Dicyanamido-metal(II) complexes. Part 4: Synthesis, structure and magnetic characterization of polynuclear Cu(II) and Ni(II) complexes bridged by μ-1,5-dicyanamide

The one pot aqueous reaction of M(ClO₄)₂ (M = Cu²⁺ or Ni²⁺) with N-methylbis[2-(2-pyridylethyl)]amine (MeDEPA) and N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethylenediamine (bpmen) and 1,4,7,10-tetraazacyclododecane (cyclen) in presence of sodium dicyanamide (Nadca) yielded dicyanamido-bridged polynuclear complex {[Cu(MeDEPA)(μ-1,5-dca)]ClO₄}_n (1), and two dinuclear complexes [Cu₂(bpmen)₂(μ-1,5-dca)]₂(ClO₄)₅dca (2) and [Ni(cyclen)(μ-1,5-dca)]₂(ClO₄)₂ (3). These complexes were characterized by IR and UV-Vis spectroscopy. Room temperature single-crystal X-ray studies have confirmed that the Cu(II) centers in 1 and 2 adopt geometries that are more close to trigonal bipyramidal (TBP) in 1 and close to square pyramidal (SP) in 2, whereas in 3, the Ni(II) centers are located in octahedral environment with doubly bridged μ-1,5-dca bonding mode. The intermolecular M···M distances in these complexes are in the range of 7.3-8.6 Å. Variable temperature magnetic susceptibility studies have confirmed that the dca-bridges mediate very weak antiferromagnetic interaction between the M(II) centers with J values of −0.35, −0.18 and −0.43 cm⁻¹ for 1, 2 and 3, respectively. The results are compared and discussed in the light of other related bridged μ-1,5-dca Cu(II) and Ni(II) complexes.     

Synthesis, structure and magnetic properties of two μ-oxo and thiocyanato-bridged manganese(II)-mercury(II) coordination polymers

Mixed-metal thiocyanate complexes [MnHg(SCN)₄(NOP)] (1) and [MnHg(SCN)₄(DMSO)] (2) (NOP = 3-methyl-4-nitropyridine-N-oxide, DMSO = dimethylsulfoxide) have been synthesized and structurally characterized by single crystal X-ray analysis. Complex 1 and 2 both contain a [Mn₂(μ₂-O)₂] lozenge, which is bridged to Hg(II) ions by end-to-end thiocyanate groups to form a 2-D and 3-D polymeric network, respectively. Magnetic studies indicate that both complexes are anti-ferromagnets, with 1 showing anti-ferrimagnetic ordering below 8.0 K.     

Syntheses, structures, and magnetic properties of two new μ-alkoxo-μ-pyrazolato bridged dicopper(II) complexes

The syntheses, structures, and magnetic properties of two new μ-alkoxo-μ-pyrazolato heterobridged compounds, [Cu ^II₂(L¹-F)(μ-prz)] (1) and [Cu ^II₂(L¹-2OMe) (μ-prz)] · 0.5 CH₃CN (2) (prz=pyrazolato; H₂L¹-F=1,3-bis(3-fluorosalicylideneamino)-2-propanol; H₂L¹-2OMe=1,3-bis(4,6-dimethoxy salicylideneamino)-2-propanol) have been reported. Compound 1 crystallizes in triclinic space group with a=8.6392(10) Å, b=10.6431(9) Å, c=11.6809(13) Å, α=85.972(8)°, β=71.492(9)°, and γ=72.221(9)°, while the unit cell parameters of 2 are as follows: space group: monoclinic C2/c, a=28.2948(5) Å, b=7.3033(2) Å, c=26.3933(5) Å, and β=96.243(1)°. The variable-temperature magnetic susceptibility measurements reveal that the metal centers in both the compounds are antiferromagnetically coupled with J=−200 cm⁻¹ for 1 and J=−175 cm⁻¹ for 2. The magnetic behaviors have been explained on the basis of two opposing factors, complementarity and countercomplementarity of magnetic orbitals.     

