Group (periodic table)

In chemistry, a group (also known as a family) is a column of elements in the periodic table of the chemical elements. There are 18 numbered groups in the periodic table; the 14 f-block columns, between groups 2 and 3, are not numbered. The elements in a group have similar physical or chemical characteristics of the outermost electron shells of their atoms (i.e., the same core charge), because most chemical properties are dominated by the orbital location of the outermost electron. There are three systems of group numbering for the groups; the same number may be assigned to different groups depending on the system being used. The modern numbering system of "group 1" to "group 18" has been recommended by the International Union of Pure and Applied Chemistry (IUPAC) since 1988. It replaces two older incompatible naming schemes, used by the Chemical Abstract Service (CAS, more popular in the United States), and by IUPAC before 1988 (more popular in Europe). The system of eighteen grou

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Crystal field, ligand field, and interorbital effects in two-dimensional transition metal dichalcogenides across the periodic table

Diego José Pasquier, Oleg Yazyev

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exist in two polymorphs, referred to as 1T and 1H, depending on the coordination sphere of the transition metal atom. The broken octahedral and trigonal prismatic symmetries lead to different crystal and ligand field splittings of the d electron states, resulting in distinct electronic properties. In this work, we quantify the crystal and ligand field parameters of 2D TMDs using a Wannier-function approach. We adopt the methodology proposed by Scaramucci et al (2015 J. Phys.: Condens. Matter 27 175503) that allows to separate various contributions to the ligand field by choosing different manifolds in the construction of the Wannier functions. We discuss the relevance of the crystal and ligand fields in determining the relative stability of the two polymorphs as a function of the filling of the d-shell. Based on the calculated parameters, we conclude that the ligand field, while leading to a small stabilizing factor for the 1H polymorph in the d(1) and d(2) TMDs, plays mostly an indirect role and that hybridization between different d orbitals is the dominant feature. We investigate trends across the periodic table and interpret the variations of the calculated crystal and ligand fields in terms of the change of charge-transfer energy, which allows developing simple chemical intuition.

IOP PUBLISHING LTD2019

Synthesis, Characterization, and Reactivity of the First Osmium beta-Diketiminato Complexes and Application in Catalysis

Paul Joseph Dyson, Andrew Douglas Phillips, Dominique Schreiber, Rosario Scopelliti

The strongly chelating anionic beta-diketiminate ligand has been employed to formulate complexes involving almost every metal of the periodic table; however, the heavier metals of the d block remain relatively unexplored. This paper describes the synthesis and characterization of the first two osmium, beta-diketiminato compounds, including a coordinatively unsaturated cationic complex. In parallel to the analogous Ru(II) complexes, the cationic (eta(6)-arene)osmium(II) complex demonstrates bifunctional behavior through [4 + 2] cycloaddition with ethylene, cleavage of dihydrogen under mild conditions, and protonation/chloride addition with [Et2OH]Cl. Metal-centered activity in both the Ru(II) and Os(II) beta-diketiminates has until now remained elusive, as the cationic Os complex is shown to readily coordinate an aryl isonitrile. The applicability of Os(II) beta-diketiminato complexes in catalytic olefin hydrogenation demonstrates significantly greater activity in terms of conversion and TOF for a range of substrates, including styrene, cyclohex-1-ene, and 1-methylcyclohex-1-ene. Moreover, selective hydrogenation of the exocyclic alkenyl group in limonene was observed, whereas the corresponding isostructural Ru(II) complexes are inactive. In contrast, the cationic (eta(6)-arene)ruthenium(II) beta-diketiminato complex proved more active for the catalytic dehydrogenation of N,N-dimethylamine borane (Me2NBH3) than the equivalent Os(II) species. A detailed DFT study of the Ru(II) and Os(II) beta-diketiminato species using charge decomposition analysis (CDA) demonstrates differences in metal ligand interactions, which in turn considerably influences the extent of bifunctional reactivity.

Amer Chemical Soc2013

Non-Precious Electrodes for Practical Alkaline Water Electrolysis

Alberto Battistel, Alejandro Nicolás Colli, Hubert Girault

Water electrolysis is a promising approach to hydrogen production from renewable energy sources. Alkaline water electrolyzers allow using non-noble and low-cost materials. An analysis of common assumptions and experimental conditions (low concentrations, low temperature, low current densities, and short-term experiments) found in the literature is reported. The steps to estimate the reaction overpotentials for hydrogen and oxygen reactions are reported and discussed. The results of some of the most investigated electrocatalysts, namely from the iron group elements (iron, nickel, and cobalt) and chromium are reported. Past findings and recent progress in the development of efficient anode and cathode materials appropriate for large-scale water electrolysis are presented. The experimental work is done involving the direct-current electrolysis of highly concentrated potassium hydroxide solutions at temperatures between 30 and 100 °C, which are closer to industrial applications than what is usually found in literature. Stable cell components and a good performance was achieved using Raney nickel as a cathode and stainless steel 316L as an anode by means of a monopolar cell at 75 °C, which ran for one month at 300 mA cm−2. Finally, the proposed catalysts showed a total kinetic overpotential of about 550 mV at 75 °C and 1 A cm−2.

2019

Periodic table

The periodic table, also known as the periodic table of the elements, arranges the chemical elements into rows ("periods") and columns ("groups"). It is an organizing icon of chemistry and is widel

Chemical element

A chemical element is a chemical substance that cannot be broken down into other substances. The basic particle that constitutes a chemical element is the atom, and each chemical element is disting

Nitrogen

Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common el

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