Trigonal-planar Complex Research Articles

Fine Structure and the Huge Zero-Field Splitting in Ni2+ Complexes.

We perform a thorough study of the ground state magnetic properties of nickel-based 3d8 complexes. This includes an in-depth analysis of the contribution of the crystal field, spin exchange and spin-orbit interactions to the ground state magnetic properties. Of particular interest to the current investigation are the presence and occurrence of non-trivial zero-field splitting. The study focuses on the cases of Ni2+ ideal octahedral, trigonal bipyramidal, square planar and tetrahedral geometries. We provide results for the complete energy spectrum, the fine structure related to the ground state and the second set of excited states, low-field magnetic susceptibility and magnetization. In addition, we examine the zero-field fine structure in square pyramidal, trigonal pyramidal and trigonal planar complexes. The obtained results unequivocally show that a moderate or highly coordinated 3d8 complex can neither exhibit spin-orbit-driven large and giant magnetic anisotropy nor a huge zero-field splitting. Moreover, in the trigonal bipyramidal coordination, a fine structure associated to the ground state cannot result from the spin-orbit coupling alone.

Low-Valent, Multiply Bonded, Trigonal-Planar Sb Complex: Rational Syntheses, Dual Acidic/Basic Properties, and Unexpected Semiconducting Characteristics.

A 4-center, 6π-conjugated, multiply bonded trigonal-planar complex, [Sb{Cr(CO)5}3]- (1), was synthesized via the hydride abstraction of [HSb{Cr(CO)5}3]2- (1-H) with HBF4·H2O, with the release of high yields of H2. The oxidation state of the Sb atom in [Et4N][1] was well-defined as 0, which was evidenced by X-ray photoelectron spectroscopy and X-ray absorption near-edge structure. The distinct color-structure relationship of this low-valent Sb complex 1 toward a wide range of organic solvents was demonstrated, as interpreted by time-dependent density functional theory calculations, allowing the trigonal-planar 1 and the tetrahedral solvent adducts to be probed, revealing the dual acid/base properties of the Sb center. In addition, 1 showed pronounced electrophilicity toward anionic and neutral nucleophiles, even with solvent molecules, to produce tetrahedral complexes [(Nu)Sb{Cr(CO)5}3]n- [1-Nu; n = 2, Nu = H, F, Cl, Br, I, OH; n = 1, Nu = PEt3, PPh3, N,N-dimethylformamide (DMF), acetonitrile (MeCN)]. On the contrary, the Fe/Cr hydride complex [HSb{Fe(CO)4}{Cr(CO)5}2]2- (2-H) was obtained by treating 1 with [HFe(CO)4]-. Upon hydride abstraction of 2-H with HBF4·H2O or [CPh3][BF4], a multiply bonded Fe/Cr trigonal-planar complex, [Sb{Fe(CO)4}{Cr(CO)5}2]- (2), was produced in which the oxidation coupling Sb2-containing complexes [Sb2Cr4Fe2(CO)28]2- (3-Cr) and [HSb2Cr3Fe2(CO)23]- (3-H) were yielded as final products. Complex 3-Cr exhibited dual Lewis acid/base properties via hydridation and protonation reactions, to form 2-H or 3-H, respectively. Surprisingly, [Et4N][1] possessed a low energy gap of 1.13 eV with an electrical conductivity in the range of (1.10-2.77) × 10-6 S·cm-1, showing that [Et4N][1] was a low-energy-gap semiconductor. The crystal packing, crystal indexing, and density of states results of [Et4N][1] further confirmed the efficient through-space conduction pathway via the intermolecular Sb···O(carbonyl) and O(carbonyl)···O(carbonyl) interactions of the 1D anionic zigzag chain of 1.

Versatile Coordination Chemistry of Hexa-tert-butyl-octaphosphine.

