Pyramidal compounds

The optical activity in above type of pyramidal compounds is explained on the basis that the unshared pair of electrons is analogous to a fourth group and is therefore different from others. Their structure approaches a tetrahedral configuration with different groups. The case of tertiary amines... 

The linear compound (XXI) is twice as active as the pyramidal compound (XXII) against Aphis fabae. 

Cobalt(II) is a d7 ion which can be high spin or low spin. Generally, it is low spin in planar or some square pyramidal compounds. The effective electron relaxation times in the low spin state are long enough (10-9-10-1° s) so that EPR spectra can be recorded at room temperature [78] and the proton NMR lines are broad. This is due to the high energy of the first excited state (Fig. 5.29). In... 

In order to study substituent effects on the epimerization rate of square pyramidal compounds of the type CsHsM(CO)2LL, we synthesized the derivatives 41-49 by using instead of pyridine carbaldehyde-(2) the corresponding aldehydes for the condensation reaction with S-(-)-a-phenyl ethyl amine. In all the cases 41-49 the resulting pair of diastereoisomers could be separated into the optically active components a and b63-65 the a series is depicted in the formulas of Scheme 21. 

Both copper(n) complexes 23(42+ and 23 ysy2"1" have electronic spectra typical of 4-coordinate and 5-coordinate species, respectively, in accordance with previously reported complexes possessing analogous ligand sets)99,1001 The same interconversion process can also be monitored by EPR,198 1011 and it was demonstrated in an unambiguous fashion that 23(42+ is a distorted tetrahedral complex and that the product of the changeover, 23 s2+, is a square pyramidal compound. 

METAL-CENTERED REARRANGEMENT IN OPTICALLY ACTIVE SQUARE-PYRAMIDAL COMPOUNDS... 

The example of 25 leads to a class of compounds the metal configuration of which is so labile that dynamic NMR spectroscopy is more appropriate than polarimetry for studying inversion of configuration of the metal atom. In this way, the fast interconversion of many square-pyramidal compounds of the type CbHsM(CO)2LX has been investigated, and the intramolecular character of the metal-centered rearrangement demonstrated, in accord with the mechanism proposed in Section X,A (74, 128-135). 

Square pyramidal compounds can be derived formally from the corresponding trigonal bipyramidal molecules by addition of an electron pair. 

Similar conditions have been used by Gusev etal. to synthesize complex 54 <20010M1001>. From OsC , they obtained the 16-electron chloride square-pyramidal compound 54, using triethylamine and methanol as solvent (Equation 22). 

Determined by X-ray crystallography in the solid phase, many structures fall between the two extremes of square pyramidal and trigonal bipyramidal, but most can be identified as lying much nearer to one than the other. Most of the few gold(III) complexes have structures closer to square pyramidal. Compounds 1-3 are typical (47). In every... 

One way of classifying phosphorus compounds is according to the configuration adopted by the chemical bonds formed by the element. Until about 60 years ago almost the whole of phosphorus chemistry was concerned with trivalent (pyramidal) compounds and pentavalent (tetrahedral) compounds (Chapter 3). Inorganic phosphorus chemistry dominated the field and the extent of known organophosphorus chemistry was still very limited. Since that time, however, many more compounds, including those with alternative combinations of valency states (X.) and coordination schemes (o), have been discovered. 

Scheme 1.4 Pyramidal inversion of a tricoordinated, pyramidal compound.

Ruthenium and Osmium.—Square-pyramidal compounds [M(X)(Y)(PPh3)3] exhibit a dynamic process in solution which according to P n.m.r. spectroscopy equilibrates apical and basal phosphine sites. The measured values of A5 (Table 5) are consistent with this being an intramolecular rearrangement (probably Berry pseudorotation) and since addition of PPh3 does not effect the process a bimolecular exchange is also excluded. 

Scheme 4 shows some optically active resolving agents used in the resolution of organometallic transition metal compounds of tetrahedral, octahedral, and square pyramidal geometry. Schemes 2 and 3 demonstrate the application of the menthoxide ion. The aminophosphine shown will be used in an example discussed later on. The pyridine imine chelate ligand has been the chiral auxiliary for the resolution of octahedral compounds [9,10], not described in detail here, and for the resolution of square pyramidal compounds to be discussed next [11]. 

While the barriers for inversion of pyramidal compounds of first-row elements are normally low so that inversion is fast, the heavier elements have much higher barriers to inversion. The preferred bonding angle at sulfur, phosphorus, and other heavier elements is about 100° for most trivalent derivatives. This means that a much greater distortion of molecular geometry is required to reach the planar transition state. Typical barriers for trisubstituted phosphines are 30-35 kcal/mol, while for sulfoxides the barriers are about 35-45 kcal/mol. Phosphines and sulfoxides can therefore be isolated in optically active form and undergo inversion only at high temperatures. ... 

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