Weak Ligand Field Research Articles

The physico-chemical properties of 4-chlorophenoxyacetates of Mn(II), Co(II), Ni(II) and Cu(II)

The complexes of 4-chlorophenoxyacetates of Mn(II), Co(II), Ni(II) and Cu(II) have been synthesized as polycrystalline solids, and characterized by elemental analysis, spectroscopy, magnetic studies and also by X-ray diffraction and thermogravimetric measurements. The analysed complexes have the following colours: pink for Co(II), green for Ni(II), blue for Cu(II) and a pale pink for Mn(II) compounds. The carboxylate group binds as monodentate and bidentate ligands. On heating to 1173K in air the complexes decompose in several steps. At first, they dehydrate in one step to anhydrous salts, that next decompose to the oxides of respective metals. Their magnetic moments were determined in the range of 76-303K. The results reveal them to be high-spin complexes of weak ligand fields.

Ligand Topology Variations and the Importance of Ligand Field Strength in Non-Heme Iron Catalyzed Oxidations of Alkanes

A series of iron(II)-bis(triflate) complexes [Fe(L)(OTf)2] containing linear tetradentate bis(quinolyl)-diamine and bis(quinolylmethyl)-diamine ligands with a range of ligand backbones has been prepared. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by 1H and 19F NMR spectroscopy. Because of the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-alpha, cis-beta, and trans) are observed with certain ligand systems. In these cases, the geometry observed in the solid-state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable-temperature (VT) magnetic moment measurements and by UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligand that generates [Fe(L)(OTf)2] complexes with a cis-alpha coordination geometry, and the corresponding [Fe(L)(CH3CN)2]2+ complex displays spin crossover behavior. The catalytic properties of the complexes for the oxidation of cyclohexane have been investigated using hydrogen peroxide as the oxidant. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst.

Weak-Field Anions Displace the Histidine Ligand in a Synthetic Heme Peptide but Not inN-Acetylmicroperoxidase-8: Possible Role of Heme Geometry Differences

We have recently reported that aquo and thioether complexes of the ferric cytochrome c heme peptide N-acetylmicroperoxidase-8 (FeIII-1) exhibit greater low-spin character than do the corresponding complexes of a synthetic, water-soluble, monohistidine-ligated heme peptide (FeIII-2; Cowley, A. B.; Lukat-Rodgers, G. S.; Rodgers, K. R.; Benson, D. R. Biochemistry 2004, 43, 1656-1666). Herein we report results of studies showing that weak-field ligands bearing a full (fluoride, chloride, hydroxide) or partial (phenoxide, thiocyanate) negative charge on the coordinating atom trigger dissociation of the axial His ligand in FeIII-2 but not in FeIII-1. We attribute the greater sensitivity of His ligation in FeIII-1 to weak-field anionic ligands than to weak-field neutral ligands to the following phenomena: (1) anionic ligands pull FeIII further from the mean plane of a porphyrin than do neutral ligands, which will have the effect of straining the His-Fe bond in FeIII-2, and (2) heme in FeIII-2 is likely to undergo a modest doming distortion following anion binding that will render the His-ligated side of the porphyrin concave, thereby increasing porphyrin/ligand steric interactions. We propose that ruffling of the heme in FeIII-1 is an important factor contributing to its ability to resist His dissociation by weak-field anions. First, ruffling should allow His to more closely approach the porphyrin than is possible in FeIII-2, thereby reducing bond strain following anion binding. Second, the ruffling deformation in FeIII-1, which is enforced by the double covalent heme-peptide linkage, will almost certainly prevent significant porphyrin doming.

Crystal-field theory ofCo2+in doped ZnO

We present a crystal field theory of transition metal impurities in semiconductors in a trigonally distorted tetrahedral coordination. We develop a perturbative scheme to treat covalency effects within the weak ligand field case (Coulomb interaction dominates over one-particle splitting) and apply it to ZnO:Co$^{2+}$ (3d$^7$). Using the large value of the charge transfer energy $\Delta_{pd}$ compared to the $p$-$d$ hoppings, we perform a canonical transformation which eliminates the coupling with ligands to first order. As a result, we obtain an effective single-ion Hamiltonian, where the influence of the ligands is reduced to the one-particle 'crystal field' acting on $d$-like-functions. This derivation allows to elucidate the microscopic origin of various 'crystal field' parameters and covalency reduction factors which are usually used empirically for the interpretation of optical and ESR experiments. The connection of these parameters with the geometry of the local environment becomes transparent. The experimentally known $g$-values and the zero-field splitting 2D are very well reproduced by the exact diagonalization of the effective single-ion Hamiltonian with only one adjustable parameter $\Delta _{pd}$. Alternatively to the numerical diagonalization we use perturbation theory in the weak field scheme (Coulomb interaction $\gg$ cubic splitting $\gg$ trigonal splitting and spin-orbit coupling) to derive compact analytical expressions for the spin-Hamiltonian parameters that reproduce the result of exact diagonalization within 20% of accuracy.