The octaphosphine {cyclo-(P4tBu3)}2 (1) possesses a multifaceted coordination chemistry. The predominant binding mode is a P,P-chelate, e.g., in the monometallic chelate complex [MLL'(1-κ2P2,P4')] in which the ligand 1 adopts a gauche conformation. Examples include square-planar (M = RhI, L = CO, L' = Cl (2), M = PdII, L = L' = Cl (3), M = PtII, L = L' = Cl (9)), tetrahedral (M = Co-I, L = NO, L' = CO (4)), and trigonal-planar complexes [ML(1-κ2P2,P4')] (M = Pd0, L = PPh3, (5), M = CuI, L = Br (6)). With 2 equiv of [CuBr(SMe2)], a dinuclear complex [(CuBr)2(1-κ2P2,P2',κ2P4,P4')] (7) was obtained which features a synperiplanar conformation of the octaphosphine. A second coordination mode was also observed in [PtCl2(1-κ2P1,P2')] (10) in which the bridge phosphorus atom in octaphosphine 1 is involved in the chelation, with the ligand in an antiperiplanar conformation. Thermolysis of selected complexes showed them to be suitable candidates for the generation of phosphorus-rich metal phosphides MPx (x > 1).

Formylglycine-generating enzyme binds substrate directly at a mononuclear Cu(I) center to initiate O2 activation

The formylglycine-generating enzyme (FGE) is required for the posttranslational activation of type I sulfatases by oxidation of an active-site cysteine to Cα-formylglycine. FGE has emerged as an enabling biotechnology tool due to the robust utility of the aldehyde product as a bioconjugation handle in recombinant proteins. Here, we show that Cu(I)-FGE is functional in O2 activation and reveal a high-resolution X-ray crystal structure of FGE in complex with its catalytic copper cofactor. We establish that the copper atom is coordinated by two active-site cysteine residues in a nearly linear geometry, supporting and extending prior biochemical and structural data. The active cuprous FGE complex was interrogated directly by X-ray absorption spectroscopy. These data unambiguously establish the configuration of the resting enzyme metal center and, importantly, reveal the formation of a three-coordinate tris(thiolate) trigonal planar complex upon substrate binding as furthermore supported by density functional theory (DFT) calculations. Critically, inner-sphere substrate coordination turns on O2 activation at the copper center. These collective results provide a detailed mechanistic framework for understanding why nature chose this structurally unique monocopper active site to catalyze oxidase chemistry for sulfatase activation.

A Multiply Bonded Trigonal-Planar Bismuth(III) Complex: Prodigious Lewis Acidity, Solvatochromism, Etherification, and Semiconducting Characteristics.

The formation of a hitherto unknown 4-center, 6π-conjugated trigonal-planar complex, [Bi{Cr(CO)5 }3 ]- (1), is reported, in which the oxidation state of the Bi atom is +3, as evidenced by XAS, XPS, and DFT calculations. The BiIII atom in 1 has dual donor and acceptor properties in its bonding mode. In contrast to the mild Lewis acidity of BiIII , the central Bi in 1 functions as a prodigious Lewis acid site to exhibit strong affinity toward F- ions, unique solvatochromic properties, intriguing etherification through the C-O bond cleavage of alcohols, and surprising semiconducting characteristics with an ultra-narrow optical band gap of 1.02 eV, which can be attributed to the intermolecular Bi⋅⋅⋅O and O⋅⋅⋅O interactions in the solid state. The tetrahedral Fe(CO)4 -adduct [{Fe(CO)4 }Bi{Cr(CO)5 }3 ]3- (1-Fe) allowed the selective demetallation to afford the isoelectronic multiply bonded BiCr3 -complex 1 and the BiCr2 Fe-complex, [Bi{Cr(CO)5 }2 {Fe(CO)4 }]- (2), which may open a novel pathway for the design of the heterometal-incorporated trigonal-planar Bi-Cr complexes.

Exploiting Pincer Ligands to Perturb the Geometry at Boron.

Boranes are ubiquitous in synthesis and materials but advancements in their development have been primarily restricted to the geometric energetic minima, trigonal planar complexes. This report discloses a class of boranes with expanded bond angles achieved by taking advantage of the structural rigidity of tridentate pincer ligands. The bonding of these novel boranes is investigated by X-ray crystallography and computationally.

Hierarchy of Supramolecular Arrangements and Building Blocks: Inverted Paradigm of Crystal Engineering in the Unprecedented Metal Coordination of Methylene Blue.

