Non-bridging Ligands Research Articles

Dramatic Variation of Magnetic Exchange through Double End-on Azide Bridges in a Series of Ladder-Like Copper(II) Coordination Polymers

Three ladder-like coordination polymers, [Cu2(phprpy)2-mu-(N3)2(N3)2], 1; [Cu2(terpy)2-mu-(N3)4Cu2-mu-(N3)2(N3)2], 2; and[Cu2(terpy)2-mu-(N3)2(N3)2Cu3-mu-(N3)4(N3)2], 3, consisting of Cu2+ ions with double end-on azide bridges were synthesized, their crystal structures and magnetic properties were determined, and spin dimer analysis was performed to explain the signs and strengths of their strong spin exchange interactions [phprpy is 4-(3-phenylpropyl)pyridine and terpy is 2,2':6,2''-terpyridine]. Although these compounds have ladder-like arrangements of Cu2+ ions, their magnetic structures are described as isolated dimers for 1 and 2 and as isolated trimers for 3. The predominant spin exchange paths in 1-3 have double end-on azide bridges linking adjacent Cu2+ ions, and the geometrical parameters of these bridging structures are similar. However, the spin dimer of 1 exhibits a strong ferromagnetic coupling; that of 2, a strong antiferromagnetic coupling; and that of 3, a weak ferromagnetic coupling. These findings are well explained by the present spin dimer analysis and show that the nature and geometry of the nonbridging ligands can have a strong influence on the sign and strength of the spin exchange interaction between Cu2+ ions connected by double end-on azide bridges.

A theoretical analysis of [M(tmtaa)] and [M(acacen)] fragments employed in the organometallic chemistry of early transition metals

The [M(tmtaa)] and [M(acacen)] fragments employed in the organometallic chemistry of titanium and vanadium have been investigated by a density functional approach. The frontier orbitals and the geometrical structure of the two macrocyclic ligands have been compared and the effect of the nature of the metal on the ligand environments has been studied. Some interesting complexes in which the metal contains a reactive organometallic functionality, such as metal–halogen (F and Cl) or metal–alkyl (CH 3) units, have been investigated. Moreover, the [Mo(acacen)] 2 dimer has been considered because of its interest as a metalmetal bonded complex with non-bridging ligands restraining the metalmetal distance.

Effect of structural and chemical parameters on magnetic coupling in hydroxo-bridged Cu(II) dimer

The influence of various structural and chemical parameters on magnetic coupling in model hydroxo-bridged Cu(II) dimers has been analyzed using the density functional theory and the broken symmetry approach. These parameters, such as the environment around copper atom, the electro negativity of the nonbridging ligands, the Cu–O–Cu angle, the Cu–O distance, the out-of-plane displacement of hydroxo group and the hinge distortion of the bridge, are all the factors that can affect magnetic coupling. The change of antiferromagnetic coupling strength does not depend on the Cu⋯Cu distance simply. In antiferromagnetic complexes, the spin delocalization from magnetic centers towards their neighbors is remarkable and weakens with ferromagnetic coupling strengthening. The effect of asymmetry at the Cu 2O 2 core has been studied also.

Structure and dynamics in the complex ion (UO2)2(CO3)(OH)3−

The structure and ligand exchange dynamics of the ternary complex (UO2)2(CO3)(OH)3− have been investigated by EXAFS and NMR spectroscopy. Very broad signals can be observed in both the 13C and the 17O NMR spectra. The EXAFS data show the presence of 1.3 ± 0.3 short uranium–oxygen distances at 2.26 A, consistent with single bonded hydroxide and 3.9 ± 0.6 distances at 2.47 A for the other ligands in the first co-ordination shell. There is also evidence for a U⋯U interaction at 3.90 A. Based on the EXAFS and NMR data we suggest the presence of three isomers with different bridge arrangements, the dominant one, C, contains 80% of the uranium and the minor ones A and B, 5 and 15%, respectively. The ligand exchange reactions between these isomers are slow. The NMR data indicate that the main reactions involve intramolecular exchanges between isomers with different positions of the non-bridging ligands in A, B and C. We suggest that these take place through water exchange as discussed earlier for other ternary uranium(VI) complexes.