Phosphorus co-doping effect on photoluminescence in Ce 3+-doped SiO 2 glasses: The formation of unique ligand field by P-co-doping

The origin of photoluminescence (PL) having UV emission from Ce 3+ ions in P 5+-co-doped SiO 2 glasses was studied through the elucidation of the coordination structure around a Ce 3+ by pulsed-EPR electron spin echo envelope modulatlion (ESEEM) spectroscopy. Simulation of the observed ESEEM pattern revealed that the number of P 5+ in the coordination sphere is ∼10 times larger than that estimated from the random distribution of P 5+, demonstrating the formation of a solvation shell structure. There are two types of the coordination manner of P–O bonds in the solvation structure, corner sharing and edge sharing. This solvation is close to that in phosphate glasses, giving a weak ligand field around Ce 3+ ions, i.e., the peak position and the lifetime of Ce 3+ ion move to a shorter side compared with the P-free SiO 2 glass.

Rhenium chemistry of pyridyldiazoles oxorhenium(V) and imidorhenium(V) species

The first diazole complexes of rhenium have been isolated and characterized in the form of the oxo and imido species of general type [ReOCl 3 (L)] and [Re(NPh)Cl 3 (L)] where L is 2,5-bis(2-pyridyl)-1,3,4-oxadiazole or 2,5-bis(2-pyridyl)-l,3,4-thiadiazole. Structure determination has revealed meridional geometry, oxo/imido triple bonding and aromatic lattice interactions. The chelates display a relatively weak ligand field band in the visible region and a quasireversible Re VI /Re V cyclic voltammetric response.

Coordination Chemistry of a New Rigid, Hexadentate Bispidine-Based Bis(amine)tetrakis(pyridine) Ligand

The hexadentate bispidine-based ligand 2,4-bis(2-pyridyl)-3,7-bis(2-methylenepyridine)-3,7-diazabicyclo[3.3.1]nonane-9-on-1,5-bis(carbonic acid methyl ester), L(6m), with four pyridine and two tertiary amine donors, based on a very rigid diazaadamantane-derived backbone, is coordinated to a range of metal ions. On the basis of experimental and computed structural data, the ligand is predicted to form very stable complexes. Force field calculations indicate that short metal-donor distances lead to a buildup of strain in the ligand; that is, the coordination of large metal ions is preferred. This is confirmed by experimentally determined stability constants, which indicate that, in general, stabilities comparable to those with macrocyclic ligands are obtained with the relative order Cu(2+) > Zn(2+) >> Ni(2+) < Co(2+), which is not the typical Irving-Williams behavior. The preference for large M-N distances also emerges from relatively high redox potentials (the higher oxidation states, that is, the smaller metal ions, are destabilized) and from relatively weak ligand fields (dd-transition, high-spin electronic ground states). The potentiometric titrations confirm the efficient encapsulation of the metal ions since only 1:1 complexes are observed, and, over a large pH range, ML is generally the only species present in solution.

X-ray Absorption Spectroscopic Investigation of the Spin-Transition Character in a Series of Single-Site Perturbed Iron(II) Complexes