The aromatic methylene blue cation (MB+) shows unprecedented ligand behavior in the X-ray structures of the trigonal-planar (TP) complexes MBMCl2 (M = CuI, AgI). The two isostructural compounds were exclusively synthesized by grinding together methylene blue chloride and MCl solids. Only in the case of AuCl did the technique lead to a different, yet isoformular, AuI derivative with separated MB+ and AuCl2- counterions and no direct N-Au linkage. While the density functional theory (DFT) molecular modeling failed in reproducing the isolated Cu and Ag complexes, the solid-state program CRYSTAL satisfactorily provided for Cu the correct TP building block associated with a highly compact π stacking of the MB+ ligands. In this respect, the dispersion interactions, evaluated with the DFT functional, provide to the system an extra energy, which likely supports the unprecedented metal coordination of the MB+ cation. The feature seems governed by subtle chemical factors, such as, for instance, the selected metal ion of the coinage triad. Thus, the electronically consistent AuI ion does not form the analogous TP building block because of a looser supramolecular arrangement. In conclusion, while a given crystalline design is generally fixed by the nature of the building block, a peculiarly efficient supramolecular packing may stabilize an otherwise unattainable metal complex.

Fabrication of Copper(I) Bipyridyl Complex Based Dye Sensitized Solar Cells

This study investigates the performance of dye-sensitized solar cells (DSSC) based on a copper(I) complex. A simple form of copper(I) complex dye was synthesized with a structure of [Cu(L)(CH3CN)], where L is the 6,6′-dimethyl-4,4′-bis(phenylethynyl)-2,2′-bipyridine ligand. The full structure of DSSC investigated in this study is as follows: FTO/TiO2/dye Cu(I) bipyridyl/3I−/I3− electrolyte/graphite/FTO. The TiO2 photoanodes were deposited from apoly(vinylpyrrolidone)-based paste using a spin coating technique. Different conditions of fabrication, such as paste dispersion time and total TiO2 thickness, were systematically studied in order to optimize the performance of the DSSC. The trigonal planar complex [Cu(L)CH3CN] was revealed to be suitable for applications in DSSC. The highest exhibited short circuit current density was found to be 0.48 mA/cm2, with an open voltage of 477 mV, a form factor of 34% and a power conversion efficiency of 0.08% for the cell with photoanodes thickness of about 2.2 μm. It was shown that the dye and the paste formulation had great potential for applications in DSSC.

Field-Induced Slow Magnetic Relaxation in the Ni(I) Complexes [NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)(PPh3)2

Direct current (dc) and alternating current (ac) magnetic measurements have been performed on the three Ni(I) complexes: [NiCl(PPh3)3], [NiCl(PPh3)2]·C4H8O, and [Ni(N(SiMe3)2)(PPh3)2]. Fits of the dc magnetic data suggest an almost similar behavior of the three compounds, which display only moderate deviations from the spin-only values. The ac magnetic investigations reveal that the two complexes with trigonal planar coordination--[NiCl(PPh3)2]·C4H8O and [Ni(N(SiMe3)2)(PPh3)2]--display slow magnetic relaxation at low temperatures under applied dc fields, whereas tetrahedral [NiCl(PPh3)3] does not. Ground and excited states as well as magnetic data were calculated by ab initio wave function based multi-configurational methods, including dynamic correlation as well as spin-orbit coupling. The two trigonal planar complexes comprise well-isolated S = (1)/2 ground states, whereas two S = (1)/2 states with a splitting of less than 100 cm(-1) were found in the tetrahedral compound.

Synthesis and Reactions of 3d Metal Complexes with the Bulky Alkoxide Ligand [OC(t)Bu2Ph

Treatment of NiCl2(dme) and NiBr2(dme) (dme = dimethoxyethane) with 2 equiv of LiOR (OR = OC(t)Bu2Ph) forms the distorted trigonal planar complexes [NiLiX(OR)2(THF)2] (THF = tetrahydrofuran) 5 (X = Cl) and 6 (X = Br). The reaction of CuX2 (X = Cl, Br) with 2 equiv of LiOR affords the Cu(I) product Cu4(OR)4 (7). The same product can be obtained using the Cu(I) starting material CuCl. NMR studies indicated that the reduction of Cu(II) to Cu(I) is accompanied by the oxidation of the alkoxide RO(-) to form the alkoxy radical RO(•), which subsequently forms tert-butyl phenyl ketone by β-scission. Treatment of compounds 1-4 ([M2Li2Cl2(OR)4], M = Cr-Co) with thallium hexafluorophosphate allowed the isolation of the distorted tetrahedral complexes of the form M(OR)2(THF)2 for M = Mn (8), Fe (9), and Co (10). Cyclic voltammetry performed on compounds 8-10 demonstrated irreversible oxidations for all complexes, with the iron complex 9 being the most reducing. Complex 9 shows a reactivity toward PhIO and Ph3SbS to form the corresponding dinuclear iron(III) complexes Fe2(O)(OR)4(THF)2 (11) and Fe2(S)(OR)4(THF)2 (12), respectively. X-ray structural studies were performed, showing that the Fe-O-Fe angle for complex 11 is 176.4(1)° and that the Fe-S-Fe angle for complex 12 is 164.83(3)°.