What Does “Through-Bond Coupling” Mean? Observations on Simple Donor−Acceptor Systems

A series of covalently linked, transition-metal donor/acceptor complexes are described in which the net donor−acceptor coupling matrix element, HDA, is independent of the extent of coupling between the donor and the bridging ligand. The bridging ligand in these complexes is a transition-metal dicyano complex with a tetraaza aliphatic nonbridging ligand, cis- or trans-M(MCL)(CN)2+ for M = Rh(III), Co(III), or Cr(III), donor = Ru(NH3)52+, and the acceptor = Ru(NH3)53+. The electronic coupling (and electron delocalization) between the donor and the central atom (M) of the bridging ligand varies from HDL ≈ 103 to ∼3 × 103 cm-1 through the series of M(MCL)(CN)2+-bridged complexes, and this variation has an effect on the energy of the Ru(II)/Ru(III) CT absorption maximum, which is expected from perturbational mixing of these electronic states. However, the usually correlated superexchange contribution to HDA is not observed and appears to be less than about 10% of the contribution predicted. This is in contrast...

X-Ray and NMR spectroscopic characterisation of cyclic titanodiphenylsiloxanes and examination of the hydrolytic stability of their Si–O–Ti bonds

Six crystalline titanodiphenylsiloxanes have been synthesised by reaction of diphenylsilanediol (DPSD) with titanium tetraisopropoxide or its complexes with acetylacetonate (acac) as ligand. Two of them show a spirocyclic structure with the formula TiO2 [O2Si2(C6H5)4 ]2A and TiO2 [O4Si4(C6H5)8 ]2B which have already been described in the literature. Two compounds C and D were identified by X-ray analysis to have the same bicyclic structure but different coordinating solvent molecules. Tetrahydrofuran acts as a non-bridging ligand at the Ti atoms in [Ti(acac)O1.5 ]2 [OSi(C6H5)2 ]3 ·2C4H8O C while dioxane acts as a bridging ligand between the Ti atoms of neighbouring molecules of [Ti(acac)O1.5 ]2 [OSi(C6H5)2 ]3 ·3C4H8O2D. The titanodiphenylsiloxanes E and F were identified by a cyclotetrameric structure and the formulas [Ti(acac)2O]2 [OSi(C6H5)2 ]2 and [Ti(acac)2O][OSi(C6H5)2 ]3 , respectively. The titanodiphenylsiloxanes A–E were characterised by 29Si and 17O NMR spectroscopy, IR and time-of-flight mass spectrometry measurements. The hydrolytic stabilities of the Si–O–Ti bonds in the titanodiphenylsiloxanes A–E have been examined mainly by means of 29Si NMR spectroscopy. The results reveal a strong influence of the structure type of the titanodiphenylsiloxanes on the hydrolytic stability of their Si–O–Ti bonds apart from the hydrolytic conditions (amount of water, Si, Ti and H+ concentration). The hydrolytic stability of the titanodiphenylsiloxanes A–E decreases in the order cyclotetramer (E)>spirocyclo (A, B)>bicyclo (C, D). Reasons for the different hydrolytic stability are discussed. The results on the different hydrolytic stabilities of Si–O–Ti bonds can contribute to a better understanding of the synthesis of homogeneous heterometal materials on a molecular scale via the sol–gel process.

