Isostructural, definition

Tbe best conditions for observing S3 are 440 C and lOmmHg when 0-20% of vapour species comprise this deep cbeny-red bent Iriatomic species like ozone, p. 607, it bas a singlet ground slate. Tbe best conditions for Sj are 450 C and 20 mmHg (concentration 20 v ) but tbe structure is still not definitely established and may, in fact be a strained ring, an unbranched diradical chain, or a branched-chain isostructural with SOi(g) (p. 703). 

If hypervalent literally means exceeding the lowest chemical valence, then even NH4+ is hypervalent, despite the fact that the Lewis-Langmuir theory easily accounts for its stability. Furthermore, if the definition of hypervalency is restricted to Groups 15-18, then this artificially excludes species such as SiFs and SiFe, which are isoelectronic and isostructural with PF5 and SFe, respectively. The ambiguities in the original definition have led to several changes. Today, no one would regard NH4+ as hypervalent, while those that consider PF5 and SFe hypervalent would classify SiFs" and SiFe in the same way. 

Raman spectra on CsF 3 XeOF4 have shown the presence of the [Xe303F,3]-ion 160). This consists of three XeOF4 molecules bonded equivalently to a central F" ion. The expected point symmetry is C3v. The proposed formulation is supported by 160-180 isotope substitution experiments. X-ray powder photography showed 160) that the compound is isostructural with CsF 3 IF5 161), but definitive confirmation of the structure has had to wait for a single crystal study, the data from which are outlined below (Fig. 10). 

Solid Europium Compounds. Solid europium oxide, chalcogenides, halides, carbonate, phosphate, etc., may be obtained by reduction of the corresponding Eu3 + compounds or, metathetically, from EuC12. The metal reacts with liquid ammonia at 50° to give the orange amide, Eu(NH2)2, which gives EuN when heated.54 The compounds are usually isostructural with the Sr2 + or Ba2+ analogs. However, definite lower fluorides and chlorides appear to exist except for La, Ce, Pr, Gd, Tb and Er.57... 

Indices for measuring the extent of isostructurality of two or more organic crystal structures can be applied not only to homomolecular crystals but also to molecular associates. such as inclusion compounds. A qualitative summary of these descriptors follows, and the reader is referred to a recent account for explicit mathematical definitions. Earlier descriptors of isostructurality included the " d-gree of isostructurahty" Ii(n) (based on the distance differences ARj between the crystal coordinates of identical nonhydrogen atoms within the same section of the asymmetric units of two or more related structures) the packing coefficient increment. A(pc) and the unit cell similarity index... 

Isostructurality of Inclusion Compounds, p. 767 Neutron Diffraction, p. 959 71-71 Interactions Theory and Scope, p. 1076 Polymorphism, p. 1129 Supramolecular Isomerism, p. 1420 Tectons Definition and Scope, p. 1484... 

The first definitive compound of this class, tetrakis(tetrahydrofurane) lithium tetrakis(2,6-dimethylphenyl) lutetiate has been prepared by the action of 2,6-dimeth-ylphenyl lithium with anhydrous lutetium trichloride in tetrahydrofurane at — 78°C, following by repeated concentration, filtration, and redissolution in tetrahydrofurane. This procedure ultimately gave, on addition of benzene, colorless needles. X-ray diffraction shows the lutetium atom to be bound to four V-aryl groups in an approximately tetrahedral array the four Lu-C distances are 2.425 A, 2.439 A, 2.442 A and 2.501 A. The four Lu-Cl-C4 angles deviate somewhat from 180° (see fig. 30), taking values from 175.6° to 167.7° the six C-Lu-C angles around lutetium are 99.4°, 101.2°, 111.0°, 112.4°, 115.5° and 118.2°. The analogous ytterbium compound has been similarly prepared and is isostructural. It smoulders instantly on exposure to air while the lutetium compound appears less readily affected (S.A. Cotton et al., 1972). 

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