Synthesis and Structure of a Trigonal Planar Complex with Pyrazole Ligands, [Cu(3,5-Ph2PzH)3][Cl

The crystal structure of a trigonal planar copper(I) complex, [Cu-(3,5-Ph2PzH)3][Cl], is reported. In the title compound, one nitrogen atom from each pyrazole ligand is coordinated to the copper(I) ion and each N–H of the pyrazoles forms a hydrogen-bond with the chloride resulting in a trigonal planar geometry about the copper(I) ion. The complex crystallizes in the trigonal space group R $$\overline 3$$ with a = b = 13.031(1) A, c = 37.3187(9) A, α = β = 90o, γ = 120o , V = 5464.5(2) A3, Z = 6. A trigonal planar copper(I) complex with three pyrazole ligands is reported in which the hydrogens of the bound pyrazole ligands are hydrogen bonded with the chloride counter ion.

Iron(ii) complexes of ditopic carbanionic carbenes

Reaction of dimesityliron(II) (Fe2(mes)4) with the N-heterocyclic carbenes 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) and 1,3-bis(2,6-dimethylphenyl)hexahydropyrimidin-2-ylidene (6-Xyl) afforded the novel trigonal planar complexes [Fe(IPr)(mes)2] (1) and [Fe(6-Xyl)(mes)2] (2), respectively. Both species were structurally characterized by single crystal X-ray diffraction and display structures and magnetic responses consistent with a quintet ground state (S = 2). Reaction of 1 with KC8 in THF afforded K(+) salts of the anionic complex [{:C[N(2,6-(i)Pr2C6H3)]2(CH)C}2Fe(mes)](-) (3) and the homoleptic organometallic anion [Fe(mes)3](-) (4). By contrast, reduction of 2 resulted in extensive decomposition and intractable product mixtures. Complex 3 is coordinated by two ditopic carbanionic carbenes via the C4/C5 position while the C2 position retains unquenched carbenic character and remains vacant for further coordination. This was corroborated by reacting solutions of 3 with one and two equivalents of triethylaluminium (AlEt3) which resulted in the formation of [{Et3Al:C[N(2,6-(i)Pr2C6H3)]2(CH)C}{:C[N(2,6-(i)Pr2C6H3)]2(CH)C}Fe(mes)](-) (5) and [{Et3Al:C[N(2,6-(i)Pr2C6H3)]2(CH)C}2Fe(mes)](-) (6), respectively. Both of these species were structurally characterized as [K(2,2,2-crypt)](+) salts.

Crystallographic and Computational Studies of Luminescent, Binuclear Gold(I) Complexes, AuI2(Ph2P(CH2)nPPh2)2I2(n= 3–6)

Four crystalline dimers of the type, Au(I)(2)(μ-PnP)(2)I(2), where PnP is PPh(2)(CH(2))(n)PPh(2) with n = 3, 4, 5, and 6 have been prepared and characterized by single-crystal X-ray diffraction and by (31)P NMR and infrared spectroscopy. Au(I)(2)(μ-P3P)(2)I(2) and Au(I)(2)(μ-P6P)(2)I(2) are centrosymmetric dimers with the planar Au(I)P(2)I units oriented in antiparallel fashion. Remarkably, noncentrosymmetric Au(I)(2)(μ-P5P)(2)I(2) has its planar Au(I)P(2)I units oriented in parallel manner. Au(I)(2)(μ-P4P)(2)(μ-I)(2) is unique, since it contains four-coordinate gold centers that are bridged by both iodide and diphosphine ligands. All four compounds are luminescent as solids at room temperature. B3LYP, B2PLYP, and spectroscopically oriented configuration interaction (SORCI) calculations have been conducted to give insight into the electronic and geometric structures of the ground and first excited triplet states of the three trigonal-planar complexes. The emission energies for the trigonal planar complexes are more strongly correlated with changes in the Au-I bond length rather than changes in the P-Au-P angle.

