Isostructurality

This was also an early example of the realization that for nearly all carbocations there exists a neutral isoelectronic isostructural boron an-alogne, which later proved itself so useful in the hands of my colleagnes R. E. Williams, G. K. S. Prakash, and L. Field. 

Whereas the proton (H ) can be considered the ultimate Bronsted acid (having no electron), the helium dication (He ) is an even stronger, doubly electron-deficient eleetron aceeptor. In a theoretical, calculational study we found that the helionitronium trication (NOaHe" ) has a minimum structure isoelectronic and isostructural... 

A range of shiny metallic compounds featuring trigonal planar anions SnX3 (X = As, Sb, Bi) have been characterized with composition M6(SnX3)Oo.5 (M = Rb, Cs) the Sn and X atoms in SnX3 (isostructural with C03 ) are coordinated by trigonal prisms of 6M+, and the ions occupy octahedral sites in the M+ lattice. 

It will be noted that Na is isosteric with the tetradeprotonated urea molecule, (H2N)2C=-0, and is also isoelectronic and isostructural with and N03. An X-ray analy-... 

Ion exchange on an acid column yields the crystalline dihydrate" H4P2O6.2H2O which is actually the hydroxonium salt of the dihydrogen hypophosphate anion [H30lJl(II0)P(0)2--P(0)2(0H)] " it is isostructural with the corresponding ammonium salt for which X-ray diffraction studies establish the staggered structure shown. 

In the vapour phase As is known to exist as tetrahedral Asa molecules with (As-As 243.5 pm) and when the element is sublimed, a yellow, cubic modification is obtained which probably also contains Asa units though the structure has not yet been determined because the crystals decompose in the X-ray beam. The mineral arsenolamprite is another polymorph, e-As it is possibly isostructural with metallic orthorhombic P. 

The compound AS2O3CI4 is polymeric and is thus not isostructural with Cl2P(0)0P(0)Cl2-... 

Table 13.11 Electronic configurations of the isostructural series of complexes containing bridging and...

In addition to the heteronuclear clusters considered in the preceding paragraphs. As, Sb and Bi also form homonuclear clusters. We have already seen that alkaline earth phosphides M3P14 contain the [Pv] cluster isoelectronic and isostructural with P4S3, and the analogous clusters [Asy] " and [Sbv] have also been synthesized. Thus, when As was heated with metallic Ba at 800° C, black lustrous prisms of BasAs were obtained, isotypic with Ba3Pi4 these contained the [Asv] anion with dimensions as shown in Fig. 13.25(a).Again,... 

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). 

The molecular and bulk properties of the halogens, as distinct from their atomic and nuclear properties, were summarized in Table 17.4 and have to some extent already been briefly discussed. The high volatility and relatively low enthalpy of vaporization reflect the diatomic molecular structure of these elements. In the solid state the molecules align to give a layer lattice p2 has two modifications (a low-temperature, a-form and a higher-temperature, yS-form) neither of which resembles the orthorhombic layer lattice of the isostructural CI2, Br2 and I2. The layer lattice is illustrated below for I2 the I-I distance of 271.5 pm is appreciably longer than in gaseous I2 (266.6 pm) and the closest interatomic approach between the molecules is 350 pm within the layer and 427 pm between layers (cf the van der Waals radius of 215 pm). These values are... 

The triatomic cations X3+ are nonlinear and thus isostructural with other 20-electron species such as XY2+ (p. 839) and SCI2 (p. 689). The contrast in bond lengths and angles between I3+ (Fig. 17.15) and the linear 22-electron anion I3 (p. 836) is notable, as is its similarity with the isolectronic Tc3 anion (p. 764). Likewise, Br3Asp6 is isomorphous with I3ASF6 and the non-linear cation has Br Br 227.0 pm and an angle of 102.5° > (cf. Br3", Table 17.15). The structures of the penta-atomic cations Brs+ (2) and I5+ (3) have been determined by X-ray analysis of their AsFe salts and shown to have centrosymmetric Cjh symmetry like the... 

This last reaction is typical of many in which F3CIO can act as a Lewis base by fluoride ion donation to acceptors such as MF5 (M = P, As, Sb, Bi, V, Nb, Ta, Pt, U), M0F4O, Sip4, BF3, etc. These products are all white, stable, crystalline solids (except the canary yellow PtFe ) and contain the [F2CIO] cation (see Fig. 17.26h) which is isostructural with the isoelectronic F2SO. Chlorine trifluoride oxide can also act as a Lewis acid (fluoride ion acceptor) and is therefore to be considered as amphoteric (p. 225). For example KF, RbF and CsF yield M [F4C10] as white solids whose stabilities increase with increasing size of M+. Vibration spectroscopy establishes the C4 structure of the anion (Fig. 17.29g). 

Products of the high temperature (typically 750-850°C) reduction of ZrX4 (X = Cl, Br, I) with Zr metal in various proportions, have provided intriguing structural problems. Black phases initially thought to be ZrX2 and made up of ZrgXi2 clusters, isostructural with the well-known [M6Xi2]"+ clusters of Nb and Ta, (p. 992), were subsequently shown to contain... 

In the solid state all three elements have typically metallic structures. Technetium and Re are isostructural with hep lattices, but there are 4 allotropes of Mn of which the o-fomi is the one stable at room temperature. This has a bcc structure in which, for reasons which are not clear, there are 4 distinct types of Mn atom. It is hard and brittle, and noticeably less refractory than its predecessors in the first transition series. 

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