Spectroscopic and Electrochemical Probes of Electronic Coupling in Some Cyanide-Bridged Transition Metal Donor/Acceptor Complexes

The effects of donor−acceptor (D/A) electronic coupling, HDA, on the spectroscopic and electrochemical properties of several series of CN--bridged transition metal complexes have been examined. The complexes employed were formed by ruthenation of M(L)(CN)2n+ parent complexes (for n = 0, M = Ru(II) or Fe(II), and L = bpy or phen; for n = 1, M = Cr(III), Rh(III), or Co(III), and L = bpy, phen, or a tetraazamacrocyclic ligand). The observed half-wave potentials of the resulting CN--bridged D/A complexes spanned a 300−350 mV range in contrast to the range of about 80 mV expected on the basis of the oscillator strength, hDA, of the D/A charge-transfer MM‘CT absorption band and the geometrical distance between donor and acceptor, rDA. Different series of complexes exhibit different correlations between E1/2 and hDA. Several factors have been found to contribute to these differences: (a) symmetry effects; (b) solvational differences that arise when nonbridging ligands are changed; (c) solvational effects arising from differences in overall electrical charges; (d) partial delocalization of electron density along the D/A axis in such a way as to reduce the effective distance between centers of charge, . To take account of the effects of the solvational factors, systematic examination has been made of (a) the metal independent shifts of E1/2 which occur when nonbridging ligands are changed; (b) the differences in E/12 that occur in closely related Ru(III)/Ru(II) couples which differ in charge; and (c) solvent peturbations of E1/2(Ru(NH3)53+,2+) and solvatochromic shifts of the central metal-to-ligand charge transfer (MLCT) and MM‘CT absorbancies of (bpy)2(CN)Ru(CNRu(NH3)5)3+ and (bpy)2Ru(CNRu(NH3)5)26+. The experimental observations indicate that changes in the nonbridging ligand of the central metal can result in a range of about 90 mV variation in E1/2(Ru(NH3)53+,2+), the effect of a one unit increase in charge of the central metal is to increase E1/2 by approximately 65 ± 15 mV, solvent perturbations of E1/2 and the electron-transfer reorganizational energy, λr, are approximately equal in magnitude, solvational corrections can be treated linearly, and the solvational contributions to E1/2 that arise from charge delocalization are less than about 10 mV in these complexes. The complexes have a very rich charge-tansfer spectroscopy, and in some complexes as many as seven different CT transitions can be identified which depend on the oxidation state of the Ru(NH3)5 moiety. There is evidence for considerable mixing between these transitions. The mixed valence (Ru(NH3)52+/Ru(NH3)53+), bisruthenates exhibit a unique Ru(NH3)5/M MM‘CT component in addition to the expected Ru(NH3)52+ → Ru(NH3)53+ CT; this relatively weak absorption tracks the dominant Ru(NH3)5/central metal MM‘CT absorption, and it is attributable to the different effects of local Mc(CN-)Ru(NH3)5 electronic coupling in the mixed valence complex. Values of E1/2(obsd), corrected for solvational effects implied by the experimental observations, correlate with hDA, corrected for symmetry effects, E1/2(corr) = E1/2(ref) ± (4.2 × 10-4) hDA/rDA, only if the “solvational correction” for Fe(II)- and Ru(II)-centered complexes is about 70% larger than suggested by the experimental observations. This may imply greater charge delocalization onto (or from) the bridging ligand for these two metal centers. For either interpretation, the correlation between E1/2(obsd) and hDA implies that ≤ 0.62rDA. This relatively small value of can be interpreted in terms of charge delocalization onto (or from) the bridging ligand, and it can be qualitatively described in terms of perturbational mixing of the ground and excited electron-transfer states with higher energy CT states. This mixing is described in terms of a multicenter (Mc−C−N−Rut) vibronic coupling model which was previously (Inorg. Chem. 1996, 34, 3463) used to account for the anamolous shifts of the CN- stretch in CN--bridged D/A complexes.

