Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

Praparative, thermochemische und strukturelle Studien lassen bei den niederen Halogeniden des Niobs und Tantals Eigentumlichkeiten im Hinblick auf Stochiometrie, Homogenitatsgebiete und Bindungsverhaltnisse erkennen. Das Beobachtungsmaterial erlaubt jetzt den Entwurf einer Gesamtschau. Hierbei werden auch niedere Oxyde und Oxydhalogenide sowie Verbindungen von Nachbaralementen berucksichtigt.

Metall‐Metall‐Bindungen bei niederen Halogeniden, Oxyden und Oxydhalogeniden schwerer Übergangsmetalle Thermochemische und strukturelle Prinzipien

Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions M X 2 and M X 3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism.

https://www.mdpi.com/2073-4352/7/5/121/pdf/1

Crystal and Magnetic Structures in Layered, Transition Metal Dihalides and Trihalides

Chemistry of hexanuclear rhenium chalcohalide clusters.

A general method for accessing the solution chemistry of cluster constituents of solid phases exhibiting extended cluster frameworks is demonstrated. The approach is described in terms of simple metal−anion (M−X) frameworks and involves the formal incorporation of AX into a parent structure, resulting in termination of the X bridges between M centers while balancing the charge of the resulting framework with external cations A. The new structures obtained display frameworks of reduced connectedness and dimensionality. By replacing single metal centers with multinuclear cluster cores, this dimensional reduction approach is extended to cluster-containing frameworks. Its utility is demonstrated via application to the phases Re6Q8Cl2 (Q = S, Se), exhibiting three- and two-dimensional arrays of face-capped octahedral [Re6(μ3-Q)8]2+ cluster cores covalently linked through extremely tight Re2Q2 rhombic interactions of the type common to many intractable cluster frameworks (including the Chevrel phases). Stoichio...

A Solid-State Route to Molecular Clusters: Access to the Solution Chemistry of [Re6Q8]2+ (Q = S, Se) Core-Containing Clusters via Dimensional Reduction

Die Molybdanchloride wurden einer erneuten chemischen und physikalischen Untersuchung unterworfen.



Die Synthese im Temperaturgefalle lieferte die Verbindungen MoCl4, α-MoCl3 und MoCl2 (≙Mo6Cl12) in reiner, kristallisierter Form. Auf gleichem Wege wurde die neue Verbindung MoCl3,08 („β-MoCl3”) gefunden.



Das im festen Zustande dimere MoCl5 verdampft monomolekular (Massenspektrometer).



Der thermische Zerfall (Thermogravimetrie, Massenspektrometer) von MoCl3 erfolgt nach wahrend MoCl2 nach .



Kristallstrukturuntersuchungen lieferten folgende Informationen:



MoCl4 kristallisiert trigonal in einem Schichtengitter mit hexagonal dichter Cl-Packung. Die Mo-Atome besetzen 75% der Metallplatze einer Trichloridstruktur, wobei im Mikrobereich Ordnungszustande auftreten.



α-MoCl3 und β-MoCl3 kristallisieren monoklin in Schichtengittern mit kubisch (α) bzw. hexagonal (β) dichter Cl-Packung. Die Mo-Atome sind paarweise als Mo2-Gruppen aneinander gebunden (MoMo = 2,76 A).



Mo6Cl12(MoCl2) kristallisiert orthorhombisch. Die Struktur enthalt [Mo6Cl8]-Gruppen, die 2-dimensional unendlich miteinander verknupft sind: {[Mo6Cl8]Cl2}Cl4/2. Die Bindungsabstande MoMo innerhalb der regularen Mo6-Oktaeder betragen 2,61 A.



Der Vergleich der Vergleich der Raumbeanspruchung („pro Cl”) zeigt, das diese beim Ubergang von den hoheren Molybdanchloriden zum Mo6Cl12 wegen dessen sperrigen Aufbaus sprunghaft groser wird.



Magnetische Messungen liefern fur MoCl5 und MoCl4 nahezu den reinen Spinwert, wahrend die fur α-MoCl3, β-MoCl3 und Mo6Cl12 gemessenen Werte wegen der MoMo-Wechselwirkungen sehr viel kleiner sing.



Mo6Br12, Mo6J12, W6Cl12, W6Br12 und W6J12 sind mit Mo6Cl12 isotyp.



Chemical and physical properties of the molybdenum chlorides have been reinvestigated.



