Metal-rich compounds

In tire transition-metal monocarbides, such as TiCi j , the metal-rich compound has a large fraction of vacairt octahedral interstitial sites and the diffusion jump for carbon atoms is tlrerefore similar to tlrat for the dilute solution of carbon in the metal. The diffusion coefficient of carbon in the monocarbide shows a relatively constairt activation energy but a decreasing value of the pre-exponential... 

Metal-rich compounds are readily obtained when a metallothermic reduction is included into the metathesis reaction by using an electropositive metal (10). In addition, metal-rich and nitrogen-rich compounds are obtained when a metal and a metal nitride are employed in reactions (11, 12) ... 

When it comes to metal-rich compounds of the alkaline earth and alkali metals with their pronounced valence electron deficiencies it is no surprise that both principles play a dominant role. In addition, there is no capability for bonding of a ligand shell around the cluster cores. The discrete and condensed clusters of group 1 and 2 metals therefore are bare, a fact which leads to extended inter-cluster bonding and results in electronic delocalization and metallic properties for all known compounds. 

Homonuclear clusters of the heavier elements of the third main-group aluminum, gallium, indium and thallium having direct element-element interactions form a fascinating new class of compounds. As discussed in the previous Chapter 2.3, in some cases their structures resemble those known with the lightest element of that group, boron, while in other cases novel, metal-rich compounds were obtained which do not have any analogue in boron chemistry. 

Some general comments on the solid-state chemistry ( From a molecular view on solids to molecules in solids ) have been reported by Simon (1995) emphasis was especially placed on the structural chemistry of metal-rich compounds formed by the metals in groups 1 to 6 and it was underlined that it is largely based on discrete and condensed clusters. In the chemistry of metals in low oxidation states, the residual valence electrons can be used for metal—metal bonding. Metal-rich compounds lie between normal valence compounds and the elemental metals themselves, with respect to their compositions, and often also with respect to their structures fragments of usual metal structures (close-packed, b.c.c., etc.) are often component units in the structures of metal-rich compounds. 

Simon, A. (1992) Metal-rich compounds. In Solid State Chemistry (Compounds), eds. Cheetham, A.K. and Day, P. (Clarendon Press, Oxford, England). 

The presence of early transition metals or rare earth elements is not necessary for the stabilization of metal-rich compounds. This is shown by the metal-rich nickel-tin sulfides Ni6SnS2 and Ni9Sn2S2, which were found during investigations of phase relations in the ternary Ni-Sn-S... 

Electronic Conductivity. Pluorides usually are good insulators owing to the wide band gap of more than 6eV in most compounds. In some cases it is possible, however, to obtain n-type semiconducting properties by doping, for example, in Cdp2(Y). Exceptional metallic conductivity is observed in the metal-rich compound Ag2p, even superconductivity in Hg3 jAsp6 - despite the presence of fluoride. Mixed-valence silver fluorides have been recently discussed as possible candidates for superconductivity. ... 

Little is yet known of the structures of metal-rich compounds of these types, but the structures of a number of anhydrous borates with 0 B ratios between 1-75 and 1-55 are described later. 

GaS also has Ga-Ga bonds (see Topics G4 and G5). Metal-rich compounds are formed by early... 

Low oxidation states (e g. +2) are of limited stability for the early elements. The unusual metal-rich compound Sc2Cl3 has a structure with extensive Sc-Sc... 

The largest clusters synthesized to date have been obtained by the reaction of silver thiolates with S (SiMe3)2 in the presence of bidentate phosphanes. Depending on the reactants, solvents and molar ratios used, clusters contaming several hundreds of silver atoms can be synthesized (Scheme 3.12). The formation of these metal-rich compounds can only be observed in the presence ofbidentate phosphanes. Although... 

The chemistry of the compounds of the early transition metals in low oxidation states is full of examples of the occurrence of metal - metal bonds. These compounds show unusual compositions in terms of the traditional valence rules metal - rich compounds contain more metal atoms than one expects for a normal valence compound (example Nbelii instead of Nbis). The transition metal elements on the left In the periodic system, and of those mainly the 4d and 5d elements, are capable of using the excess valence electrons, not needed to complete the octets of the anions, to fbnn metal -metal bonds. 

In this section the structures of binary crystal compounds are considered, including those which contain metal or non-metal atoms of different types. The limited volume of the book does not allow to pay attention to stmctures of inter-metallic compounds (see reviews [81, 82]) and of metal-rich compounds, which are in detail discussed by Rao [83] andFranzen [84]. 

Franzen HF (1978) Structure and bonding in metal-rich compounds Pnictides, chalcides and halides. Progr Solid State Chem 12 1-39... 

The structures and properties of metal rich compounds are well suited to illustrate the sometimes different yet complimentary views in molecular and solid state chemistry. The examples already mentioned illuminate important facets of the discussion to come. Metal rich compounds contain a larger number of metal atoms than necessary to saturate the maximum valencies of the nonmetal atoms. The excess of metal centered valence electrons can be involved in bonding, non-... 

Further reduction of the nonmetal to metal ratio creates a qualitatively new situation as now not only nonmetal atoms but also metal atoms have to be shared between clusters. Such condensation can occur via corners, edges, or faces of the M( octahedron. In fact, numerous structures of metal rich compounds composed of d metals and p elements can be interpreted in terms of condensed clusters which form characteristic partial structures. A more comprehensive treatise of the concept has been given earlier. [102] At that time, emphasis was laid on a scheme designed to order a large body of related structures. The concept was looked upon as a rather didactic tool. [103] In the years since, the concept... 

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