Base hydrolysis in homometallic dinuclear chromium(III) complexes bridged by hydroxide and carboxylate

Base hydrolysis of dinuclear complexes [Cr2(µ-OH)(µ-RCO2)(en)4]4+ and [(nta)Cr(µ-OH)(µ-RCO2)Cr(en)2]+(en = ethane-1,2-diamine, nta = nitrilotriacetate; R = H, Me, Et, Prn, CH2Cl, CH2ClCH2, MeOCH2 or Ph) was studied by using the UV/VIS absorption and 2H NMR spectral changes and chromatography. The reactions were found to occur in two stages: the first-stage rates (k1) varied with R, but the second-stage ones (k2) were almost the same throughout the series. Analysis of the kinetic data for the two types of complexes disclosed the effects of the substituent groups R and/or the non-bridging ligands on the carboxylato bridge-cleavage reaction mechanism. Examination of the substituent effects in terms of Taft's equation with the electronic (ρ*) and steric (δ) parameters gave a critical clue to establishing the difference in bond-cleavage sites. The values were ρ*= 2.29 ± 0.20 and δ= 1.41 ± 0.13 for [Cr2(µ-OH)(µ-RCO2)(en)4]4+ and ρ*= 0.01 ± 0.11 and δ= 0.32 ± 0.08 for [(nta)Cr(µ-OH)(µ-RCO2)Cr(en)2]+. The carboxylato bridge-cleavage reaction is influenced by both the inductive and steric effects of the substituents in [Cr2(µ-OH)(µ-RCO2)(en)4]4+, and both effects contribute to smaller extents for [(nta)Cr(µ-OH)(µ-RCO2)Cr(en)2]+; the former complexes undergo cleavage at the carboxylic acyl C–O bond and the latter at the Cr–O bond in the Cr(en)2 site.

Substituent effects on the physical properties of (μ-oxo)(μ-carboxylato)bis{tris(2-pyridylmethyl) aminechromium(III)} dimers

In order to document the influence of carboxylate substituents on the physical properties of [(tmpa)Cr(μ-O)(μ- RCO 2)Cr(tmpa)] 3+ dimers, electronic spectra, Cr(III,IV/III,III) half-wave reduction potentials, μ-OH − ionization constants and antiferromagnetic coupling constants are reported for R=CH 2Cl, CHCl 2, C(CH 3) 3, C(C 6H 5) 3, 1- adamantyl and Ph-4-X (X=NMe 2, OH, OCH 3, CH 3, CF 3, F, Cl, CN, NO 2); tmpa=tris(2-pyridylmethyl)amine. Inductive effects of bridging benzoato para substituents are transmitted effectively to the μ-O 2− group in [(tmpa)Cr(μ-O)(μ-4-X-PhCO 2)Cr(tmpa)] 3+ dimers. A linear free energy relationship between E 1/2 Cr(III,IV/ III,III) and p K a(Cr(μ-OH)Cr) indicates a high degree of cooperativity among bridging oxo, complementary bridging and non-bridging ligands with regard to π-donation towards the Cr(III) centers. Trends in antiferromagnetic coupling constants are interpreted through the influence of configuration interaction between ligand- and metal- centered b 1 symmetry orbitals on the HOMO(b 1)-LUMO(a 2) singlet-triplet energy gap.

Correlation of CrOCr .pi.-bonding strength with .pi.-donor effectiveness of bridging and nonbridging ligands in (tris(2-pyridylmethyl)amine)chromium(III) dimers

The preparations of two [Cr(tmpa)L] 2 O 2+ dimers in which the pseudohalide ligand is a weaker r-donor than NCS - are reported (tmpa=tris(2-pyridylmethyl)amine): [Cr(tmpa)(NCSe)] 2 O(ClO 4 ) 2 and [{Cr(tmpa)(NCS)} 2 (O)Pd(CA)] (ClO 4 ) 2 .2CH 3 CN; CA 2- =2,5-dioxo-3,6-dichloro-1,4-benzoquinone. The NCS - ligands are constrained to lie on the same side of the CrOCr bond axis by chelation of the Pd(II) center in the latter complex.