By synthesis in a temperature gradient crystalline samples of MoCl4, α-MoCl3 and MoCl2 (≙Mo6Cl12) were prepared. The new compound MoCl3,08 (“β-MoCl3”) was found in the same way.



MoCl5, being dimeric in the solid state, is monomeric in the vapour (mass spectrum).



Thermal dissociation (TGA, mass spectrum) of MoCl3 proceeds according to



2 MoCl3 MoCl2 + MoCl4,g; P(MoCl4, 800°C) = 12 atm, whereas MoCl2 decomposes according to



2 MoCl2 Mo + MoCl4,g; P(MoCl4, 860°C) = 0,4 atm.



Crystal structure analyses submitted the following informations:



MoCl4 (trigonal) forms a layer structure with a hexagonal closepacked Cl sequence. Three quarters of the metal positions of a corresponding trichloride structure are randomly occupied by Mo atoms.



α-MoCl3 and β-MoCl3 (both monoclinic) have layer structures with cubic (α) and hexagonal (β) close Cl arrangements and with certain adjacent octahedral holes occupied by molybdenum forming Mo2 pairs (MoMo = 2,76 A).



Mo6Cl12 (MoCl2) (orthorhombic) is built up by [Mo6Cl8] clusters, linked to a 2-dimensional arrangement: {[Mo6Cl8]Cl2}Cl4/2. The MoMo distance in the regular octahedral Me6 group is 2,61 A.



Comparing the volumes per one Cl atom, it can be seen, that these are abruptly increased on going from the molybdenum chlorides of higher oxidation state to Mo6Cl12 with its cumbersome structure.



The magnetic moments of MoCl5 and MoCl4 nearly correspond to the spin-only values, whereas the moments of α-MoCl3, β-MoCl3, and Mo6Cl12 are much smaller, caused by MoMo interaction.



Mo6Br12, Mo6I12, W6Cl12, W6Br12, and W6J12 are isotypic with Mo6Cl12.

Neue Untersuchungen über die Chloride des Molybdäns

Beim Erhitzen von Nb3Cl8 in Niobgefasen entstehen Kristalle von Nb6Cl14. Diese Verbindung ist das neben elementarem Niob thermodynamisch stabile Niobchlorid.



Nb6Cl14 wird durch Angabe chemischer, thermischer und magnetischer Eigenschaften charakterisiert. Thermodynamische Orientierungswerte werden angegeben.



Die Verbindung kristallisiert orthorhombisch in der Raumgruppe DBbam (a = 12,252; b = 11,019; c = 13,494 A). Die Kristallstruktur von Nb6Cl14 wurde mit Hilfe von Patterson- und Fourier-Synthesen aufgeklart. Besondere Beachtung verdient das Vorliegen von Nb6-Oktaedern im Kristallverband (MeMe-Bindungen).



Der fruher als „NbCl2” angesehene Stoff besteht in Wirklichkeit aus Nb6Cl14, das wegen storender Reaktionen mit der Wand der Quarzampullen verunreinigt ist. Bereits kleine Sauerstoffgehalte im metallischen Niob erschweren dessen rontgenographischen Nachweis neben Nb6Cl14 erheblich, weil die Nb-Reflexe dadurch — selbst auf hochauflosenden Guinier-Aufnahmen — mit Nb6Cl14-Reflexen koinzidieren.



Crystals of Nb6Cl14 are prepared by heating Nb3Cl8 in a niobium capsule. Nb6Cl14 is thermodynamically stable in the presence of metallic niobium.



Thermal, chemical and magnetic properties of Nb6Cl14 are described and approximate thermodynamic values are given.



Nb6Cl14 has an orthorhombic unit cell (a = 12.252; b = 11.019; c = 13.494 A), space group DBbam. The crystal structure has been determined by means of Patterson and Fourier syntheses. Metal—metal bonds are responsible for the occurrence of octahedral Nb6clusters characterizing the structure of Nb6Cl14. The phase formerly thought to be „NbCl2” is, in fact, Nb6Cl14. Due to reaction with the quartz wall, the excess of metallic niobium in „NbCl2” contains oxygen. In powder photographs therefore — even when applying the Guinier technique — lines of niobium often coincide with lines of Nb6Cl14, which fact may cause errors.

Horst Wöhrle

Papers

Neue Untersuchungen über die Chloride des Molybdäns

Beiträge zur Chemie der Elemente Niob und Tantal. 44. Nb6Cl14 Synthese, Eigenschaften, Struktur

Die Kristallstruktur der Verbindung Nb 3 Cl 8

Die Kristallstruktur des MoCl3