Oxygen Donor Ligands Research Articles

Dinuclear Copper(II) Complex with Intramolecular O–H⋅⋅⋅O Hydrogen Bonding: Magneto‐Structural Correlation for Acylhydrazone‐Based Phenoxido Bridged Copper(II) Complexes

The μ‐phenoxido‐bridged dinuclear copper(II) complex, [Cu2(L1)2(dmso)(MeOH)] (1), has been synthesized using N′‐(4‐(diethylamino)‐2‐hydroxybenzylidene)‐1‐naphthohydrazide (H2L1) as ligand. The structural analysis of 1 reveals that the coordination geometry at the copper ions is best described as a square pyramidal with a Cu···Cu distance in the dinuclear complex of 303 pm which is supported by hydrogen bonding between the two apical oxygen donor ligands (O···O 277 pm). Magnetic studies indicate that 1 exhibits a strong antiferromagnetic interaction between the two copper(II) ions with a coupling constant of J = −450 cm−1. The magnetic exchange is transmitted through the central [Cu2(OR)2]2+ core, a fragment for which different magneto‐structural correlations have been reported depending on the substituent at the bridging oxygen atom. However, the properties of complex 1 cannot be explained by any of the established correlations. Nevertheless, complex 1, together with the series of compounds based on an acylhydrazone ligand framework reported so far, shows a magneto‐structural correlation as a function of the Cu−O−Cu bridging angle that significantly differs from all previously reported.

Understanding Stabilization Factors in Heterodinuclear Ln-Al Complexes from DFT Simulations on Thermochemistry Data: A Counterintuitive Conclusion.

This study validates a computational protocol to predict the stability of heterodinuclear complexes by varying ligands on both moieties and analyzing the reaction Gibbs free energy (ΔGr) values. To this purpose, a series of Eu-Al complexes with the general formula [Eu(LEu)3Al(LAl)3], where LEu is the ligand of europium and LAl is an oxygen donor ligand of aluminum, is used. The nature of the bridging bonds and thermochemical characteristics (ΔGr, enthalpy, and entropy) of the complexes were evaluated via DFT calculations. We demonstrated that both entropic and enthalpic effects play a relevant role in the stability. The analysis of the series allows us to identify three ΔGr ranges where heterodinuclear complexes are (i) stable and easy to characterize, (ii) fragile and difficult to characterize, and (iii) not observed (unreacted precursors are recovered). To rationalize the trend of the stability and correlate it with the chemical nature of the ligands, we investigated the condensed Fukui function on the Al fragment. Results suggest that to obtain stable heteronuclear complexes, it is necessary to consider ligands with small condensed Fukui function values. This corresponds to a less nucleophilic oxygen site, yet counterintuitively, it allows the ligand to delocalize the received electronic charge and stabilize the complex.

Stability Constants of Tetravalent Germanium Complexes in Aqueous Solution: Small Organic Oxygen and Nitrogen Donor Ligands.

The formation constants of about 100 different complexes of Ge(IV) with some 50 organic, low-molecular-weight ligands, principally bidentate oxygen and/or nitrogen donors, are determined by potentiometric titrations at 25 °C and an ionic strength of 0.1 mol L-1 (KNO3). In the few cases for which literature data are available, these older data are critically discussed and completed in light of our new experimental data. The overall picture emerging from our work is that such complexes are comparatively weak, except when a chelate effect comes into play, as is observed with aminopolyols, α-hydroxocarboxylic acids, and carbohydrate derivatives, where even quantitative chelation of Ge is possible. No binding to amino acids or peptides is detected. An important finding is our evidence for the existence of a bridged dinuclear germanium(IV) tartrate complex in the aqueous phase, whose occurrence in solids was confirmed by X-ray diffraction beforehand. Our work has implications for the general understanding of the aqueous coordination chemistry of Ge, its geochemical cycling, and its behavior in environmental and biological systems. Potentiometric raw data for use by other researchers are publicly accessible in the repository Zenodo at CERN.

Coordination chemistry effects of the space-demanding solvent molecule N,N'-dimethylpropyleneurea.