Synthesis and characterization of tetraammineruthenium(II) complexes bound to the bridging ligand 3,6-bis(2-pyridyl)1,2,4,5-tetrazine (bptz): comparative effects of σ and π peripheral ligands on electronic absorption and electrochemical properties

Tetraammineruthenium(II) complexes with the nitrogen aromatic heterocyclic bridging ligand 3,6-bis(2- pyridyl)1,2,4,5-tetrazine (bptz) have been prepared and characterized. The [{(NH 3) 4Ru}(bptz)] 2+ ion exhibits intense electronic MLCT transitions at 560(sh) and 479 nm, and for the [{(NH 3) 4Ru} 2(bptz)] 4+ ion MLCT transitions occur at 850(sh), 603 and 370 nm. Reversible metal oxidations for the bimetallic complex are at +1.56 and +0.72 V versus SCE and Δ E(2−1)=840 mV indicates a large degree of ruthenium-ruthenium communication occurs through the bptz bridging ligand. The energies of the MLCT transitions and electrochemical potentials are compared with those for the analogous bis- bipyridyl ruthenium bptz complexes to assess the effect of NH 3 σ versus bpy π non-bridging ligands. In these complexes, as well as others, the results show the ruthenium centers coordinated with NH 3 are comparatively π electron rich when compared with the analogous bpy complexes. 1H and 13C NMR data are reported for the bptz ligand and mono and bimetallic complexes.

Kinetics of the reduction of Ru 2(CH 3COO) 4+ and of Ru(NH 3) 5pyridine 3+ by N-(2-hydroxyethyl)-ethylenediaminetriacetatoaquotitanium(III)

The kinetics of the reaction of Ti(HEDTA)(H 2O) with Ru 2(CH 3COO) 4 +and with Ru(NH 3) 5py 3+ (py = pyridine) have been studied in LiCl media of 1.0 M ionic strength. Both reactions follow second-order kinetics with the rate constant dependent on acidity according to the rate law: k 2 = kK/([H +] + K), where K is close to 0.1 M [the acid-dissociation constant of Ti(HEDTA)(H 2O)], and k is 1.6 x 10 4 M −1 s −1 for Ru 2(CH 3COO) 4 + at 25°C and k is 2.0 x 10 5 M −1 s −1 for Ru(NH 3) 5py 3+ at 4.5°C. The rate constant for reaction of Ti(HEDTA)(OH) − and Ru 2(CH 3COO) 4 + is about one order of magnitude higher than the rate constant for reduction of Ru 2(CH 3COO) 4 + by Ti(OH) 2+, as would be expected on the basis of the shift in Ti III redox potential that is caused by ligation of HEDTA and the known influence of change in driving force on rates of outer-sphere Ru III-Ti III redox reactions. The rate of reduction of Ru(NH 3) 5py 3+ by Ti(HEDTA)(OH) − at 4.5°C is more than two orders of magnitude higher than the reduction rate of the same oxidant by Ti(OH) 2+ at 25°C. This non-bridging ligand effect is at least 100 times larger than can be understood on the basis of change of redox potential, and is best ascribed to a large increase in the stability constant of the precursor complex.

Cyclic voltammetric studies of dioxygenbridged dinuclear cobalt(III) complexes

Cyclic voltammetric studies were carried out for dioxygen-bridged dinuclear cobalt(III) complexes using platinum electrodes at different values of pH and ionic strength. Reversible electrode reactions occur particularly for the dioxygen complexes [(phen)2Co(μ-O2,NH2)Co(phen)2] (ClO4)42H2O and [(bpy)2Co(μ-u-O2, NH2)Co(bpy)2] (ClO4)4H2O with 1,10-phenanthroline (phen) and 2,2-bipyridine (bpy) as the non-bridging ligands. For complexes with ammonia (NH3) ethylenediamine (en) as the terminal ligands, there is quasi-reversible electrode behaviour at pH < 3 due to protonation of the peroxo complexes which subsequently undergo equilibration with isomeric forms. Such behaviour is well-marked for the peroxo complex [(en)2Co(μ-O2 NH2)Co(en)2](NO3)3. The decaammine complex [(NH35Co(μ-O2Co(NH3)5](NO3)5 shows only a cathodic peak due to intramolecular charge transfer decomposition in solution after reduction at the electrode. Diffusion coefficients for all the dioxygen complexes were determined from the plots ofIp vs.v1/2.