Magnetostructural correlations in μ-pyrazolato-μ-acetato dinickel(II) complexes: Subtle effects of acetate tilting

A series of dinickel acetato complexes [LNi₂(OAc)(solvent)_x](ClO₄)₂ (1-3, 5) as well as related complexes [LNi₂(OAc)₂](OAc) (4) and [LNi₂(OAc)(NO₃)₂] (6), all derived from three pyrazolate-based binucleating ligands, have been prepared and characterized by X-ray crystallography. The solid state structures reveal different acetate binding modes (μ_1,3-bridging and bidentate chelating) plus severe twisting and tilting of the μ_1,3-acetate bridges with respect to their bimetallic scaffolds, reflecting the great flexibility of carboxylate coordination. Magnetic properties of all six complexes have been investigated, and the strength of antiferromagnetic coupling is discussed in the light of the structural differences, suggesting a magnetostructural correlation for acetato-bridged complexes.     

Substitution of trans ligands in μ-oxo-bis(μ-acetato)diruthenium(III) complexes: Synthesis and kinetic studies

[Ru₂O(L)₆(acetate)₂](PF₆)₂ {L = pyridine 1; 4-picoline 2} undergo aquation in acetone-water (60:40 v/v) mixed solvent to form diaquo complexes in solution as shown by proton NMR studies. Ligands trans to the μ-oxo group are substituted. These diaquo complexes react with substituted pyridines and imidazoles to form respective disubstituted complexes. Rate constants for aquation and complexation under pseudo first order conditions of ligand are reported. Rate constants increase with increase in the basicity of incoming ligand. Disubstituted complexes proposed to be formed in solution have been isolated and characterized by elemental analyses, visible spectra, proton NMR. Single crystal X-ray structures of 4-picoline and 4-methylimidazole disubstituted complexes are reported. All the isolated complexes exhibit a strong peak between 570 and 585 nm in their visible absorption spectra. λ_max varies linearly with ∑pk_a of terminal ligands. In disubstituted complexes of 1 with 2-methyl and 4-methyl imidazole deprotonation of N(1)H of methylimidazoles takes place in solution.     

Synthesis and structure of the complex tetra-μ-prolinatodirhodium(II)

The dinuclear Rh^IIRh^II complex with proline [Rh₂(pro₄][NEt₄]₂ was synthesized and its structure studied by means of spectroscopic (IR, EPR and ESCA) and magnetochemical methods. It was shown that two proline molecules serve as bridging ligands, while the other two are only axially coordinated through their N atoms.     

Effect of anionic co-ligands on structure and magnetic coupling of bis(μ-phenoxo)-bridged dinuclear copper(II) complexes

Two phenoxo bridged dinuclear Cu(II) complexes, [Cu₂L₂(NO₂)₂] (1) and [Cu₂L₂(NO₃)₂] (2) have been synthesized using the tridentate reduced Schiff-base ligand 2-[(2-dimethylamino-ethylamino)-methyl]-phenol (HL). The complexes have been characterized by X-ray structural analyses and variable-temperature magnetic susceptibility measurements. The structures of the two compounds are very similar having the same tridentate chelating ligand (L) and mono-dentate anionic ligand nitrite for 1 and nitrate for 2. In both complexes Cu(II) is penta-coordinated but the square pyramidal geometry of the copper ions is severely distorted (Addison parameter (τ) = 0.33) in 1 while the distortion is quite small (average τ = 0.11) in 2. These differences have marked effect on the magnetic properties of two compounds. Although both are antiferromagnetically coupled, the coupling constants (J = −140.8 and −614.7 cm⁻¹ for 1 and 2, respectively) show that the coupling is much stronger in 2.     

Synthesis, structure and properties of unsymmetrical μ-alkoxo-dicopper(II) complexes: biological relevance to phosphodiester and DNA cleavage and cytotoxic activity