Transition Metal Complexes of Anionic N‐Heterocyclic Dicarbene Ligands

A tale of two carbenes: Reaction of :C[N(2,6-iPr2C6H3)CH]2 (IPr) with Mn3(mes)6 (mes=2,4,6-trimethylphenyl) yielded the trigonal planar complex [Mn(IPr)(mes)2]. Reduction of this species with potassium/graphite in THF afforded the polymeric dicarbene-bridged species K[{:C[N(2,6-iPr2C6H3)]2(CH)C}2Mn(mes)(thf)]⋅THF (see picture). The anionic moiety in this complex is the first reported example of a transition metal complex containing an N-heterocyclic dicarbene ligand. Gray C, blue N, red Mn.

On the Solid Solution Series Er15[(Fe1–xRux)8C25]: Site Preference for Substitution in the Range 0 ≤ x ≤ 0.625?

AbstractThe solid solution series Er15[(Fe1–xRux)8C25] was prepared by arc‐melting of pressed pellets of mixtures of the elements, followed by annealing at 1173 K and quenching. Powder X‐ray patterns reveal the presence of a limited solid solution phase within the composition range 0 ≤ x ≤ 0.75 (trigonal, P321). Its crystal structure contains polyanionic slabs $\rm^2_\infty$[Fe1–xRux)8(C25)45–] stacked along [001] with Er3+ ions in between. The substitutions concern a planar triangular cluster made up of six transition metal atoms and a trigonal planar complex of composition [(Fe1–xRux)(C2)3]. The single crystal structure determination of Er15[Fe4.93(1)Ru3.07C25] clearly shows that the iron atoms forming the outer triangle of the planar cluster are not affected by substitution, resulting in the chemical composition Fe3(Fe0.19(3)Ru0.81)3for the intermetallic cluster unit. According to the chemical bonding analyses, the formation of Ru–Ru bonds seems to be the driving force for this preferential site occupation. The trigonal planar complex, again, is affected by substitution: [(Fe0.68(4)Ru0.32)(C2)3]. Assuming complete substitution except for the outer triangle of the intermetallic cluster, the limiting chemical composition for the series results inEr15[(Fe3Ru5)C25] (x = 0.625). The difference to the experimental value (x = 0.75) gives reason for further investigations. The end member Er15[Ru15C25] was not observed.

Synthesis, Structure, and Reactivity of Organo-Iron(II) Complexes with N-Heterocyclic Carbene Ligation

The synthesis, structure, and reactivity of some organo-iron complexes with monodentate N-heterocyclic carbene (NHC) ligation were studied. Mononuclear ferrous complexes [(IEt)2FeR2] (IEt = 2,5-diethyl-3,4-dimethylimidazol-1-ylidene, R = Me (2a), CH2TMS (2b)) and [(IPr)FeMes2] (3, IPr = 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene) were prepared in good yields via salt elimination reactions of [(NHC)2FeCl2] (1) with alkylation reagents. The interaction of 1 with PhLi gave a mixture of dinuclear complexes [Cl(IEt)Fe(IEt′)2Fe(IEt)Cl] (4a) and [Ph(IEt)Fe(IEt′)2Fe(IEt)Ph] (4b) (IEt′ = 3-Et-4,5-Me2-2-ylideneimidazolyl anion), in which N−C(ethyl) bond cleavage of the NHC ligand was involved. Complexes 2a−4b were characterized by 1H NMR, elemental analyses, and single-crystal X-ray diffraction studies. Solution magnetism measurement by Evan’s method revealed the high-spin electronic configuration for the mononuclear organo-iron(II) complexes 2a, 2b, and 3. Reactivity studies showed the tetrahedral complex 2a was inert toward many unsaturated organic substrates, whereas the trigonal-planar complex 3 could react with CO and carbodiimide PriN═C═NPri to yield dimesityl ketone and [(IPr)Fe(Mes)(η2-PriNC(Mes)NPri)] (5), respectively. Relevant to iron-catalyzed Kumada couplings, both complexes 2b and 3 were found reactive with PhI to yield the corresponding carbon−carbon bond formation products Ph−CH2TMS and Ph−Mes.