Crystallographic investigations of eight homoleptic N,N'-dimethylpropyleneurea (dmpu) coordinated metal ions in the solid state, [Mg(dmpu)5]I2 (1), [Ca(dmpu)6]I2 (2), [Ca(dmpu)6](ClO4)2 (3), [Ca(dmpu)6](CF3SO3)2 (4), [Sr(dmpu)6](CF3SO3)2 (5), [Ba(dmpu)6](CF3SO3)2 (6), [Sc(dmpu)6]I3 (7), and [Pr(dmpu)6]I(I3)2 (8), and the complex [CoBr2(dmpu)2] (9) as well as the structures of the dmpu coordinated calcium, strontium, barium, scandium(III) and cobalt(II) ions and the cobalt(II) bromide complex in dmpu solution as determined by EXAFS are reported. The methyl groups in the dmpu molecule are close to the oxygen donor atom, causing steric restrictions, and making dmpu space-demanding at coordination to metal ions. The large volume required by the dmpu ligand at coordination contributes to crowdedness around the metal ion with often lower coordination numbers than for oxygen donor ligands without such steric restrictions. The crowdedness is seen in M⋯H distances equal to or close to the sum of the van der Waals radii. To counteract the space-demand at coordination, the dmpu molecule has an unusual ability to increase the M-O-C bond angle to facilitate as large coordination numbers as possible. M-O-C bond angles in the range of 125-170° are reported depending on the crowdedness caused by the coordination figure and the M-O bond distance. All reported structures of dmpu coordinated metal ions in both the solid state and dmpu solution are summarized to study the relationship between the M-O-C bond angle and the crowdedness around the metal ion. However, highly symmetric complexes seem to be favoured in the solid state due to favourable lattice energies. As a result, the dmpu coordinated lanthanoid(III) ions are octahedral in the solid state, while they, except lutetium, are seven-coordinate in the dmpu solution.

Experimental and theoretical studies on new organotin(IV) complexes with oxygen donor ligand: DNA binding, molecular docking and antimicrobial activity

Experimental and theoretical studies on new organotin(IV) complexes with oxygen donor ligand: DNA binding, molecular docking and antimicrobial activity

Cu(III) Trifluoromethyl Complexes with 1,3‐Diketonate Ligands and Their Versatile Reactivity in C−H Trifluoromethylation

AbstractHigh‐valent Cu(III) trifluoromethyl complexes with 1,3‐diketonates as bidentate oxygen donor ligands LCu(III)(CF3)2 were prepared for the first time and fully characterized, including by X‐ray crystallography. These complexes are bench stable and soluble in most organic solvents. Promising reactivity of Cu(III) trifluoromethyl diketonates in radical and radical‐polar crossover trifluoromethylation reactions was demonstrated. Direct C−H trifluoromethylation of electron‐rich arenes and indoles using Cu(III) trifluoromethyl diketonates under blue light irradiation is described. Azolo‐ and oxy‐trifluoromethylation of terminal alkynes and S‐trifluoromethylation of aromatic thiol can be achieved using LCu(III)(CF3)2 in mild conditions.

Synthesis and characterization of heterobinuclear Schiff base nickel(II) complexes involving nitrogen and oxygen donor ligand

This study reports the synthesis of five novel Schiff base heterobinuclear nickel(II) and tin(II) complexes. The heterobinuclear complexes were synthesized by the interaction of N,N′-ethylenebis(2-hydroxyacetophenoniminato) nickel(II) with tin(II) salts of 2,4,6-trinitrophenol, 1-nitroso-2-naphthol, o-nitrophenol, 8-hydroxyquinoline and 2,4-dinitrophenol. Characterization of these complexes was carried by physico-chemical techniques such as elemental analyses, conductivity measurements, magnetic measurements, infrared spectra and electronic spectral studies. The molar conductance data reveals that the complexes are non-electrolytic in nature. The magnetic susceptibility data and the spectral studies propose square planar geometry of the complexes around nickel(II) and coordination to the metal centre via azomethine nitrogen and deprotonated phenolic oxygen atoms.

