Steric pressure

All ligands and metal complexes were characterized spectroscopically. The 31P-NMR spectra of ligands 1-4 appear in a region of 5 111.6-140.6. Upon metal complexation an upfield shift is observed. Table 1 summarizes the selected spectroscopic properties of the new phosphinite ligands (1-4) and corresponding metal complexes (5-12). With exception of 8 and 12 all complexes have a cis coordination geometry as judged from JP.Pt and the IR spectra (341 cm"1 and 355 cm"1 for 8 and 12, respectively). Due to the steric pressure imposed on the... 

Kenworthy, A.K., Hristova, K., Needham, D., and McIntosh, T.J. (1995) Range and magnitude of the steric pressure between bilayers containing phospholipids with covalently attached po/y(ethylene glycol). Biophys. J. 68, 1921-1936. 

For PR3/P(OR)3-stabilized nickel complexes, there are two borderline cases known from the experimental investigation of Heimbach et al. 1 which, unlike the usual behavior, redirect the cyclo-oligomerization reaction into the Ci2-cyclo-oligomer production channel. Catalysts bearing either strong a-donor ligands that must also introduce severe steric pressure (e.g., PBu Pr2) or sterically compact n-acceptors (like P(OMe)3) are known to yield CDT as the predominant product. From a statistical analysis it was concluded,8a,8c that the C8 Ci2-cyclo-oligomer product ratio is mainly determined by steric factors (75%) with electronic factors are less important. 

The 2p orbital radius may be considered anomalously small (of the same order as the 2s orbital radius) because there is no inner shell of the same angular symmetry that exerts outward steric pressure due to the Pauli exclusion principle. (A similar exception causes the first transition series to appear anomalous compared with later lanthanides, since 3d orbitals form the innermost d shell.) The 2p -> 3p expansion therefore appears to be relatively more pronounced than 2s —> 3s expansion. 

The filled (Is)2 cores lead to additional steric repulsions with the incoming donor hybrid. These combine with nuclear Coulomb repulsions to oppose high electronic overlap. Figure 3.6 displays the calculated increase in steric repulsion as each filled 2s spin-orbital of Li2 collides with the filled core Is of the opposite atom. The paired one-electron steric repulsions shown in Fig. 3.6 are similar to the ionic two-electron steric repulsions of Fig. 2.11, and the wave-mechanical origin of the steric pressure would be analogous to that described in the discussion surrounding Fig. 2.12. 

The complexes [Mo(CO)2(py)2(OR)2] (R = Pr or Bu ) have been prepared by treatment of [Mo2(OR)8] with CO in presence of pyridine and their octahedral structure established by X-ray diffraction 66 they have low v(CO) values (1906, 1768 cm"1). The (/i-OR) dimers [Mo2(OR)8(CO)2] also have low v(CO) values.66 Because of steric pressure in the potentially seven-coordinate complexes, a chelating ligand may also act as a bridge, to form dinuclear complexes, e.g. [Mo2(CO)4(dppm)3Br4].67... 

Attempts to prepare Ta(NEt2)5 by metathetic exchange reactions were found to lead to a complex in which activation of a )3-CH bond of an NEt2 ligand occurs with the elimination of diethylamine (equation 92), 244 345 Presumably steric pressure at the tantalum center promotes the cyclometallation reaction to generate the iminomethyl group. 

This type of C-H transformation is also feasible at the notoriously unreactive t-butyl group, as outlined in Scheme 3. After a slight increase of the temperature compared with the reaction conditions for C-H transformation at methoxy groups the process proceeds smoothly. The terphenyl 8 is obviously analogous to compound 2, whereas the benzocydobutene 7 as the main product results from reductive elimination of a palladacyde, a step which minimizes steric pressure while obviously building up ring strain. 

Scheme 3. Proposed reaction pathway for the formation of the Wj-jr-allyl,

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