The reaction of ruthenium atoms with buta-1,3-diene: synthesis of [RuL(η4-C4H6)2] where L = PF3, CO and (CH3)3 CNC

Condensation of ruthenium vapour with a mixture of pentane and buta-1,3-diene vapours at ?196°C, followed by warming to ?40°C, yields an unstable brown solution which reacts above ?40°C with PF₃, CO or (CH₃)₃ CNC to give the corresponding complex [RuL(η⁴-C₄H₆)₂] in > 40% yield. The ease of isolation and purification of these air sensitive complexes decreases in the order PF₃ > (CH₃)₃CNC > CO.     

The structures of higher boron halides B8X12(X = F, Cl, Br and I) by gas-phase electron diffraction and ab initio calculations

The structure of B8F12 has been shown by gas electron diffraction and computational methods (up to MP2/6-31+G*) to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2 group, bridged by two BF2 groups to give a central B4 core that is folded, not planar, and with a very short bond [164.3 pm calculated, 164.2(19) pm experimental] along the fold line. There are also four terminal BF2 groups. One of the other four bonds in the core is consistently 20-30 pm longer than the others. This asymmetry has been attributed to many intra-molecular B...F interactions, particularly those between core boron atoms and fluorines of the terminal BF2 groups. Calculations for the chloro analogue lead to a structure similar to that for B8F12, but with the long core bond extended so that one of the bridging BCl2 groups may now be regarded as terminal. With bromine as the halogen the structure changes again, with one bromine atom taking up a bridging position. With iodine, this process continues further, and there are three bridging iodine atoms. However, in this case this is not the lowest energy structure, and instead a loosely associated dimer of B4I6 is preferred. In all these cases, and particularly with the heavier halogens, there are huge differences between the results obtained with different computational methods.     

Force free fields in type II superconductors

It seems possible that, under certain field and current conditions, the critical current of a type II superconductor is not determined simply by application of a Bean-London model, but by the joint use of the latter and a second equation.     

A study of the alkyl borazoles and of the silicon-germanium hydrides

Summary Mixtures of alkyl borazoles have been synthesised and separated by gas liquid chromatography. Individual components have been identified by means of their characteristic retention times on two columns, one containing squalane and the other a donor solvent (carbowax) as column liquids. Silicon-germanium hydrides, together with pure silanes and germanes, have been prepared by the hydrolysis of magnesium-silicon-germanium alloys. The resulting complex mixtures have been separated into individual hydrides by gas-liquid chromatography except in the case of Position isomers. The hydrides have been analysed in a number of ways, including chlorination, pyrolysis, and adsorption on molecular sieves. With both the alkyl borazoles and the silicon-germanium hydrides, it has been found possible to interpolate or extrapolate retention times with considerable reliability.

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Zusammenfassung Gemische von Alkylborazolen wurden synthetisiert und gaschromatographisch getrennt. Die einzelnen Bestandteile wurden mit Hilfe der Retentionszeiten auf zwei Säulen (mit Squalane bzw. Carbowax) identifiziert. Weiterhin wurden Silicium-Germanium-Hydride zusammen mit reinen Silicium- und Germaniumwasserstoffen durch Hydrolyse von Magnesium-Silicium-Germanium-Legierungen dargestellt. Aus dem Gemisch wurden die einzelnen Hydride gaschromatographisch abgetrennt, ausgenommen im Falle von Stellungsisomerie. Die Hydride wurden auf verschiedene Art analysiert, einschließlich Chlorierung, Pyrolyse und Adsorption an Molekularsieben. Sowohl bei Alkylborazolen als auch bei Silicium-Germanium-Hydriden war es möglich, Retentionszeiten durch Interpolieren oder Extrapolieren zuverlässig zu berechnen.

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We are indebted to The Harmsworth Trust, Merton College, (P. P.), the Salters' Company and Merton College (J. A. S.) and the D. S. I. E. (P. P. and P. L. T.) for senior scholarships and maintenance grants. We thank I. C. I. for the loan of recorders, the Germanium Center for a sample of Ge, and Gow-Mac Ltd., for the gift of a gas-density balance.     

Evidence from ESR studies for [Co(eta-C2H4)3] produced at 77 K in a rotating cryostat

Co atoms were reacted with ethene at 77 K and the paramagnetic products studied by electron spin resonance (ESR) at X- and K-bands. The ESR spectra of the major product at both frequencies showed eight cobalt multiplets (ICo=7/2) indicating a mono-cobalt complex. The spectra have orthorhombic g and cobalt hyperfine tensors and were simulated by the parameters; g1=2.284, g2=2.0027, g3=2.1527; A1<-25 MHz, A2=-109 MHz, A3=-198 MHz. Proton and 13C (1% natural abundance) hyperfine couplings were lower than the line widths (<2 MHz) indicating less than 0.5 spin transfer to the ethene ligands. We assigned the spectrum to a Jahn-Teller-distorted planar trigonal mono-cobalt tris-ethene [Co(eta-C2H4)3] complex in C2v symmetry. The SOMO is either a 3dx2-y2 (2a1) orbital in a T-geometry or a 3dxy (b1) orbital in a Y-geometry but there is only a spin density, a2, of 0.30 in these d orbitals. The spin deficiency of 0.70 is attributed to two factors; spin transfer from the Co to ethene pi/pi* orbitals and a 4p orbital contribution, b2, to the SOMO. Calculations of a2 and b2 have been made at three levels of spin transfer, theta. At theta=0.00a2 is 0.23 and b2 is 0.78, at theta=0.25a2 is 0.25 and b2 is 0.52 and at theta=0.50a2 is 0.28 and b2 is 0.23. The other possible assignment to a mono-cobalt bis-ethene complex [Co(eta-C2H4)2] cannot be discounted from the ESR data alone but is considered unlikely on other grounds. The complex is stable up to approximately 220 K indicating a barrier to decomposition of approximately 50 kJ Mol-1     

Promotion of Sandmeyer hydroxylation (homolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst: bidentate ligands

Relative to the rate observed for the hexa-aqua ion, Cu(OH(2))(6)(2+), chelation of the copper catalyst by certain bidentate ligands enhances the rate of hydroxydediazoniation reaction (Sandmeyer hydroxylation); the ligands also provide a source of hydrogen in competitive hydrodediazoniation (H-transfer) reactions. By using the cyclisation of 2-benzoylphenyl radical as a radical clock, it has been possible to evaluate absolute rate constants for both processes effected by a variety of complexes involving one or two bidentate ligands (2-aminocarboxylate, 2-hydroxycarboxylate, 1,3-dicarboxylate, 1,2-diamine). The radical exhibits electrophilic character in both processes. The pattern of behaviour observed suggests the rate determining step in hydroxylation is reaction of the aryl radical at the metal centre to form an organocopper adduct which is rapidly converted into products. The relative reactivities of different complexes are explained qualitatively in terms of variations in the ligand field and Jahn-Teller distortion splittings of the copper d orbitals. Hydrodediazoniation is an S(H)2 H-abstraction process. Generally, coordination by Cu(2+) deactivates the first added ligand relative to its reactivity as a free species in the same state of protonation. For the majority of complexes studied, the relative reactivity as H-donors of 1 : 1 and 1 : 2 complexes is statistically determined but an additional electronic effect is discerned for doubly charged ions.