Synthesis and Characterization of Fe (II), Co (II), Ni (II), Cu (II), Cd (II) Mono-Complexes with Mannich Base 4- hydroxy- 3-(4'-morpholinyl methyl) quinoline

Mannich bases have rececived great attention in recent years because of their proven antimalarial, antifungal and anticancer activities, coordination certain metal ions to them have interested use in synthesizing and studying structural aspects of metal complexes with oxygen and nitrogen donor ligands.
 The chelate ligand 4-hydroxy-3-(4'-morpholinyl methyl) quinoline was prepared by condensation of the formaldehyde with morpholine and 4- hydroxyquinoline in ethanol.
 A methanolic solution of the prepared ligand was reacted with a methanolic solution containing metal ions Fe (II), Co (II), Ni (II), Cu (II), Cd (II) to give metal complexes.
 The reaction of Fe (II), Co (II), Ni (II), Cd (II) ions with chelate ligand gave the mono-nuclear tetrahedral complexes, and mono-nuclear square planar complex with Cu (II).
 Their molecular structures were studied by FTIR, 1H-NMR and UV-Visible spectra, atomic absorption technique, elemental analysis have been performed using CHN, magnetic susceptibility and conductivity measurements.

Lanthanide extraction using a thiodiglycolamic acid extractant: effect of S-donor on lanthanide separation.

Liquid-liquid extraction of lanthanide (Ln) ions was investigated using N,N-dioctylthiodiglycolamic acid (DOTDGAA), which is a sulfur donor ligand with an amide group and a carboxyl group connected by a thioether chain. The extraction performance and selectivity of DOTDGAA for Ln ions were compared with those of N,N-dioctyldiglycolamic acid (DODGAA), which is also an oxygen donor ligand with a similar chemical structure, to assess the effect of the soft/hard donor atom on Ln separation. DOTDGAA quantitatively extracted all Ln ions while being selective toward the light and middle Ln ions, in contrast to the selectivity of DODGAA for heavier Ln ions. Slope analysis demonstrated that the Ln3+ transfer using DOTDGAA proceeded through a proton-exchange reaction, forming a 1:3 complex, Ln(DOTDGAA)3. The back-extraction of Ln ions from the extracting phase was successfully achieved under acidic conditions.

Heterometallic Iridium, Rhodium and Ruthenium Nitrido Complexes Supported by Oxygen and Sulfur Donor Ligands

Heterometallic μ-nitrido complexes have been synthesized from organometallic complexes containing O/S-donor ligands and a ruthenium(VI) nitride. The treatment of [Ir(tpip)(L)2] (tpip−=[N(Ph2PO)2]−; L=cyclooctene (coe) or 2,6-dimethylphenylisocyanide (xylNC)) and [Ir{N(iPr2PS)2}(coe)] with [Ru(LOEt)(N)Cl2] (LOEt−=[Co(η5-C5H5){P(O)(OEt)2}3]−) (1) afforded [(L)(tpip)Ir(μ-N)Ru(LOEt)Cl2] (L=coe (2) or xylNC (3)) and [(coe){N(iPr2PS)2}Ir(μ-N)Ru(LOEt)Cl2] (4), respectively, in which the Ir−N(nitride) bonds show double bond character. The bonding in 2–4 can be described by two resonance forms: Ir(I)−N≡Ru(VI) and Ir(III)=N=Ru(IV). The treatment of [Ph4P][Ir(CO)2Cl2] with 1 afforded (PPh4)[(CO)Cl2Ir(μ-N)Ru(LOEt)Cl2] (5) that reacted with [Tl(SR)] to give (PPh4)[(CO)(RS)2Ir(μ-N)Ru(LOEt)Cl2] (R=xyl (6), C6F4H (7)). The reaction of [M(acac)(CO)2)] or [Ru(acac)2(coe)2] (acac−=acetylacetonate) with 1 gave [(CO)(acac)M(μ-N)Ru(LOEt)Cl2] [M=Ir (8), Rh (9)] or [(H2O)(acac)2Ru(μ-N)Ru(LOEt)Cl2] (10), whereas the treatment of [Ru(acac)2(MeCN)2] with 1-azidoadamantane (AdN3) yielded a Ru(II) tetrazene complex, [Ru(acac)2(N4Ad2)] (11).

Study of Hydrazine Carboxamide, Characterization Method and Biological Activities with Transition Metal Using Cu (II) and Mn (II) as Central Metal

The co-ordination chemistry of transition metals and their derivatives has received much attention in the recent years, primarily because of their biological importance1-3, Nitrogen, Oxygen and Sulphur donor ligands possess a range of biological applications like antitomour4, antiviral5, antimaterial7 and antifungal activities. The legends of Hydrazine with their metal complexes with Cu (II) and Mn (II). Fungicidal activity of the ligand and their metal complexes were also done to establish any relation with the metal and ligands.