Select ferrous spin-transition complexes with the pentadentate ligand 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine (PY5) were examined using variable-temperature solution solid-state magnetic susceptibility, crystallography, X-ray absorption spectroscopy (XAS), and UV/vis absorption spectroscopy. Altering the single exogeneous ligand, X, of [Fe(PY5)(X)]n)+ is sufficient to change the spin-state of the complexes. When X is the weak-field ligand Cl-, the resultant Fe complex is high-spin from 4 to 300 K, whereas the stronger-field ligand MeCN generates a low-spin complex over this temperature range. With intermediate-strength exogenous ligands (X = N3-, MeOH), the complexes undergo a spin-transition. [Fe(PY5)(N3)]+, as a crystalline solid, transitions gradually from a high-spin to a low-spin complex as the temperature is decreased, as evidenced by X-ray crystallography and solid-state magnetic susceptibility measurements. The spin-transition is also evident from changes in the pre-edge and EXAFS regions of the XAS Fe K-edge spectra on a ground crystalline sample. The spin-transition observed with [Fe(PY5)(MeOH)]2+ appears abrupt by solid-state magnetic susceptibility measurements, but gradual by XAS analysis, differences attributed to sample preparation. This research highlights the strengths of XAS in determining the electronic and geometric structure of such spin-transition complexes and underscores the importance of identical sample preparation in the investigation of these physical properties.

Magnetic, thermal and spectroscopic features of 2, 3- and 3, 5-dimethoxybenzoates of Co(II), Ni(II) and Cu(II)

The 2,3- and 3,5-dimethoxybenzoates of Co(II), Ni(II) and Cu(II) were synthesized as solids and their magnetic, spectral and thermal properties studied. The complexes are hydrated or anhydrous compounds which possess colors typical of the M(II) ions. Their thermal stabilities were examined in air and nitrogen and the gaseous and solid state decomposition products were also identified. The magnetic susceptibilities of the complexes were measured over the temperature range 4.4?300 K and the magnetic moments were calculated. The results show that the 2,3- and 3,5-dimethoxybenzoates of Co(II) and Ni(II) are high-spin complexes with weak ligand fields, and that the complexes of Cu(II) form dimers.

Effects of Peripheral Substituents and Axial Ligands on the Electronic Structure and Properties of Iron Phthalocyanine

The effects of peripheral substituents and axial ligands on the electronic structure and properties of iron phthalocyanine, H(16)PcFe, have been investigated using a DFT method. Substitution by electron-withdrawing fluorinated groups alters the ground state of H(16)PcFe and gives rise to large changes in the ionization potentials and electron affinity. For the six-coordinate adducts with acetone, H(2)O, and pyridine, the axial coordination of two weak-field ligands leads to an intermediate-spin ground state, while the strong-field ligands make the system diamagnetic. The electronic configuration of a ligated iron phthalocyanine is determined mainly by the axial ligand-field strength but can also be affected by peripheral substituents. Axial ligands also exert an effect on ionization potentials and electron affinity and can, as observed experimentally, even change the site of oxidation/reduction.

Realization of unusual ligand binding motifs in metalated container molecules: synthesis, structures, and magnetic properties of the complexes [(LMe)Ni2(mu-L')]n+ with L'=NO3-, NO2-, N3-, N2H4, pyridazine, phthalazine, pyrazolate, and benzoate.

A series of dinickel(II) complexes with the 24-membered macrocyclic hexaazadithiophenol ligand H(2)L(Me) was prepared and examined. The doubly deprotonated form (L(Me))(2-) forms complexes of the type [(L(Me))Ni2II(mu-L')](n+) with a bioctahedral N(3)Ni(II)(mu-SR)(2)(mu-L')Ni(II)N(3) core and an overall calixarene-like structure. The bridging coordination site L' is accessible for a wide range of exogenous coligands. In this study L'=NO(3)(-), NO(2)(-), N(3)(-), N(2)H(4), pyrazolate (pz), pyridazine (pydz), phthalazine (phtz), and benzoate (OBz). Crystallographic studies reveal that each substrate binds in a distinct fashion to the [(L(Me))Ni(2)](2+) portion: NO(2)(-), N(2)H(4), pz, pydz, and phtz form mu(1,2)-bridges, whereas NO(3)(-), N(3)(-), and OBz(-) are mu(1,3)-bridging. These distinctive binding motifs and the fact that some of the coligands adopt unusual conformations is discussed in terms of complementary host-guest interactions and the size and form of the binding pocket of the [(L(Me))Ni(2)](2+) fragment. UV/Vis and electrochemical studies reveal that the solid-state structures are retained in the solution state. The relative stabilities of the complexes indicate that the [(L(Me))Ni(2)](2+) fragment binds anionic coligands preferentially over neutral ones and strong-field ligands over weak-field ligands. Secondary van der Waals interactions also contribute to the stability of the complexes. Intramolecular ferromagnetic exchange interactions are present in the nitrito-, pyridazine-, and the benzoato-bridged complexes where J=+6.7, +3.5, and +5.8 cm(-1) (H=-2 JS(1)S(2), S(1)=S(2)=1) as indicated by magnetic susceptibility data taken from 300 to 2 K. In contrast, the azido bridge in [(L(Me))Ni(2)(mu(1,3)-N(3))](+) results in an antiferromagnetic exchange interaction J=-46.7 cm(-1). An explanation for this difference is qualitatively discussed in terms of bonding differences.