The unsymmetric dinucleating ligand N-(2-hydroxybenzyl)-N,N′,N′-tris(2-pyridylmethyl)-1,3-diaminopropan-2-ol (L = H₂btppnol) and the corresponding copper(II) complex [Cu₂(Hbtppnol)(μ-CH₃COO)](ClO₄)₂ (1) have been recently reported in part in a short communication [Inorg. Chem. Commum. 8 (1999) 334]. In this study, we investigated the ability of complex 1 to promote the hydrolysis of P-O phosphate diester bonds in bis(2,4-dinitrophenyl) phosphate (2,4-BDNPP) and the cleavage of genomic and plasmid DNA molecules. Reaction of 1 with excess of the diester 2,4-BDNPP, at pH 7.0, results in the formation of the monoester phosphate coordinated [Cu₂(Hbtppnol)(μ-((NO₂)₂-C₆H₃)PO₄)]ClO₄ (3) complex, which was also characterized by X-ray crystallography. In addition, the stable μ-phosphate complex [Cu₂(Hbtppnol)(μ-(NO₂-C₆H₄)PO₄)](ClO₄) (2) obtained from the reaction of 4-nitrophenyl phosphate with complex 1 was also characterized by X-ray crystallography, indicating that 1 is unable to cleave monoester-phosphate bonds. The kinetics for the promotion of bis(2,4-dinitrophenyl) phosphate (2,4-BDNPP) hydrolysis by complex 1 was investigated as a function of pH, catalyst concentration and substrate concentration. On the basis of kinetic and potentiometric studies, the deuterium isotope effect (k^H/k^D ∼ 1) and the X-ray structure of the monoester phosphate coordinated [Cu₂(Hbtppnol)(μ-((NO₂)₂-C₆H₃)PO₄)]ClO₄ (3) complex as the product of the reaction, we demonstrated that the aquo/hydroxo complex is the active species and the reaction occurs through the formation of a ternary complex in which one Cu^II binds the substrate and the second copper center has a terminal bound hydroxide to attack the phosphorus atom, at physiological pH. A rate enhancement factor of ∼100 was calculated relative to that measured for the uncatalyzed reaction under identical conditions. Complex 1 effectively promotes the cleavage of double-stranded genomic and plasmid DNA, at physiological pH, probably through a hydrolytic mechanism in agreement with that proposed for the reaction of 1 with 2,4-BDNPP. Finally, cytotoxic activity of 1 in a human small cell lung carcinoma cell line (GLC4) and its cisplatin resistant subline (GLC4/CDDP) was studied and the IC₅₀ values were determined.     

Synthesis and non-linear optical properties of (η-pentaphenylcyclopentadienyl)dicarbonylruthenium(II) σ-alkenyl complexes

The complexes [Ru{(Z)-HCCHPh}(CO)₂(η⁵-C₅Ph₅)] (1) and [Ru{(Z)-HCCHC₆H₄NO₂}(CO)₂(η⁵-C₅Ph₅)] (2) have been synthesized and their identity confirmed by single-crystal X-ray diffraction studies. Reaction of 2 with PMe₂Ph and Me₃NO in tetrahydrofuran afforded [Ru{(Z)-HCCHC₆H₄NO₂}(CO)(PMe₂Ph)(η⁵-C₅Ph₅)] (3). Cyclic voltammetry confirms the expected increase in ease of oxidation on proceeding from 2 to 1 and from 2 to 3. Hyper-Rayleigh scattering studies at 1064 nm reveal a dramatic increase in quadratic non-linearity on co-ligand replacement of CO by PMe₂Ph, in proceeding from 2 to 3. Z-scan studies at 800 nm are consistent with significant contribution from two-photon states, and with an increase in γ_real on co-ligand replacement of CO by PMe₂Ph in proceeding from 2 to 3.     

Synthesis,X-ray structures,and magnetic properties of heterometal dinuclear Cr(III)–M(II) complexes

New 3,5-bis(2-pyridyl)pyrazolato (bpypz) bridged heterometal dinuclear complexes [(nta)Cr(μ-bpypz)M^II(picen)]⁺ (M = Mn(II), Ni(II)) and [(acac)₂Cr(μ-bpypz)Ni^II(picen)]²⁺ (nta = nitrilotriacetate, picen = N,N′-bis(2-pyridylmethyl)ethylenediamine, acac = acetylacetate) were synthesized and characterized by the X-ray analysis, ESI-MS and the magnetic measurements, and/or ²H NMR spectra. The molecular structures were compared from a viewpoint of the conformation of the picen depending on M^II ionic radii or different modes of hydrogen bonds. The picen in [(nta)Cr(μ-bpypz)Mn^II(picen)]BF₄ takes an abnormal conformation with intramolecular bifurcated three-center hydrogen bonds between two carboxylate oxygens of nta and an amine proton of the picen as found for the previously reported corresponding Fe(II) complex [K. Ni-iya, A. Fuyuhiro, T. Yagi, S. Nasu, K. Kuzushita, S. Morimoto, S. Kaizaki, Bull. Chem. Soc. Jpn. 74 (2001) 1891]. On the other hand, for both Ni(II)-nta and Ni(II)-acac complexes, the picen takes a normal conformation with only a two-center hydrogen bond between non-bridging ligands. The magneto-structural relation is discussed for the Cr(III)–Ni(II) complexes in connection with the orthogonality or orbital overlap arising from the difference in distortion around Cr(III) moiety.     