Electron Paramagnetic Resonance of Ni2+ in BaZnF4 Single Crystals

X-band EPR measurements of Ni-doped BaZnF4 crystals revealed presence of three centers. Well-resolved hyperfine structure from 19F nuclei at suitable orientations allowed assignment of the most intense spectrum to single ions on Ba sites. The occupation of Zn sites is considerably lower. The third spectrum is assigned to pairs of Ni2+ ions on adjacent Ba sites. Attempts to correlate the zero-field splitting (ZFS) patterns of the single ion spectra with the distortions of the first coordination spheres of Ba and Zn resp. with a single intrinsic ZFS parameter for both bridging and nonbridging ligands were equally unsuccessful as in the case of Mn2+.

Complexes of Oxamic Acid with La(III), Gd(III), Tb(III), Er(III), Tm(III) and Lu(III)

The preparation, for the first time, of the deprotonated complexes of oxamic acid with La(III), Gd(III), Tb(III), Er(III), Tm(III) and Lu(III) is reported. Analytical results, conductometric measurements, magnetic moments and spectral data (IR and diffuse reflectance spectra) are discussed in terms of possible structural types. The oxamate anion acts as a N, O bidentate non-bridging ligand.

Redox kinetics of metal complexes in non-aqueous solutions. XII. Effect of non-bridging ligands on intramolecular electron transfer in the FeL 3+5/Fe(Me 4phen) 2+3 reaction in acetonitrile

In acetonitrile the reductions of a series of iron-(III) solvates, FeL 3+ 6 (L = TMP, DMF, DEF (= diethyl formamide), AA (= acetamide), and DMSO), by Fe(tmphen) 2+ 3 (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) proceed via formation of a kinetically-detectable intermediate. All evidence available suggests its form as L 5Fe(acetonitrile)Fe(tmphen) 5+ 3 wherein acetonitrile features a bridge between phenanthroline and FeL 3+ 5 in that the CN triple bond interacts with the π orbitals on the peripheries of the phenanthroline complex and the nitrile N coordinates to FeL 3+ 5. The decomposition rate constant of that intermediate, signifying the rate of the actual electron transfer, has been measured in dependence of ligands and temperature. For the series of related reactions the isokinetic relationship holds. As the donor number of the ligands increases, the activation energy is decreased according to the increase in electronic coupling, over the wide range of 50 kJ mol −1. At the same time the activation entropy shifts to more negative values, due to increased steric crowding in the successor complex, over an even wider range from −41 to the extremely negative value of −286 J K −1 mol −1. Consequently the decrease in ΔS≠ strongly overcompensates the favorable change in ΔH≠: The reaction series is entropy-controlled, as is recognized by an isokinetic temperature which is below the experimental temperature range T iso ∼ −70 °C.

V. Molecular structures of acetate-bridged dimers of a 2-arylpyridine and a 2,6-diarylpyridine cyclometallated by palladium(II)

