Interstitial phosphorus atoms

Cobalt clusters containing semi-interstitial phosphorus atoms have been prepared using W(CO)4(PH3)2 and Co2(CO)g at room temperature. Reaction with equimolar concentrations affords octanuclear Cog(p6-P)2(p-CO)(CO)ig (3), consisting of four cobalt and two phosphorus atoms in a distorted octahedral arrangement with four faces capped by Co(CO)3 units. The same reaction using a four-fold excess of dicobalt octacarbonyl affords small amounts of [Coio(p7-P)2(p-CO)6(CO)ig] (4) along with the hexa-nuclear product (5). ... 

The octaruthenium phosphide cluster anion [Ru8(/t8-P)(/i-CO)2(CO)2o] 310 is produced in around 30% yield from the thermolysis of Ru3(jU-H)(/i-NCsH4)(CO)io and chlorodiphenylphosphine in chlorobenzene. The square-antipris-matic cluster contains an interstitial phosphorus atom (5p 600-800), and 114 c.v.e., four electrons less than expected. Cyclic voltammetric studies show an uptake of four electrons in three steps the third two-electron reduction step is irreversible, suggesting that a structural change may occur. 

Transition metal phosphido clusters usually result from pyrolysis reactions in the presence of an external source of the phosphorus atom. PCI3, white phosphorus, PH3, and, more commonly, PPh3 have all been used in this respect, and it has been shown that the thermal decomposition of triphenylphosphine by successive loss of phenyl radicals can result in the formation of naked phosphorus atoms in interstitial cavities. This phosphorus metalation is thought to prevail in cluster chemistry because of the presence of several transition metal atoms neighboring the arylpho-sphine coordination site which aid the P-C bond cleavage process. 

The reaction of [Rh(acac)(CO)2] and PPh3 in the presence of cesium benzoate at high temperature and pressure results in the formation of [Rh9P(CO)2i] and [RhioP(CO)22] J the phosphorus atom being derived from triphenylphosphine. This reaction is difficult to follow and seems to proceed in a non-specific, non-stoichiometric manner, probably because the harsh conditions employed in the reaction do not allow for the isolation of intermediate species. The trigonal prismatic cluster [Os6P(CO)i8] has, however, been prepared in a far more systematic fashion and some key intermediates have been isolated these have helped elucidate the mechanism of interstitial phosphido atom formation. ... 

Another source of departure from stoichiometry occurs when cations are reduced, as for example in tire reduction of zinc oxide to yield an oxygen-defective oxide. The zinc atoms which are formed in tlris process dissolve in the lattice, Zn+ ions entering interstitial sites and the coiTesponding number of electrons being released from these dissolved atoms in much the same manner as was found when phosphorus was dissolved in the Group IV semiconductors. The Kroger-Viirk representation of dris reduction is... 

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

It has been seen in the previous section that the ratio of the onsite electron-electron Coulomb repulsion and the one-electron bandwidth is a critical parameter. The Mott-Hubbard insulating state is observed when U > W, that is, with narrow-band systems like transition metal compounds. Disorder is another condition that localizes charge carriers. In crystalline solids, there are several possible types of disorder. One kind arises from the random placement of impurity atoms in lattice sites or interstitial sites. The term Anderson localization is applied to systems in which the charge carriers are localized by this type of disorder. Anderson localization is important in a wide range of materials, from phosphorus-doped silicon to the perovskite oxide strontium-doped lanthanum vanadate, Lai cSr t V03. 

Impurities in zirconium and zirconium alloys and compounds are often determined by emission spectroscopy. Both carrier distillation techniques and poiat-to-plane methods are available (91,92). Several metaUic impurities can be determined instantaneously by this method. Atomic absorption analysis has been used for iron, chromium, tin, copper, nickel, and magnesium (93). The interstitial gases, hydrogen, nitrogen, and oxygen are most often determiaed by chromatography (81). Procedures for carbon, chloride, fluoride, phosphorus, siUcon, sulfur, titanium, and uranium in zirconium are given in the hteiatuie (81,94—96). 

As would be expected, there are exceptions to these rules, for example phosphorus has been located in a trigonal prismatic cavity in the cluster [Os6P(CO)ig] f l and arsenic in a square antiprismatic cavity in [RhioAs(CO)22] , even though theoretically it seems unlikely. The aim of this section is to hi ight the different interstitial sites in which main-group atoms have been located, and to illustrate the effects they have on the polyhedral cluster core. 

The clusters [RugP(CO)i9( - 7 / -CHjCsHs)], " [Rh9P(CO)2i] - (Fig. 8b), and [RhioP(CO)22] , provide examples of P-atoms encapsulated in un-, mono-and Z /-capped square antiprismatic geometries, respectively. The replacement of phosphorus by arsenic in the cluster [RhioAs(CO)22] (Fig. 8c),I shows the capacity of the basic square antiprismatic cluster polyhedron to accommodate interstitial atoms of this size. Elongation of the interplanar Rh-Rh contacts results, however, suggesting that the increase in the steric demands of the central atom is accommodated, at least in part, by expansion of the metal polyhedron. 

Interstitial alloys are formed between metals and non-metaUic or semi-metallic elements such as boron, phosphorus, carbon and nitrogen the latter occupy holes in the metal structure, which may however have to expand or re-arrange to accommodate them. Carbides and nitrides of metals of the first Transition Series form spontaneously during catalytic reactions where the reactants contain these atoms, and are themselves catalytically active. They do not exist as stable compounds of the noble metals of Groups 8 to 10. 

Unlike the interstitial nitrides, the covalent nitrides are not metallic compounds. The differences in electronegativity and atomic size between the nitrogen and the other element are small and their electronic bonding is essentially covalent. In this respect, they are similar to the covalent carbides. They include the nitrides of Group mb (B, Al, Ga, In, Tl) and those of silicon and phosphorus. Of these, only three are considered refractory boron nitride, silicon nitride, and aluminum nitride. These are reviewed in Chs. 12 and 13. 

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