Protein Binding of a Novel Platinum-Based Anticancer Agent BP-C1 Containing a Lignin-Derived Polymeric Ligand

Platinum (Pt) antineoplastic agents remain indispensable for the treatment of oncological disease. Pt-based drugs are mainly used in the therapy of ovarian cancer and non-small-cell lung carcinoma. A novel platinum-containing antineoplastic agent BP-C1 is a complex of diamminoplatinum with an oxygen-donor polymeric ligand of benzene-polycarboxylic acids, isolated from natural lignin. The aim of the study was to investigate ex vivo protein binding of BP-C1. Protein binding of BP-C1 was tested using equilibrium dialysis. Pooled blood plasma was used in the study. Control solutions contained the same dosages of BP-C1 in PBS (pH 7.2). Plasma and control solutions were submitted to equilibrium dialysis across a vertical 8 kDa cut-off membrane for 4 h at 37 °C under gentle shaking. Platinum was quantified in dialysis and retained fractions using inductively coupled plasma mass spectrometry after microwave digestion. The dialysis system was tested and validated; this showed no protein saturation with platinum. A medium degree of binding of platinum to macromolecular species of ca. 60% was observed. The study showed the maintenance of a high fraction of free BP-C1 in the bloodstream, facilitating its pharmacological activity.

Solvent free synthesis of hydrazine carbothioamide derivatives as a precursor in the preparation of new mononuclear Mn(II), Cu(II), and Zn(II) complexes: Spectroscopic characterizations

ABSTRACT. Over the last few years considerable attention has been devoted to the study of Schiff base complexes of metal(II) containing nitrogen, oxygen and sulfur donor ligands due to their diverse biological activities. Therefore, the Schiff base thiosemicarbazone derivatives of 2-(2-hydroxy-3-methoxybenzylidene) hydrazine carbothioamide (HL1) and 2-(4-bromo-2-hydroxy-3-methoxybenzylidene) hydrazine carbothioamide (HL2) were synthesized with solvent free technique using silica gel as material support for the reaction. The structures of the precursor’s organic products confirmed with their 1H, 13C NMR, IR and microanalysis data. Six new manganese(II), copper(II), and zinc(II) complexes of HL1 and HL2 2-(2-hydroxy-3-methoxybenzylidene) hydrazinecarbothioamide derivatives with the general formulation [ML1Cl] and [ML2Cl], where M = Mn2+, Cu2+, and Zn2+ were prepared and well discussed by using elemental analyses, magnetic moments, molar conductance, infrared and electronic spectral techniques. The assignments data of spectroscopic analysis confirm that the synthesized thiosemicarbazone Schiff bases of acts as a tridentate ligand as sulfur-oxygen-nitrogen donating atoms. The values of magnetic moments agreed with the data of electronic spectra which both suggested a four-coordination geometry of the synthesized complexes.
 
 KEY WORDS: Schiff base, Thiosemicarbazone, Manganese(II) complex, Copper(II) complex, Zinc(II) complex
 
 Bull. Chem. Soc. Ethiop. 2021, 35(2), 413-424.
 DOI: https://dx.doi.org/10.4314/bcse.v35i2.14

BUTADIENE RUBBER: SYNTHESIS, MICROSTRUCTURE, AND ROLE OF CATALYSTS

ABSTRACTButadiene rubber (BR) is one of the most useful and second most produced rubber worldwide. Polymerization of 1,3-butadiene (BD) is a highly stereospecific reaction that offers a wide variety of BR with different microstructures and influences the fundamental properties of the rubber. Since the first successful polymerization of conjugated diene using the Ziegler–Natta–based catalyst (TiCl4 or TiCl3 with aluminum alkyls) in 1954, the research on producing synthetic rubber with an appropriate catalyst system has been accelerated. Subsequently, various research groups are actively engaged in designing active catalyst systems based on a suitable combination of transition metal complexes with alkyl-aluminum and successfully using them in BD polymerization. Although various scientific inventions have proven their significance for the production of high-quality BR, with the rising demands in improving the quality of the product, research on developing new catalyst systems with enhanced catalytic activity and high stereoselectivity is still in progress. The present review focuses on the synthesis of BR using various transition metal catalysts and discusses their microstructures. The catalysts based on new-generation metal complexes with phosphorus, nitrogen, and oxygen donor ligands (e.g., phosphines, imines, 1,10-phenanthroline, and imino-pyridines) have been introduced. The role that catalysts play in the production of BR with different microstructures (i.e., high-cis, high-trans or low-cis, low-trans polybutadiene) has also been described. The combination of catalyst (transition metal complex) and suitable co-catalyst (alkyl-aluminum) is the major factor influencing the reaction and microstructure of the resulting polymer. This report focuses on the effect of transition metal catalysts (i.e., lithium [Li], titanium [Ti], zirconium [Zr], iron [Fe], cobalt [Co], nickel [Ni], and neodymium [Nd]) on the activity and stereoselectivity of polymers such as 1,4-cis-, 1,4-trans-, and 1,2-vinyl-polybutadiene.