Novel acetate binding modes in [Na2Cu(CH3COO)4(H2O)].H2O.

A new copper(II) acetate, [Na(2)Cu(CH(3)COO)(4)(H(2)O)].H(2)O (1), has been crystallized from an aqueous solution containing sodium acetate and copper(II) acetate monohydrate in a 4:1 ratio and the structure determined by X-ray crystallography. 1 crystallizes in the monoclinic space group P2(1)/c, with a = 16.638(3) A, b = 11.781(2) A, c = 15.668(3) A, beta = 90.11(3) degrees, V = 3071.0(11) A(3), and Z = 4. In the asymmetric unit, sodium ions bridge two crystallographically unique square planar [Cu(CH(3)COO)(4)](2-) units to their symmetry-generated neighbors to form corrugated 2D sheets of Na(2)Cu(CH(3)COO)(4), which are held together by H-bonding interactions involving the waters of crystallization. In contrast, the structures of known sodium copper acetates are better described as 3D frameworks. The metal centers are bridged by a number of acetates in novel coordination modes. The square planar Cu(II) geometry generated by oxygen atoms from four different acetates is an unexpected feature given the weak ligand field provided by the acetate ligands.

Structural and spectroscopic characterisation of bis-ligand complexes of iron(ii), nickel(ii) and nickel(iii) with the hydrotris(methimazolyl)borate anion: soft S6 donor sets generating a weak ligand field

Reaction of iron(II) and nickel(II) halides with sodium hydrotris(methimazolyl)borate (NaTm) results in the formation of 2 ∶ 1 complexes [M(Tm)2]. Crystallographic characterisation reveals in both cases trigonally distorted octahedral geometry in an S6 donor set. Spectroscopic properties indicate that the Tm ligand generates a weak ligand field, with Dq between that of H2O and Cl− and this is confirmed by the high-spin configuration of the iron(II) complex. Both complexes display classical paramagnetic behaviour. Mössbauer spectroscopy of the iron complex is also consistent with a high-spin complex. Reaction of NiCl2·6H2O with [Tl(Tm)] results in a small quantity of a material which is crystallographically demonstrated to be [Ni(Tm)2]Br.

A spectrochemical walk: single-site perturbation within a series of six-coordinate ferrous complexes.

A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5) was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe(II)(PY5)(X)](n+), X = MeOH, H(2)O, MeCN, pyridine, Cl-, OBz-, N(3)-, MeO-, PhO-, and CN-. The spin-states of these ferrous complexes are extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate with their high-spin (hs) or low-spin (ls) electronic ground state. Systematic metrical trends within six crystallographic structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding affinity; the [Fe(II)(PY5)](2+) unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-field ligands are preferentially bound over weak-field ligands. In MeOH, the pK(a) of the exogenously ligated MeOH in [Fe(PY5)(MeOH)](2+) (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate [Fe(PY5)(MeOH)](2+) to yield [Fe(PY5)(OMe)](1+) rather than ligate to the ferrous center. Exogenous ligation by a strongly basic ligand, however, can be achieved in polar aprotic solvents.

Reparameterization of hybrid functionals based on energy differences of states of different multiplicity

Low-spin/high-spin energy splittings for Fe(II) transition-metal complexes – particularly in weak ligand fields – cannot be well described by density functional methods. Different density functionals yield results which differ by up to 1 eV in transition-metal complexes with sulfur-rich first coordination spheres. We attribute this failure to the fact that the high-spin state is systematically favoured in Hartree–Fock-type theories, because Fermi correlation is included in the exact exchange, while Coulomb correlation is not. We thus expect that the admixture of exact exchange to a given density functional will heavily influence the energy splitting between states of different multiplicity. We demonstrate that the energy splitting depends linearly on the coefficient of exact exchange admixture. This remarkable result is found for all the Fe(II)–S complexes studied. From this observation we conclude in connection with experimental results that Becke's 20% admixture should be reduced to about 15% if meaningful energetics are sought for transition-metal compounds. We rationalize that this reduction by 5% will not affect the quality of the hybrid functional since we arrive at a slightly modified functional, which lies between the pure density functional and the hybrid density functional, which both give good results for “standard” systems.