The crystal and molecular structures of dimeric cyclopalladated acetato(OAc)-bridged complexes of a 2-arylpyridine ( 1) and a 2,6-diarylpyridine ( 2), [Pd( 1)OAc] 2 ( 3) and [Pd( 2)OAc] 2 ( 4) have been determined by X-ray diffraction. In each. two planar acetates, nearly perpendicular to each other, cis-bridge two Pd II atoms, while the other two square planar sites on each metal are occupied by the cyclometallated aryl-substituted pyridine. The two metal coordination planes are in near parallel registry with one another, giving a “boat” molecule shape of exact ( 3 or near ( 4) two-fold symmetry. The non-bonding Pd⋯Pd distance (δ) strongly correlates with the angle (α) subtended by the metal coordination planes, which in turn seems related to the steric requirements of the non-bridging ligands. In 3, with the planar nitrophenylpyridine ligand, α 25.7° and δ 2.822(6) Å, the shortest Pd⋯Pd distance yet observed in dimers of this type. The PdC lengths, 1.94 Å in 3 and 1.944 Å in 4, are significantly shorter than the calculated length, 2.05 Å, suggesting partial multiple bond character in these shortest PdC (aryl) bonds so far observed. PdN bond lengths are near expected single bond lengths. Failure of trans-dimetallation to occur in 4, analogous to that recently observed with certain 2,6-dialkylpyridines or to the tridentate behavior of terpyridine in [Pd terpy X] + compounds, appears due to a combination of the shortened PdC bond in the first-formed metallated ring and the larger constraining angles (near 120° for sp 2 carbons) in the remaining (unmetallated) ring.

Complexes of oxamic acid with Au(III) and Rh(III)

The preparation and some properties of the deprotonated complexes of oxamic acid with Au(III) and Rh(III) are reported. On the basis of analytical results, conductometric measurements, magnetic moments and spectral data (IR and UV-visible), a square planar structure is proposed for K[AuL(OH)2] and octahedral for K3[RhL 3] 3H2O (whereLH2=oxamic acid).L 2− acts as a bidentate, non-bridging ligand.

Reaction of carbon monoxide with hemocyanin: Stereochemical effects of a non-bridging ligand

The carbon monoxide binding equilibria and kinetics of a number of molluscan and arthropodal hemocyanins have been investigated employing the visible luminescence of the carbon monoxide-copper complex. Proteins from both phyla, in oligomeric and monomeric form, bind carbon monoxide non-co-operatively; the reaction is largely enthalpy driven is associated with a small unfavourable entropy change. Molluscan hemocyanins display a carbon monoxide affinity ( p 50 = 1 to 10mm Hg) higher than that of arthropodal hemocyanins ( p 50 = 100 to 700mm Hg), and only Panulirus interruptus hemocyanin, among those studied here, exhibits a small Bohr effect. The observed differences in equilibrium constant are kinetically reflected in differences in the carbon monoxide dissociation rate constant, which ranges from 20 to 70 s −1 for molluscan hemocyanins and from 200 to 9000 s −1 for arthropodal hemocyanins; on the other hand the differences in the combination rate constants between the two phyla are considerably smaller. A comparison of the equilibrium and kinetic results shows some discrepancies between the two sets of data, suggesting that carbon monoxide binding may be governed by a complex mechanism. The correlation between the ligand binding properties and the stereochemistry of the active site is discussed in the light of the knowledge that, while oxygen is bound to both copper atoms in a site, carbon monoxide is a “non-bridging” ligand, being bound to only one of the metals.

Complexes of 1,2,4-Triazoles, Part XVI Binuclear Complexes of Transition Metal(II)thiocyanates with 4-Ethyl-1,2,4-triazole

Abstract Coordination compounds of 4-ethyl-1,2,4-triazole (Ettrz) with Mn, Fe, Co, Ni, Cu and Zn are reported. The compounds have been identified and characterized by chemical analyses, infrared and ligand field spectra, electron paramagnetic resonance (e.p.r.) measurements, magnetic susceptibility measurements and thermoanalytical experiments. Except for the Cu compound, which is polynuclear, all compounds appear to be binuclear, with both bridging bidentate and non-bridging triazole ligands. The overall formula is M(Ettrz)2(NCS)2(H2O)3/2 for Mn, Fe, Co and Ni. The copper and zinc compounds have the formula Cu(Ettrz)2(NCS)2 and Zn(Ettrz)3/2(NCS)2.