Low temperature X-ray crystallography

The tantalum(IV) hydrides [TaH2Cl2(PMe3)4] (51) and [TaH2Cl2(dmpe)2] (50) were characterized by low temperature X-ray crystallography. SSS,69Z Complex (51) adopts a distorted dodecahedral geometry in the solid state, while (50) is better described as a distorted square antiprismatic complex. The hydrogen atoms have been located. 

In practice, the marked temperature variability of the ESR spectra of these temperature variable copper(II) structures has proved a useful criterion of their fluxional stereochemistry,396 which has then been confirmed by low temperature X-ray crystallography (Chapter 53.4.2.lx). 

The structures of the CuL6 chromophores may be temperature variable with bond-length changes of the order of the Jahn-Teller radius, Fig. 1(A), Rjj = 0.1-0.4 A, accessible by low temperature X-ray crystallography, which if determined at low enough temperature may yield AE values. 

Fig. 2. Molecular structure and packing diagram of (F3C)p2SiONMe2 obtained by low-temperature X-ray crystallography.

The formation of acyl halide-Lewis acid complexes can be demonstrated readily. Acetyl chloride, for example, forms both 1 1 and 1 2 complexes with AICI3 that can be observed by NMR. Several Lewis acid complexes of acyl chlorides have been characterized by low-temperature X-ray crystallography. For example, the crystal stmctures of PhCOCl-SbCl and PhCOCl-GaClj and [PhCOCl-TiCl4]2 have been determined. In all of these complexes, the Lewis acid is bound to the carbonyl oxygen. Figure 9.11 shows two examples. 

Experimental electron densities in the NjP3(NC2H4) C5H4 clathrate have been established using low temperature X-ray crystallography coupled with X-X and deformation density calculations. Both the aziridine lone pair and the jt system restricted to the PNP endocyclic "island" can be observed . 

Fig. 3. Molecular structure of [F3SiCH2NMe2 ]2Sip4 as obtained by low-temperature X-ray crystallography. Important bond lengths and angles are SicN -F 1.648(1)-1.655(1), Sicj -F 1.572(1)-1.589(1), SicN N 2.000(1), SicN4-C 1.855(1), C-N 1.501(1) k, Sicm-C-N 121.1(1) .

B. Post. Low-Temperature X-Ray Diffraction. Bibliography 2, 1964. (International Union of Crystallography.) Available in the U.S.A. from Polycrystal Book Service (see [5.1]). 

B. Post, Low-Temperature X-ray Diffraction, International Union of Crystallography (1964). 

Research into these compounds has followed the usual pattern for the study of very reactive compounds.584 585 First, the compounds are obtained only as transient intermediates that are characterized by their reactions, then longer-lived compounds are obtained that can be studied in solution at low temperature finally, compounds are designed, usually by building in steric protection, that can be obtained as crystalline solids, the structures of which can be determined by X-ray crystallography. These advances have been made alongside related studies of the chalcogenides of the other group 14 metals.584,585... 

It is also clear that the delineation of these three cases is based on the lower limit to measurability of torsional barriers. In practice, it is difficult to go below 7 kcal/mol with push-pull ethylenes, since these rather polar compounds tend to aggregate at low temperatures and give very broad bands below -120 to - 130°C. As will be discussed later, several compounds that show a Case 1 type of NMR spectrum in solution are shown by X-ray crystallography to be twisted... 

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