Metal binding studies with macrocyclic hexathioethers: Transition metal complexes of 22S6 and 20S6

AbstractThe two macrocyclic hexathioethers, 1,4,7,12,15,18‐hexathiacyclodocosane (22S6) and 1,4,7,11,14,17‐hexathiacycloeicosane (20S6) have been examined with respect to their complexation behavior towards a variety of transition metals ions. In all of these complexes, the two ligands generate relatively strong ligand field, but their field strengths fall between the smaller ring and stronger field hexathioether ligands (such as 18S6) and larger ring and weaker field ligands, such as 24S6. The stability of the complexes towards hydrolysis parallels their reduced field strengths with first row transition metal complexes of both ligands showing enhanced sensitivity to solvolysis reactions compared to the analogous 18S6 complexes. In general, the electrochemical behavior of the hexathioether complexes shows metal centered oxidations which occur at higher potentials than those complexes involving trithioethers. We suggest that the hexathioether ligand is less able to expand and contract during the reduction and oxidation of the metal center. The complex [Pd(22S6)]2+ does not display the unusual spectroscopic and electrochemical properties observed in the analogous 18S6 complex.

Real and Hypothetical Intermediate-Valence AgII /AgIII and AgII /AgI Fluoride Systems as Potential Superconductors.

With the aim of gauging their potential as conducting or superconducting materials, we examine the crystal structures and magnetic properties of the roughly one hundred binary, ternary, and quaternary AgII and AgIII fluorides in the solid state reported up to date. The AgII cation appears in these species usually in a distorted octahedral environment, either in an [AgF]+ infinite chain or as [AgF2 ] sheets. Sometimes one finds discrete square-planar [AgF4 ]2- ions. The AgIII cation occurs usually in the form of isolated square-planar [AgF4 ]- ions. Systems containing AgIII (d8 ) centers are typically diamagnetic. On the other hand, the rich spectrum of AgII (d9 ) environments in binary and ternary fluorides leads to most diverse magnetic properties, ranging from paramagnetism, through temperature-independent paramagnetism (characteristic for half-filled band and metallic behavior) and antiferromagnetism, to weak ferromagnetism. AgII and AgIII have the same d-electron count as CuII (d9 ) and CuIII (d8 ), respectively. F- and O2- ions are isoelectronic, closed-shell (s2 p6 ) species; both are weak-field ligands. Led by these similarities, and by some experimental evidence, we examine analogies between the superconducting cuprates (CuII /CuIII -O2- and CuII /CuI -O2- systems) and the formally mixed-valence AgII /AgIII -F- and AgII /AgI -F- phases. For this purpose we perform electronic-structure computations for a number of structurally characterized binary and ternary AgI , AgII , and AgIII fluorides and compare the results with similar calculations for oxocuprate superconductors. Electronic levels in the vicinity of the Fermi level (x2 -y2 or z2 ) have usually strongly mixed Ag(d)/F(p) character and are Ag-F antibonding, thus providing the potential of efficient vibronic coupling (typical for d9 systems with substantially covalent bonds). According to our computations this is the result not only of a coincidence in orbital energies; surprisingly the Ag-F bonding is substantially covalent in AgII and AgIII fluorides. The electron density of state at the Fermi level (DOSF ) for silver fluoride materials and frequencies of the metal-ligand stretching modes have values close to those for copper oxides. The above features suggest that properly hole- or electron-doped AgII fluorides might be good BCS-type superconductors. We analyze a comproportionation/disproportionation equilibrium in the hole-doped AgII fluorides, and the possible appearance of holes in the F(p) band. It seems that there is a chance of generating an AgIII -F- /AgII -F0 "ionic/covalent" curve crossing in the hole-doped AgII -F- fluorides, significantly increasing vibronic coupling.

Photophysics of pyridylporphyrin Ru(II) adducts: heavy-atom effects and intramolecular decay pathways.

