First-row Transition Elements Research Articles

Two modifications of cobalt dipotassium tetrachloride

High-temperature K2COC14: orthorhombic, a=26.838 (3), b=12.406 (1), c=7.262 (1 )~ , space group Pna21, Z = 1 2 , adopts a slightly modified fl-KzSO4 structure. Low-temperature K2COC14: monoclinic, a=6.801 (1), b=9.569 (1), c=12.757 (1)/~,, ,8=107.0 (1) °, space group P21/c, Z = 4 , adopts a strongly distorted ,8-K2SO4 structure. Introduction. The structure determination of KzCoC14 is part of an investigation of structural relationships and magnetic properties of ternary halides AxByXz with A an alkali metal, B a first-row transition element or an alkaline-earth metal and X = C1, Br or I. The phase diagram of the system KCI-CoCIz was constructed by Seifert (1961). Apart from K2COC14 (m.p. 436°C) it shows another compound, KCoC13, but not the phase transition we observe for K2CoCI 4. By melting stoichiometric amounts of KC1 and CoClz in an evacuated quartz tube and annealing the mixture at 400°C for one week, the orthorhombic modification of KzCoCI4 is obtained. After a few months this phase changes at room temperature into the monoclinic modification. The former is again obtained after annealing the latter at 400°C. The two phases are easily recognized by their X-ray powder diagrams. We were unable to measure the transition temperature by means of DTA, but visual inspection of samples after heating suggests a phase transition between 350 and 360°C. Lamotte & Vermeire (1975) report that powder diffractograms of K2COC14 and (NHa)zZnCI 4 can be indexed on the basis of an orthorhombic cell with axes a=8.90, b=12.39 and c=7.25 (.~) for KzCoCI4 and a=9.20, b = 12.56 and c=7.17 (•) for (NH4)2ZnCI 4. In both cases our diagrams show powder lines between d=3.47 and 3.51 A, which cannot be indexed with these cell parameters. On tripling the a parameter these reflexions can be indexed as 202, 231 and 112. Klug & Sears (1945) also report a tripled a parameter for KzZnCI4, based on single-crystal measurements. Dry KCI was obtained by heating in vacuo at 400 °C, and dry COC12 by heating COC12.6HzO in a stream of dry HCI gas at 400°C. Lowand high-temperature KzCoCI4 were obtained by melting a stoichiometric mixture of KCI and CoCIz in an evacuated sealed quartz tube and slowly lowering the temperature over the course of a few days to room temperature with a temperature gradient of about 50°C, with the higher temperature at the top of the melt. All manipulations of KzCoCI 4 were carried out in a glovebox filled with dry N2 or under dry paraffin oil. The crystallized blue material was broken into pieces and crystals were selected. A series of Weissenberg photographs showed some crystals with orthorhombic and some with monoclinic symmetry. One suitable crystal of each phase was collected and measured by means of an Enraf-Nonius three-circle single-crystal diffractometer. Intensities were recorded by the 0-20 scan method for all reflexions with 0 between 3 and 30 °. Monochromatic (graphite) Mo K0~ radiation was used for measuring the intensities. Background inteni ° sities were determined at 0+~zl , with z1=0.7+1.3 tan 0 °. The mean counting time was 25 s for each background and 50 s for the scan. Standard deviations were calculated from counting statistics. Absorption corrections were applied with a computer program written by de Graaff (1973), and new standard deviations (av) were calculated taking into account the inaccuracy of the absorption correction and attenuation factors of the filters used. Non-equivalent significant reflexions were reduced to F values and Wilson plots were calculated, yielding approximate values for the scale factors and overall isotropic temperature parameters, B. Table 1 gives the crystal and diffraction data. According to the reflexion conditions for orthorhombic KzCoCI4, another space group (Pnam) is possible, which the Patterson synthesis proved to be incorrect. Table 1. Crystal and diffraction data for K2CoCI 4

Interpretation of structure and magnetism in transition-metal pnictides M 2X and (M 1− xM′ x) 2X

The transition-metal pnictides M 2X and (M 1− x M′ x ) 2X containing first-row transition elements and X = P, As or Sb tend to crystallize in three related structures that permit metal-metal bonding via partially filled 3 d-shell cores. It is argued that in the phosphides, and probably in most arsenides and antimonides, of the first-row transition elements, the X-atom p bands are filled and the cation 4 s bands are empty, so that the number of 3 d electrons per metal atom are known unambiguously. Furthermore, some of the phosphides are magnetic and some are not, so that the width of the 3 d-electron bands can be varied by As substitution or by hydrostatic pressure to provide critical information about changes in magnetic order, magneticordering temperatures, and the magnitudes of the atomic moments as a function of bandwidth and band occupation in the critical region where the transition from spontaneous magnetism to Pauli paramagnetism occurs. General conceptual phase diagrams are developed from physical arguments about the influence of electron-electron correlations on quasidegenerate, narrow d bands. This discussion, which leads to an explanation of the Slater-Pauling curve of magnetization vs electron/atom ratio in the transition metals, is then applied to an interpretation of available magnetic data for the transition-metal pnictides M 2X and (M 1− x M′ x ) 2X. Prediction of individual atomic moments requires that allowance be made for distinguishable cation sites and for the transfer of 3 d-electron charge from lighter to heavier elements, but the crystal-field effects appear to be manifest only in the signs of the interatomic exchange interactions. Significantly, in alloys an equal and integral number of majority-spin electrons tend to be stabilized at each atomic constituent that is magnetic.