Synthesis, crystal structures and magnetic properties of two bis(μ-phenoxido)dicopper(II) complexes derived from reduced Schiff base ligands

Two phenoxido bridged dinuclear Cu(II) complexes, [Cu₂(L¹)₂(NCNCN)₂] (1) and [Cu₂(L²)₂(NCNCN)₂]·2H₂O (2) have been synthesized using the tridentate reduced Schiff-base ligands 2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol (HL¹) and 2-[1-(3-methylamino-propylamino)-ethyl]-phenol (HL²), respectively. The complexes have been characterized by X-ray structural analyses and variable-temperature magnetic susceptibility measurements. Both the complexes present a diphenoxido bridging Cu₂O₂ core. The geometries around metal atoms are intermediate between trigonal bipyramid and square pyramid with the Addison parameters (τ) = 0.57 and 0.49 for 1 and 2, respectively. Within the core the Cu–O–Cu angles are 99.15° and 103.51° and average Cu–O bond distances are 2.036 and 1.978 Å for compounds 1 and 2, respectively. These differences have marked effect on the magnetic properties of two compounds. Although both are antiferromagnetically coupled, the coupling constants (J = −184.3 and −478.4 cm⁻¹ for 1 and 2, respectively) differ appreciably.     

Ligand substitution reactions of bis(μ-acetato)dichlorodicarbonyldiiridium(II)

Seven diiridium(II) complexes were synthesized by ligand substitution reactions of [Ir₂(μ-O₂CMe)₂Cl₂(CO)₂] (1) and [Ir₂(μ-O₂CMe)₂Cl₂(CO)₂(py)₂] (2).The reaction of 2 with the silver salt of a less coordinating anion, AgSbF₆, gave a cationic complex [Ir₂(μ-O₂CMe)₂Cl(CO)₂(py)₃]SbF₆ (3).A tricarbonyl cationic complex [Ir₂(μ-O₂CMe)₂(CO)₃Cl(py)₂]SbF₆ (4) was obtained under a CO atmosphere.Complex 2 reacted with AgO₂CCF₃ to give [Ir₂(μ-O₂CMe)₂Cl(O₂CCF₃)(CO)₂(py)₂] (5) in toluene.[Ir₂(μ-hiq)₂(CO)₂Cl₂] (Hhiq = 1-hydroxyisoquinoline, 6) was synthesized by the bridging-ligand substitution of 1 with Hhiq.Its axial adducts [Ir₂(μ-hiq)₂Cl₂(CO)₂L₂] (L = Mepy (4-methylpyridine), 7 or PPh₃, 8) were synthesized by addition of the ligands to a suspension of 6.In the structures of 7 and 8, two iridium atoms are bridged by two hiq ligands in a head-to-tail arrangement.The reaction of 1 with Hmhp (2-hydroxy-4-methylpyridine) led to triply bridged [Ir₂(μ-mhp)₃(CO)₂Cl(Hmhp)] (9).In complex 9, all the mhp ligands bridge between the Ir atoms in a head-to-head manner.The Ir-Ir distances of 3, 4, 5, 7 and 8 are 2.6047(7), 2.6216(9), 2.5899(9), 2.5933(5) and 2.634(2) Å, respectively, which are similar to those observed in[Ir₂(μ-O₂CMe)₂Cl₂(CO)₂L₂]. The Ir-Ir distance of 2.5512(4) Å in 9 is shorter than in the other complexes.     

