low rate of triplet-singlet intersystem crossing within the manifold of states of the geminate radical pair implies that charge recombination toward the ground state is a spin-forbidden process, favoring large cage escape yields that are not influenced by dielectric effects. Geminate charge recombination in open-shell metal complexes, such as the two Fe(III) photosensitizers studied herein, is no longer a spin-forbidden process and becomes highly sensitive to solvent effects. Altogether, this study provides general guidelines for factors influencing bimolecular excited-state reactivity using prototypical photosensitizers but also allows one to foresee a great development of Fe(III) photosensitizers with the 2LMCT excited state in photoredox catalysis, providing that solvents with low dielectric constants are used.

Antituberculosis, antimicrobial, antioxidant, cytotoxicity and anti-inflammatory activity of Schiff base derived from 2,3-diaminophenazine moiety and its metal(II) complexes: structural elucidation, computational aspects, and biological evaluation

Abstract Enticed by the present scenario of infectious diseases, four new Co(II), Ni(II), Cu(II), and Cd(II) complexes of Schiff base ligand were synthesized from 6,6′-((1E-1′E)(phenazine-2,3-dielbis(azanylidene)-bis-(methanylidene)-bis-(3-(diethylamino)phenol)) (H 2 L) to ascertain as effective drug for antituberculosis, anti-inflammatory, antioxidant, cytotoxic and antimicrobial activities. The organic ligand and its metal(II) complexes were characterized by numerous physical and spectroscopic methods, which showed that the complexes have a general formula, [ML], (where M = Co(II) (C1), Ni(II) (C2), Cu(II) (C3) and Cd(II) (C4)), for metal complexes have been proposed and have a square planar geometry, are amorphous in nature, and are thermally stable. Data highlight obtained from activity testing against tuberculosis, inflammation, and oxidants that all compounds are significantly active against these symptoms. Also, was to evaluate the effectiveness of various compounds against bacterial and fungal strains. Specifically, four bacterial strains (Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa) and two fungal strains (Aspergillus flavus and Candida albicans) were tested and compared to the results of some standard drugs. The results revealed that compound C4 was more effective against bacterial strains than the comparison standard drugs. In addition, C3 was found to be the most effective of the comparison antibiotics against fungi, while the other compounds showed moderate antifungal activity. Moreover, to support the vitro results, certain computational studies as molecular docking studies, DFT, MESP, and AMEDT were also conducted to confirm the effectiveness of an organic ligand and its complexes against tuberculosis. These studies revealed that C4 is the most effective against tuberculosis and has desirable effects such as absorption, no degradation and no hepatotoxicity, etc.

Design, Characterization, and DFT exploration of new mononuclear Fe(III) and Co(II) complexes based on Isatin-hydrazone derivative: Anti-inflammatory profiling and molecular docking insights

This research focuses on the synthesis and detailed analysis of novel compounds encompassing Fe(III) (C1) and Co(II) (C2) ions, featuring 4-(2-oxo-1,2-dihydro-3H-indol-3-ylidene)hydrazono]methylphenyl 4-methylbenzenesulfonate ligand. The structural elucidation of these freshly prepared compounds was meticulously pursued employing a battery of spectroscopic techniques including NMR, UV–vis, IR, mass spectroscopy, magnetic susceptibility, conductivity, and thermal analysis (TG, DTA). Furthermore, the determination of complex stoichiometry and identification of their octahedral structural configuration were facilitated through the application of Job's techniques and molar ratio analysis. Complementing the experimental analyses, computational investigations via Density Functional Theory (DFT) were conducted to garner deeper insights into the structural, orbital, and electronic attributes of the compounds. Moreover, the anti-inflammatory properties of these synthetic compounds were scrutinized through in vitro assays, notably highlighting the superior efficacy of the C1 complex among the examined compounds. To validate their therapeutic potential, a comparative assessment was made against established anti-inflammatory agents. The hierarchy of anti-inflammatory activity among the metal complexes is established as follows: C1 > C2 > L, with the C1 exhibiting the highest potency. Additionally, a molecular docking study was executed to elucidate the compounds' interaction with the cyclooxygenase-2 (COX-2) enzyme (PDB ID: 5IKT), further corroborating their potential pharmacological relevance.

Synthesis and Reactivity of the Rare-Earth Metal Complexes Bearing the Indol-2-yl-Based NCN Pincer Ligand.

The pincer rare-earth dialkyl complexes [κ3-LRE(CH2SiMe3)2 (RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e))] with the indol-2-yl-based NCN pincer ligand were synthesized by the reactions of the proligand HL (L = 1-Me2NCH2CH2-3-(2-iPrC6H5N═CH)C8H4N) with RE(CH2SiMe3)3(THF)2. These complexes exhibited a variety of reactivities toward organic compounds such as amines, triphenylphosphine ylide, N-phenylimidazole, pyridine derivatives, and o-carborane leading to σ-bond metathesis, migration insertion, and redox reaction products. The reactions of the dialkyl rare-earth metal complexes with o-carborane afforded the novel NCN pincer-ligated carboryne-based metallacyclopropanes which reacted with diphenyl ketone to give insertion products of the RE-C2-ind and one of the RE-Ccage bonds, while the reaction of the carboryne-based metallacyclopropanes with diphenyldiazomethane produced the di-aza-metallacyclopentanes via the insertions of the N═N bond of the diphenyldiazomethane into two RE-Ccage bonds and the RE-C2-ind bond. The reactions of the dialkyl complexes with 2 equiv of 2,2'-bipyridine afforded the pincer-ligated bis(2,2'-bipyridyl monoanionic radical) complexes via the homolytic redox reaction.