Eight adducts between different pyridylporphyrins and ruthenium complexes, MPyP[RuCl(2)(DMSO)(2)(CO)], c-DPyP[RuCl(2)(DMSO)(2)(CO)](2), TrPyP[RuCl(2)(DMSO)(2)(CO)](3), TPyP[RuCl(2)(DMSO)(2)(CO)](4), (MPyP)(2)[RuCl(2)(DMSO)(2)], [c-DPyP[RuCl(2)(DMSO)(2)]](2), MPyP[RuCl(2)(CO)(3)], and [c-DPyP[RuCl(2)(CO)(2)]](2), have been investigated. The results show that in all the adducts the porphyrin singlet is quenched, to a greater or lesser extent, relative to the parent-free molecule. This study provides insight into the mechanisms of singlet quenching in the adducts. Two mechanisms for singlet quenching, both related to the "heavy-atom effect" of the ruthenium center and experimentally distinguishable by transient spectroscopy, are examined. Enhanced intersystem crossing within the porphyrin chromophore is demonstrated for the series of adducts MPyP[RuCl(2)(DMSO)(2)(CO)], c-DPyP[RuCl(2)(DMSO)(2)(CO)](2), TrPyP[RuCl(2)(DMSO)(2)(CO)](3), and TPyP[RuCl(2)(DMSO)(2)(CO)](4), where a nice correlation is observed between the magnitude of the effect and the number of ruthenium centers attached to the pyridylporphyrin chromophore. Singlet-triplet energy transfer from the pyridylporphyrin chromophore to the ruthenium center(s) is an additional efficient quenching channel for adducts containing ruthenium centers with weak field ligands and low triplet energies, such as (MPyP)(2)[RuCl(2)(DMSO)(2)] and [c-DPyP[RuCl(2)(DMSO)(2)]](2).

Transition metal complexes with macrocyclic oxathiaethers

Complexation studies with Ni(II), Co(III), Co(II), Cd(II), and Cu(II) for the two mixed oxathia crown ligands 1-oxa-4,7-dithiacyclononane (9S2O) and 1,10-dioxa-4,7,13,16-tetrathiacyclooctadecane (18S4O2) are reported. These ten complexes have been characterized by a variety of means including electronic spectroscopy, cyclic voltammetry, and nuclear magnetic resonance spectroscopy. Furthermore, the complexes [Ni(18S4O2](BF4)2, [Cu(9S2O)2](BF4)2 and [Ni(9S2O)2](ClO4)2, have been characterized by single crystal X-ray diffraction. For the complex [Ni(18S4O2](BF4)2 the diastereoisomer obtained is the one which contains the two oxygen atoms trans to each other and a meridional positioning of the diethylene S-O-S moiety. Selectivity for this particular stereoisomer is also supported by NMR data for the Cd(II) and Co(III) complexes, and this selectivity arises from the conformational preferences of the individual C-O and C-S bonds in the macrocycle. Both the nickel-sulfur and nickel-oxygen bonds in the complex are highly compressed due to the rigid macrocyclic structure and are among the shortest of these types of bonds found in any crown Ni(II) complex. The copper(II) complex, [Cu(9S2O)2](BF4)2, shows an interesting Jahn–Teller distortion from an octahedral geometry resulting in coordinate bonds which are all remarkably similar in length (Cu-S(4) 2.3293(6); Cu-S(7) 2.3336(6); and Cu-O(1) 2.355(2) Å). The oxygen atoms are found in a trans position around the copper(II) center, and the axial Cu-O bonds are elongated due to the Jahn–Teller distortion. In all of these complexes, the two oxathiaether ligands function as much weaker field ligands than do their crown thioether analogs. Also, cyclic voltammetric experiments reveal that the oxathia crowns do not have the ability to stabilize less common metal oxidation states, a common property of many crown thioether ligands.

Simple overlap model and crystal field analysis of Cs 2ZnCl 4:Co 2+. Correlation with optical and magnetic properties

The optical (energy level scheme) and magnetic properties (paramagnetic susceptibility as a function of temperature and g values) of Co 2+ in a Cs 2ZnCl 4 single crystal were reproduced simultaneously according to the crystal field theory involving a set of free ions and B k q crystal field parameters. The Co 2+ are placed in a weak tetrahedral ligand field. The crystal field parameters calculated from the structure are in good agreement with the experimental values. The correlation between the g values and B k q parameters is discussed.