Synthesis and antimicrobial activity of copper(II) and manganese(II) α,ω-dicarboxylate complexes

Copper(II) ,-dicarboxylate complexes of general formulae, [Cu(O₂C(CH₂)_nCO₂)]·xH₂O, [Cu(O₂C(CH₂)_nCO₂) (phen)₂]·xH₂O and [Cu(O₂C(CH₂)_nCO₂)(bipy)_y]·xH₂O (n=1–8; y=1, 2; phen = 1,10-phenanthroline; bipy = 2,2-bipyridine) were synthesised. These copper complexes, some related manganese(II) complexes and the metal-free ligands were screened in vitro for their ability to inhibit the growth of Candida albicans. Metal-free 1,10-phenanthroline and all of the copper(II) and manganese(II) phenanthroline complexes were potent growth inhibitors, with only one bipyridine complex, [Cu(O₂C(CH₂)CO₂)(bipy)₂]·2H₂O, having moderate activity. The remaining substances were effectively inactive. Complexes which were active against C. albicans also proved effective against C. glabrata, C. tropicalis and C. kreusi with the manganese complexes retaining superior activity. For the phenanthroline complexes the active drug species is thought to be the dication [M(phen)₂(H₂O)_n]²⁺ (M = Cu, Mn). Escherichia coli and Staphylococcus aureus were resistant to all of the metal complexes and also to metal-free 1,10-phenanthroline. Only the copper phenanthroline complexes showed intermediate activity against Pseudomonas aeruginosa.     

Synthesis, crystal structure, magnetic properties and EPR spectra of copper(II) acetate derivatives: tetra-(μ-acetato)bis(2-methylaminopyridine)copper(II) and catena-poly(aquadiacetato-μ-3-aminomethylpyridine)copper(II). Sheets formed by chains connected through hydrogen-bonds

The derivatives of Cu(OAc)₂ · H₂O with 2-methylaminopyridine and 3-aminomethylpyridine, [Cu₂(μ-OAc)₄(MeNHpy)₂] (1) and [Cu(OAc)₂(μ-NH₂CH₂py)(H₂O)]_n (2), respectively, have been synthesized and characterized. Compound 1 shows the dimer structure of [Cu₂(μ-OAc)₄(H₂O)₂], with four syn-syn bridging acetate groups and the MeNHpy ligand coordinated in the axial positions. It is antiferromagnetic (2J=−285 cm⁻¹). Signals of the triplet state are observed in its EPR spectrum and the zero-field splitting parameter has been calculated (D=0.36 cm⁻¹; g_∥=2.35; g_⊥=2.07). Otherwise, the ligand 3-aminomethylpyridine acts as bridging bidentate ligand in compound 2, forming infinite zig-zag chains. Each copper atom lies in a square-planar pyramidal coordination, determined by two nitrogen atoms of two bridge ligands, two oxygen atoms of two monodentate terminal acetate groups and a water molecule. The parallel chains form a sheet because of the hydrogen bonds between them. The shortest Cu-Cu distances are: 5.1270, 6.0952 and 6.2163 Å (inter-chains) and 7.875 Å (intra-chain). Compound 2 shows a slight antiferromagnetic effect below 30 K. The EPR spectra show an orthorhombic signal (g₁=2.26; g₂=2.08; g₃=2.06).     

Studies on metal-drug complexes. Crystal structure and characterization of μ-sulfato bromazepam copper(II) complex

The compound Cu(Bromazepam) SO₄ having been synthesized, its crystal structure shows distorted octahedral environment for the Copper(II) ion. Because of the long Cu-O(4) length (2.90 Å), we can consider that there is a semicoordinative interaction with the sulfate group acting as tridentate bridging ligand. The structure is a polymeric chain where dimeric units are linked by the oxygen (C-O(5)) atom of the Bromazepam carbonyl group. The drug consequently acts as a tridentate ligand in this compound. The magnetic results show a very weak antiferromagnetic interaction.     

Theoretical description of the magnetic properties of μ3-hydroxo bridged trinuclear copper(II) complexes