Ultra-stretchable and adhesive hydrogel based on double network structure as flexible strain sensor for human motion detection

In recent years, conductive hydrogels were widely applied as flexible strain sensor due to its outstanding stretchability, high flexibility and conductivity. However, most conductive hydrogels would reduce the sensitivity and accuracy of hydrogel-based strain sensor for the lack of self-adhesion. Herein, a highly stretchable and excellent adhesive hydrogel based on chitosan network and ionic liquid crosslinking network was prepared. Chitosan (CS) worked as network skeleton, which could enhance the mechanical properties of hydrogel by participating in the construction of double network structure. Furthermore, dimethylaminoethyl methacrylate maleate (DM-M) as cross-linker, the hydrogel was imparted with high ionic conductivity (0.29 S/m) while it also enhanced the mechanical properties of the hydrogel. The ionic bonding conferred high toughness to the hydrogel for avoiding stress concentration during tensile process. Additionally, the hydrogel could form strong adhesion to a variety of substrate surfaces through electrostatic interaction, hydrogen bond and metal complexation. Meanwhile, the hydrogel remained strong adhesion under different pH, solvent and temperature condition. Therefore, the hydrogel with excellent adhesive property, mechanical properties and high electrical conductivity have been designed as flexible strain sensor, which could detect limb movement and physiological signals with real-time feedback electrical signals. Thus, this highly stretchable, tough and adhesive hydrogel based on double network structure would promote the development of flexible electronic device.

Rigid Macrocycle Metal Complexes as CXCR4 Chemokine Receptor Antagonists: Influence of Ring Size

Rigid Macrocycle Metal Complexes as CXCR4 Chemokine Receptor Antagonists: Influence of Ring Size

Exploring antimalarial and antioxidant properties of hydrazone ligands and their transition metal complexes: Insights through molecular docking and ADMET studies

Malaria is a leading cause of death in many parts of the world and has a profound impact on public health and poses significant challenges to healthcare systems in endemic regions. Malaria is a severe and sometimes fatal disease caused by Plasmodium parasites and its one significant aspect of malaria pathophysiology is its relationship with oxidative stress, a condition characterized by an imbalance between the production of free radicals and the body’s ability to detoxify these reactive products or repair the resulting damage. So, in the present research, to identify effective agents for malaria and oxidative stress, microassay and DPPH protocols were applied to previously synthesized and well characterized octahedral hydrazone ligands (1–2) and their Co(II), Ni(II), Cu(II), Zn(II) metal complexes (3–10), which were derived from 2-methoxy-1-naphthaldehyde, 3,5-bis(trifluoromethyl)benzohydrazide, 3-bromo-5-ethoxy-4-hydroxybenzaldehyde. The biological assessment indicated that complexes (8, 9, 10) were particularly effective in inhibiting malaria and oxidative infections as compared to other compounds. Among these, zinc(II) complex (10) exhibited the highest efficacy for malaria and oxidative stress, with an IC50 values of 0.59 ± 0.13 and 2.12 ± 0.17 µM which are comparable with quinine and ascorbic acid, correspondingly. Additionally, molecular docking studies against the 8E1Z and 1U5A proteins along with ADMET investigations were also support the superior efficacy of complex (10), demonstrating the lowest binding affinity, favorable binding modes and drug likeness ability.

Recent developments in metal-catalysed transamidation of quiescent amides

Amides played a vital role in the development of organic synthesis in recent years. These structural motifs are found in several biologically important synthetic and naturally occurring compounds. Developing efficient methods for their synthesis is crucial for drug discovery. Transamidation is a prominent tool in organic and medicinal chemistry, enabling the replacement of the amine group in an existing amide with new functionalised amines. Additionally, Transamidation process is used significantly in the total synthesis of few naturally occurring compounds. Transamidation reaction has been achieved significantly by using metal catalysis but there are several metal-free approaches have been employed to achieve the same reaction. This review focuses on recent literature concerning metal-catalysed transamidation reactions, highlighting the use of alkaline earth metals, transition metals, lanthanides, actinides, alkali metal salts or complexes, and metalloids. Both catalytic and stoichiometric quantities of catalysts are discussed for their roles in enhancing reaction efficiency.

A Data-driven Approachfor Enhanced CO2 Capturewith Ruthenium Complexes.

To attain carbon neutrality, significant efforts have been made to capture and use CO2. The homogeneous hydrogenation of CO2 catalyzed by transition metal complexes, particularly ruthenium complexes, has demonstrated significant advantages and is regarded as a viable approach for practical application. Insertion of CO2 into the Ru-H bond, producing the Ru-formate product, is the key step in the hydrogenation of CO2. In order to parameterize the catalytic activities in the CO2 insertion into the Ru-H bond, the concept of simplified mechanism-based approach with data-driven practice (SMADP) has been introduced in this paper. The results showed that the hydricity of the Ru-H complex (ΔGH-) might serve as a single active descriptor in the process of CO2 insertion, and that a novel Ru complex in CO2 catalysis may not be easily obtained by mere modification of the auxiliary ligand at the ruthenium metal site.