On the validity of the core-point-charge approximation for inner subshells of atoms

In order to test the validity of approximating the effects of various atomic inner subshells by means of point charges at the nuclei, the electron repulsion and exchange integrals involving these inner electrons and valence electrons were computed for a series of atoms. The point-charge approximation was found to be very poor for the 3s, and 3p subshells of the first-row transition elements.

Fluorinated Alkoxides Part VII. Complexes of 2-Acetamidohexafluoro-2-propanol with First-row Transition and Lanthanide Elements

2-Acetamidohexafluoro-2-propanol (HAcfp), CH3CONHC(CF3)2OH, may act as a bidentate, uninegative, ligand to give complexes of the type M(Acfp)2 (M = Co2+, Ni2+, Cu2+, Zn2+); M(Acfp)3 (M = Fe3+); M(Acfp)4 (M = Ce4+); or [M(Acfp)4]− (M = lanthanide(III)). Coordination is through the two oxygen atoms, except in the case of copper, where nitrogen also enters into coordination. In the complex Cu(Acfp)2, the amido proton may be removed with NaH to give Na2[Cu(Acfp)2], which undergoes addition reactions with carbonyl groups.Formamide similarly undergoes reaction with hexafluoroacetone to give HCONHC(CF3)2OH, which chelates to copper in the same manner as HAcfp, while succinic acid diamide reacts with hexafluoroacetone to give the compound [—CH2CONHC(CF3)2OH]2, which acts as a tetradentate, dinegative, ligand.Possible structures of the above complexes are discussed.

Complexes of some first-row transition elements with (–)-spartein

Complexes of cobalt(II), nickel(II), copper(II), and zinc(II), of the general formula [M(spartein)X2], where X = Cl–, Br– or I–, have been prepared and characterised. Magnetic moments, electronic spectra, and the circular dichroism absorption (down to 5000 cm–1) of [Ni(sp)Cl2] are reported. The electronic structures of the cobalt(II) and nickel(II) complexes are discussed in terms of an effective C2v chromophoric symmetry. Complexes of manganese(III) and iron(III) have been obtained, but are less well characterised.

Photopolarographic studies of some first-row transition elements. Part I

Photopolarographic signals over a range of wavelengths within the near-u.v. and visible portion of the spectrum have been obtained for a number of transition-metal ions. The intensity variation of such signals with wavelength enable characteristic ‘spectra’ to be drawn, the intensities being a function of depolarizer concentration. It is suggested that such spectra arise as a result of electronic transitions between levels induced by both photo- and electrode-field excitation of the depolarizer.

Complexes of Di-2-pyridyl ketone. I. Complexes of first-row transition elements in normal oxidation states

All the complexes of di-2-pyridyl ketone (DPK) described appear to involve N,N-chelation of the ligand. In CU(DPK)2SO4,2H2O the water molecules are added across the ketone groups yielding gem-dihydroxy functions. Several previously reported solvated DPK-cupric complexes which were assigned N,O-chelated structures probably have analogous N,N-chelated structures with solvent molecules (H2O or alcohols) added across the ketone group. The compounds M(DPK)2Cl2 (M = MnII, FeII, CoII, NiII) are tentatively assigned cis-octahedral structures, and a number of mono- and bis-DPK-cupric complexes appear to have the common tetragonal or square-planar geometries. The compounds of stoicheiometry Co(DPK)2(OH)2Cl,�H2O and CO(DPK)2(OH)2NO3, prepared from the corresponding cobalt(11) salts, are cobalt(111) derivatives in which one or both of the DPK units is reduced and probably acts as a tridentate ligand.

Magnetic Properties of Some Binuclear Complexes of Chromium and Iron

WE have been investigating the magnetic properties of a series of polynuclear complexes of the first-row transition elements. Following the procedure of Kambe1 and Abragam2, the general interaction between the two metal ions in a binuclear complex may be formulated as , where J is the exchange interaction, s1 and s2 are the spins of the two metal ions, and is the Hamiltonian for the system. In the case of a binuclear chromium complex, this leads to the following expression for the atomic susceptibility per ion (χa): . where K = g2Nβ2/3K; x = J/KT and N(α) is the high-frequency contribution to the susceptibility. Fig. 1 shows the calculated and experimental curves, for the susceptibilities of two such two such chromium complexes, between liquid-nitrogen and room temperatures. The value of K was taken such that N (α) was about 50 × 10−6 C.G.S. units as, from, spectral measurements, the nearest quartet-level for which interaction could occur was of the order of 15,000 cm.−1. The values of J, g and N(α) obtained are contained in Table 1. These values are of a similar order to those obtained by Kambe1 in the interpretation of Welo's data on some trinuclear complexes of chromium. The increase in interaction in the mono-bridged complexes may be due to the fact that the metal–oxygen–metal angle is not restricted, as in the diol.