A theoretical study of the magnetic properties, using density functional theory, of a family of trinuclear μ₃-OH copper(II) complexes reported in the literature is presented. The reported X-ray crystal structures of [Cu₃(μ₃-OH)(aat)₃(H₂O)₃](NO₃)₂·H₂O (HUKDUM), where aat: 3-acetylamine-1,2,4-triazole; [Cu₃(μ₃-OH)(aaat)₃(H₂SO₄)(HSO₄)(H₂O)] (HUKDOG), where aaat: 3-acetylamine-5-amine-1,2,4-triazole; [Cu₃(μ₃-OH)(PhPyCNO)₃(tchlphac)₂] (HOHQUR), where PhPyCNO: phenyl 2-pyridyl-ketoxime and tchlphac: acid 2,4,5-trichlorophenoxyacetic; [Cu₃(μ₃-OH)(PhPyCNO)₃(NO₃)₂(CH₃OH)] (ILEGEM); [Cu₃(μ₃-OH)(pz)₃(Hpz)₃(ClO₄)₂] (QOPJIP), where Hpz?=?pyrazole; [Cu₃(μ₃-OH)(pz)₃(Hpz)(Me₃CCOO)₂]?2Me₃CCOOH (DEFSEN) and [Cu₃(μ₃-OH)(8-amino-4-methyl-5-azaoct-3-en-2-one)₃][CuI₃] (RITXUO), were used in the calculations. The magnetic exchange constants were calculated using the broken-symmetry approach. The calculated J values are for HUKDUM J₁?=??68.6 cm^?1, J₂?=??69.9 cm^?1, J₃?=??70.4 cm^?1; for HUKDOG, J₁?=??73.5 cm^?1, J₂?=??58.9 cm^?1, J₃?=??62.1 cm^?1; for HOHQUR J₁?=??128.3 cm^?1, J₂?=??134.1 cm^?1, J₃?=??120.4 cm^?1; for ILEGEM J₁?=??151.6 cm^?1, J₂?=??173.9 cm^?1, J₃?=??186.9 cm^?1; for QOPJIP J₁?=??118.3 cm^?1, J₂?=??106.0 cm^?1, J₃?=??120.6 cm^?1; for DEFSEN J₁?=??74.9 cm^?1, J₂?=??64.0 cm^?1, J₃?=??57.7 cm^?1 and for RITXUO J₁?=??10.9 cm^?1, J₂?=?+14.3 cm^?1, J₃?=??35.4 cm^?1. The Kahn-Briat model was used to correlate the calculated magnetic properties with the overlap of the magnetic orbitals. Spin density surfaces show that the delocalization mechanism is predominant in all the studied compounds.

Binuclear metal complexes. 59. Synthesis and magnetism of dicopper(II) and copper(II)-nickel(II) complexes with N,N′-ethylenedisalicylamidatocuprate(II)

The complexes [Cu(samen)Cu(L)] and [Cu(samen)Ni(L)₂] (Lbpy, phen) have been synthesized by the reaction of sodium N,N′-ethylenedisalicylamidatocuprate(II) pentahydrate (Na₂- [Cu(samen)]·5H₂O), a divalent metal ion, and 2,2′- dipyridyl or 1,10-phenanthroline. Cryomagnetic data for the CuCu complexes did not fit the Bleaney- Bowers equation; but the data did fit a modified Bleaney-Bowers equation

with a large negative J and a significant negative θ, suggesting that a considerable magnetic interaction operates between essentially planar [Cu(samen)Cu(L)] molecules. The magnetisms of the CuNi complexes were well interpreted in terms of the susceptibility equation based on the Heisenberg model. An antiferromagnetic spin-exchange interaction (J= −13∼−14 cm⁻¹) was suggested between the metal ions.     

Synthesis and spectroscopic studies of platinum(II) complexes of N,N′-dicyclohexylethylenediamine

The platinum(II) complexes of the formula [Pt(DCHEDA)X₂], where DCHEDA is N,N′-dicyclohexylethylenediamine and X⁻ is CL⁻, Br⁻, I⁻, 0.5C₂O₄²⁻ (oxalate), 0.5C₃H₂O₄²⁻ (malonate), 0.5C₉H₄O₆²⁻ (4-carboxyphthalate), 0.5S₂O₃²⁻ or 0.5SO₄²⁻, have been synthesized and characterized by UVVis, IR, and ¹H NMR spectral techniques. All the above complexes are non-electrolytes in DMF/H₂O, except the sulphate complex which becomes a 1:1 electrolyte after incubation for 24 h at 28 °C. The halide complexes were also studied by X-ray photoelectron spectroscopy and these data suggest that there is π-bonding from platinum to halide in these complexes. The oxalate, malonate and sulphate bind in their complexes as bidentate ligands to platinum through two oxygen atoms whereas the thiosulphate in its complex binds as a bidentate ligand to platinum through one oxygen atom and one sulphur atom.