Forty years of Umpolung in organometallic chemistry: from carbanionic nucleophiles to metallic electrophiles

The evolution of mechanistic insight into the nature of organometallic reactions over the last forty years is recounted from the personal perspective of a chemist trained in the empirical tradition of organometallic chemistry. Starting from the viewpoint prevailing in the 1950s of organometallics as potential carbanionic nucleophiles, this investigator has become persuaded by his researches with Group 13 organometallics of the merits of treating these reagents as organometallic electrophiles. The profound effects that a tricoordinate boron center can exert on the structure and reactivity of boracyclopolyenes is a telling illustration of such boron electrophilicity operating in an intramolecular fashion. The elucidation of the mechanisms of both the carbalumination and the hydroalumination of olefins and acetylenes has adduced cogent evidence for the rate-determining step being the electrophilic attack of tricoordinate aluminum on the carbon-carbon π-electron cloud of the substrate. Finally, in an investigation of the molecular basis for Ziegler-Natta catalysis, the Breslow-Natta soluble catalyst for the polymerization of ethylene, Cp₂TiCl₂-R_nAlCl_3−n, was examined in detail and compelling evidence has been adduced that the active catalyst site is the solvent-separated ion-pair, [Cp₂TiR]⁺ [R_nAlCl_4−n]⁻.. Here again, the polymerization reaction is initiated by an organometallic electrophile, indeed by an even more powerful cationic electrophile. The net effect of these studies has been an Umpolung in the manner with which this chemist and many of his colleagues view organometallic reaction mechanisms.     

Gallium and oxygen accumulations on gallium nitride surfaces following argon ion milling in ultra-high vacuum conditions

Metallic gallium was observed on the surfaces of GaN commercial samples following argon ion milling. SIMS measurements confirmed that the commercial GaN had approximately 0.02% bulk oxygen present. The SIMS signal was standardized using a specimen of known oxygen content, as determined by elastic recoil detection analysis using 200 MeV heavy ions of ¹⁹⁷Au. Despite this 2–5% oxygen was observed by XPS in the bulk of the GaN after the argon ion milling. This oxygen is believed to be from the original surface oxide that re-cycles on the GaN surface during the ion milling.     

解联立方程组法校正火焰原子吸收光谱干扰

本文用解联立方程组法校正了火焰原子吸收分析中镓４０３．３０ｎｍ谱线对锰４０３．３１ｎｍ谱线的重叠干扰。实验结果表明：解联立方程组法是解决原子吸收光谱干扰的可能方法，该法在应用于合成的锰和镓测定中结果令人感到满意。     

Komplexometrische Bestimmung von Gallium und Gadolinium in GaGd Granat Kristallen

Ohne Zusammenfassung

---

Complexometric determination of gallium and gadolinium in GaGd garnet crystals

     

Preconditioned conjugate gradients for solving the transient Boussinesq equations in three-dimensional geometries

In this paper we present a new version of the ‘modified finite element method’ (MFEM) presented by Gresho, Chan, Lee and Upson.¹ The main modification of the original algorithm is the introduction of a cost-effective and memory-saving iterative solver for the discretized Poisson equation for the pressure. The vectorization of the preconditioner has been especially considered. For low Prandtl number problems we also split the advection-diffusion operator of the energy equation into explicit and implicit parts. In that sense the present approach is related to the recent implicitization of the diffusive terms introduced by Gresho and Chan² and by Gresho.³ The algorithm is applied to the study of buoyancy-driven flow oscillations occuring in a horizontal crucible of molten metal under the action of a horizontal temperature gradient.     

Determination of oxygen in gallium using spark-source mass spectrometry

Summary Spark-source mass spectrometry was used for the determination of oxygen in gallium. Specially formed electrodes made of oxygen standard in gold matrix provided the possibility of baking the source together with the sample and obtaining a reliable comparison of line blackenings for the evaluation of results. The method was used for the detection of oxygen in gallium in the range of 8 to 200 ppmw.

---

Bestimmung von Sauerstoff in Gallium mit der Funken-Massenspektrometrie

Zusammenfassung Die Funken-Massenspektrometrie ist für die Bestimmung des Sauerstoffgehalts in Gallium eingesetzt worden. Mit speziell geformten Elektroden aus Gold mit bekannten Anteilen an Sauerstoff war es möglich, die Ionenquelle zusammen mit der Probe auszuheizen und über einen zuverlässigen Vergleich der Linienschwärzungen den Sauerstoffanteil der Probe zu erhalten. Diese Methode wurde eingesetzt, um Sauerstoffgehalte in Gallium zwischen 8 und 200 ppmw zu bestimmen.