Synthesis, spectral and sol-gel behavior of mixed ligand complexes of titanium(IV) with oxygen, nitrogen and sulfur donor ligands

A new route to synthesize nano-sized Ti(IV) mixed ligand complexes have been investigated by the reaction of titanium(IV) chloride with ammonium salts of dithiophosphate and 3(2'-hydroxyphenyl)-5-(4-substituted phenyl) pyrazolines. The resultant complex is then treated with H2S gas to get sulfur bridged dimer of Ti(IV) complex, a precursor of TiS2. The morphology of the complexes was studied by employing XRD which shows that all the complexes are amorphous solid. Molecular weight measurements, elemental analysis in conjugation with spectroscopic (IR, 1H NMR, 13C NMR and 31P NMR) studies revealed the dimeric nature of the complexes in which pyrazoline and dithiophosphate are bidentate. Scanning electron microscopic image and XRD indicate that the particles are in the nano range (50 nm). Putting all the facts together, coordination number six is proposed for titanium with octahedral geometry.
 
 KEY WORDS: Titanium(IV), Dithiophosphate, Pyrazoline, Nano-sized, Sol-gel, Mixed ligand complexes
 
 Bull. Chem. Soc. Ethiop. 2021, 35(1), 61-76.
 DOI: https://dx.doi.org/10.4314/bcse.v35i1.5

Crystal structures of zinc(II) complexes with β-hydroxypyridinecarboxylate ligands: examples of structure-directing effects used in inorganic crystal engineering.

The coordination properties of four hydroxypyridinecarboxylates, designed for the treatment of iron-overloading conditions as bidentate O,O'-donor ligands, have been studied with ZnII in the solid state. The coordination compounds [Zn(A1)2(H2O)2] (1), [Zn(A2)2(H2O)] (2), [Zn(A3)2(H2O)]·2H2O (3) and [Zn2(B1)4(H2O)2]·4H2O (4), where the ligands are 1-methyl-4-oxidopyridinium-3-carboxylate (A1, C7H6NO3), 1,6-dimethyl-4-oxidopyridinium-3-carboxylate (A2, C8H8NO3), 1,5-dimethyl-4-oxido-pyridinium-3-carboxylate (A3, C8H8NO3) and 1-methyl-3-oxidopyridinium-4-carboxylate (B1, C7H6NO3), have been synthesized and analysed by single-crystal X-ray diffraction. The ligands were chosen to probe (i) the electronic effects of inverting the positions of the O-atom donor groups (i.e. A1 versus B1) and (ii) the electronic and steric effects of the addition of a second methyl group in different positions on the pyridine ring. Two axially coordinated water molecules resulting in a six-coordinated symmetrical octahedron complement the bis-ligand complex of A1. Ligands A2 and A3 form five-coordinated trigonal bipyramidal complexes with one additional water molecule in the coordination sphere, which is a rarely reported geometry for ZnII complexes. Ligand B1 shows a dimeric structure, where the two Zn2+ dications have slightly distorted octahedral geometry and the pyridinolate O atom of the neighbouring complex bridges them. The coordination spheres of the Zn2+ dications and the supramolecular structures are discussed in detail. The packing arrangements of 1-3 are similar, having alternating hydrophilic and hydrophobic layers, however the similarity is broken in 4. The obtained coordination geometries are compared with their previously determined CuII analogues. The study of the individual complexes is complemented with a comprehensive analysis of ZnII complexes with oxygen donor ligands with data from the Cambridge Structural Database.