Theoretical investigation of dinitrosyl complexes in Cu-zeolites as intermediates in deNOx process

The structure and stability of nitrosyl complexes formed in Cu-FER zeolite were investigated using a periodic DFT model. The reliability of both DFT methods and cluster models when describing the Cu(+) interaction with NO molecules was examined. The relative stabilities of mononitrosyl complexes on various Cu(+) sites in Cu-FER are governed by the deformation energy of the particular site. Three types of dinitrosyl complexes with different coordination on the Cu(+) cation were identified: (i) four-fold tetrahedral, (ii) four-fold square-planar and (iii) three-fold trigonal-planar complexes. The most stable dinitrosyl complex, formed when the two NO molecules interact with Cu(+)via the N atom, has a tetrahedral coordination on Cu(+). The cyclic adsorption complex, having a square-planar arrangement of ligands on Cu(+) and interaction via O atoms, is only about 10 kJ mol(-1) less stable than the N-down dinitrosyl complex. This cyclic dinitrosyl complex is suggested to be the key intermediate in the deNO(x) process taking place in Cu-zeolites.

The Remarkable Solvent‐Dependent Crystallization of the Mono‐ and Bis(4‐pyrrol‐1‐ylbenzonitrile) Adducts of Bis(pentafluorophenyl)zinc

AbstractTreatment of a toluene solution of Zn(C6F5)2 with p‐C4H4N–C6H4CN yields (p‐C4H4N–C6H4CN)2Zn(C6F5)2, which has been structurally characterised following recrystallization from 1,2‐difluorobenzene by single‐crystal X‐ray diffraction and possesses the expected distorted tetrahedral molecular geometry. However, crystallization from dichloromethane yields a trigonal planar complex with only one p‐C4H4N–C6H4CN ligand. The three‐coordinate complex forms an intermolecular dimer in the solid state through a cooperative combination of zinc–arene, offset homoaromatic and dipole–dipole interactions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis, crystal structures and multinuclear NMR spectroscopy of copper(I) complexes with benzophenone thiosemicarbazone

Reactions of benzophenone thiosemicarbazone (Hbztsc, Ph 2C N-NH-C( S)-NH 2) with copper(I) chloride/bromide in the presence of two moles of PPh 3, formed monomeric tetrahedral complexes, [CuX (η 1-S-Hbztsc)(PPh 3) 2] · CH 3CN (X = Cl, 1; Br, 2). It did not form similar complex with copper(I) iodide; rather it formed a trigonal planar complex [CuI(η 1-S-Hbztsc) 2] ( 3) with two moles of Hbztsc in absence of PPh 3. All the complexes have been characterized with the help of elemental analyses, IR, 1H, 13C, and 31P NMR spectroscopy, and single crystal X-ray crystallography. The crystal structure of ligand is also described. In all the complexes, benzophenone thiosemicarbazone is acting as a neutral S donor ligand in η 1-S bonding mode. NMR data support that the complexes remain stable in solution phase.

Soft metal ions, Cd(II) and Hg(II), induce triple-stranded alpha-helical assembly and folding of a de novo designed peptide in their trigonal geometries.

We previously reported the de novo design of an amphiphilic peptide [YGG(IEKKIEA)4] that forms a native-like, parallel triple-stranded coiled coil. Starting from this peptide, we sought to regulate the assembly of the peptide by a metal ion. The replacement of the Ile18 and Ile22 residues with Ala and Cys residues, respectively, in the hydrophobic positions disrupted of the triple-stranded alpha-helix structure. The addition of Cd(II), however, resulted in the reconstitution of the triple-stranded alpha-helix bundle, as revealed by circular dichroism (CD) spectroscopy and sedimentation equilibrium analysis. By titration with metal ions and monitoring the change in the intensity of the CD spectra at 222 nm, the dissociation constant Kd was determined to be 1.5 +/- 0.8 microM for Cd(II). The triple-stranded complex formed by the 113Cd(II) ion showed a single 113Cd NMR resonance at 572 ppm whose chemical shift was not affected by the presence of Cl- ions. The 113Cd NMR resonance was connected with the betaH protons of the cysteine residue by 1H-113Cd heteronuclear multiple quantum correlation spectroscopy. These NMR results indicate that the three cysteine residues are coordinated to the cadmium ion in a trigonal-planar complex. Hg(II) also induced the assembly of the peptide into a triple-stranded alpha-helical bundle below the Hg(II)/peptide ratio of 1/3. With excess Hg(II), however, the alpha-helicity of the peptide was decreased, with the change of the Hg(II) coordination state from three to two. Combining this construct with other functional domains should facilitate the production of artificial proteins with functions controlled by metal ions.