Symmetry in the solid

We have introduced the term "symmetry" in the solid. Let us now examine exactly what is meant by that term. To illustrate this concept, examine the symmetry elements of our cube, given in the following diagram, given on the next page as 2.1.6. 

A).537 ESR, IR and Raman spectroscopies support a configurational change from symmetry in the solid to D4d (antiprism) in solution. 

Remarkably, the two complexes 104zi and 104N display C3 symmetry in the solid state, albeit with rather different degrees of helicity 6 (19° for 104N vs. 27° for 104ZI). This further substantiates the propensity of the TPE ligands (E = group 13 element) to form three-blade propeller complexes even in the absence of a central M-E interaction. 

As shown in Figure 4, the anions have nominal D4d symmetry in the solid state, and NMR studies are consistent with the same structure in solution. Tungsten-183 NMR spectra (Fedotov et al., 1989a, 1989b, 1996 Shiozaki et al., 1996a Bartis et al.,... 

Nine derivatives of 1, [RSb9W2i086]16 (R = La-Gd, Dy, Yb), have been isolated and characterized by elemental analysis, magnetic susceptibility, and infrared spectra. The four-line 183W-NMR spectrum of the lanthanum derivative is consistent with the D3h symmetry of the parent structure, but the emission spectrum of the europium derivative suggests a lower symmetry in the solid state (Liu et al., 1992). No structural determinations of these derivatives have yet been reported. 

In contrast to its ready reaction with acetylenes, Cp2Mo2-(CO)i+ does not react with simple olefins or dienes, e.g., C2Ht+, butadiene, C2H3CN, norbornadiene, etc. With TONE (C2(CN)iJ, 1 is oxidized and [CpMo(C0)il]+ [TONE]7 may be isolated (6). Allene does react, however, to form a complex, ll, in which each of the orthogonal C=C n-bonds donates two electrons to each molybdenum. This fluxional molecule has C2 symmetry in the solid state and has a Mo-Mo bond length of 3.117(1)A (32). 

A subsidiary goal from the outset was to identify those systems in which the lone-pair electrons present in these subvalent species show no stereochemical activity that is, in which the fourth-group atoms occupy sites of perfect symmetry in the solid state. Our search brought forth various scattered examples of such structural systems, and reading of their unusual properties heightened our interest. However, with the fortuitous synthesis by C. Janiak in our own laboratory... 

In the gas phase, the dinitramide ion N(N02)2 has symmetry. In the solid state, the symmetry is often distorted by interactions with the counterion, for example, hydrogen bonding with the ammonium ion in ammonium dinitramide (see Table 33). The N-NO2 rotation barrier is very low. In fact, though the bond lengths of the nitramide ion remain fairly constant, no two-dinitramide crystal structures show the same geometry of the dinitramide anion. The crystal stmcture of lithium dinitramide shows the symmetric anion. 

Decamethylferrocene, [FeCpf] (Cp = // -CjMes) [26], is the most common alkylated ferrocene derivative used for the preparation of CT complexes. [FeCpf] and its oxidized form, as is the case for most other decamethylmetallocenes [27], has Djj symmetry in the solid state. For use as an electron-donor it has the important advantage that it is resistant towards substitution reactions, ring exchange, and hydrolysis. It is also much more electron-rich than its parent compound, as reflected by the redox data reported in Table 8-1. 

The homoleptic, bis(trispyrazolylborate)titanium(ll) complex, Tp2Ti (Tp = trispyrazolylborate 203), has been prepared by reaction of 2 equiv. of KTp with TiCl2(TMEDA) (TMEDA = Ar,Ar,Ar, A -tetrarncthylcthylcncdiarninc).122 The dark red, paramagnetic compound has idealized /)v/ symmetry in the solid state and is easily oxidized to the corresponding Ti(m) derivative both electrochemically and chemically with AgPF, 204. Two-electron oxidation to Ti(rv) chalcogenido complexes has been accomplished with pyridine-N-oxide as well as with elemental sulfur and selenium (205, Scheme 31). 

The ditungsten complex, W2(C3H5)2(NMe2)4 (79), has Ci symmetry in the solid state with bridging allyl ligands that exhibit an unusual /n-jj i/j -Z-propenyl coordination mode, and the reaction of W2(OR)g with allene generates the... 

All of these data show that in frozen solutions the immediate environment of the central atom of the mixed complexes has almost the same symmetry as the tris parent complex. That is, the pentacoordinated mixed complex of square pyramidal symmetry in the solid state will presumably assume in solution a distorted octahedral, hexacoordinated structure as a result of the coordination of a solvent molecule. 

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