Elemental distribution and chemical speciation of copper and cobalt in three metallophytes from the copper-cobalt belt in Northern Zambia.

Three metallophyte species, Persicaria capitata, P. puncata (Polygonaceae), Conyza cordata (Asteraceae) from mineral wastes in the Zambian copper-cobalt belt were studied. This study focused on the elemental distribution in the roots, stems and leaves, using a range of techniques: micro-PIXE, SEM-EDS synchrotron XFM and XAS. The species differed in their responses to growing on Co-Cu-enriched soils: Persicaria puncata is a Co hyperaccumulator (up to 1060 μg g-1 in leaves), while Persicaria capitata and Conyza cordata are Co-excluders. All three species are Cu-accumulators. The highest concentrations of Cu-Co are in the epidermal cells, whereas in Persicaria puncata Co was also enriched in the phloem. The Co coordination chemistry shows that an aqueous Co(ii)-tartrate complex was the predominant component identified in all plants and tissues, along with a minor component of a Co(iii) compound with oxygen donor ligands. For Cu, there was considerable variation in the Cu speciation in the various tissues and across the three species. In contrast to hyperaccumulator plants, excluder and accumulator type plants have received far less attention. This study highlights the different biopathways of transition elements (Cu, Co) in hyper-tolerant plant species showing different responses to metalliferous environments.

Crystal structures of chlorido-[dihy-droxybis-(1-imino-eth-oxy)]arsanido-κ3 N,As,N']platinum(II) and of a polymorph of chlorido-[dihy-droxybis-(1-imino-prop-oxy)arsanido-κ3 N,As,N']platinum(II).

Each central platinum(II) atom in the crystal structures of chlorido-[dihy-droxybis-(1-imino-eth-oxy)arsanido-κ3 N,As,N']platinum(II), [Pt(C4H10AsN2O4)Cl] (1), and of chlorido-[dihy-droxybis-(1-imino-prop-oxy)arsanido-κ3 N,As,N']platinum(II), [Pt(C6H14AsN2O4)Cl] (2), is coordinated by two nitro-gen donor atoms, a chlorido ligand and to arsenic, which, in turn, is coordinated by two oxygen donor ligands, two hydroxyl ligands and the platinum(II) atom. The square-planar and trigonal-bipyramidal coordination environments around platinum and arsenic, respectively, are significantly distorted with the largest outliers being 173.90 (13) and 106.98 (14)° for platinum and arsenic in (1), and 173.20 (14)° and 94.20 (9)° for (2), respectively. One intra-molecular and four classical inter-molecular hydrogen-bonding inter-actions are observed in the crystal structure of (1), which give rise to an infinite three-dimensional network. A similar situation (one intra-molecular and four classical inter-molecular hydrogen-bonding inter-actions) is observed in the crystal structure of (2). Various π-inter-actions are present in (1) between the platinum(II) atom and the centroid of one of the five-membered rings formed by Pt, As, C, N, O with a distance of 3.7225 (7) Å, and between the centroids of five-membered (Pt, As, C, N, O) rings of neighbouring mol-ecules with distances of 3.7456 (4) and 3.7960 (6) Å. Likewise, weak π-inter-actions are observed in (2) between the platinum(II) atom and the centroid of one of the five-membered rings formed by Pt, As, C, N, O with a distance of 3.8213 (2) Å, as well as between the Cl atom and the centroid of a symmetry-related five-membered ring with a distance of 3.8252 (12) Å. Differences between (2) and the reported polymorph [Miodragović et al. (2013 ▸). Angew. Chem. Int. Ed. 52, 10749-10752] are discussed.

O-I-O halogen bond of halonium ions.

The reactivity of halonium ions is conveniently modulated by three-center, four-electron halogen bonds. Such stabilized halonium complexes are valuable reagents for oxidations and halofunctionalization reactions. We report the first example of the stabilization of a halenium ion in a three-center, four-electron halogen bond with two oxygen ligands. The influence of electron density and solvent on the stability of the complexes is assessed. O-I-O halogen bond complexes are applicable as synthetic reagents and as supramolecular synthons.

Studies on structural and biological aspects of active oxygen donor ligands oxazolines and iron metal complexes

Studies on structural and biological aspects of active oxygen donor ligands oxazolines and iron metal complexes