Lewis donor

Cationic low-coordinated n-bonded phosphorus compoimds add Lewis donors, such as amines or phosphines. In contrast to the trigonal bipyramide formed in donor-addition to the silico-nium cation, the phosphorus cations add donors in a perpendicular fashion, depending on the nature of the n-bonds toward phosphorus. According to quantum chemical calculations the various cations reveal different stabilities and hence a strong variation in donor addition abilities. 

Electron density is transferred from a Lewis donor to a Lewis acceptor, here shown for an amine (donor) interacting with a phosphenium cation (acceptor). The lone pair orbital at the amine dives into the emptyp-orbital of the phosphenium cation, hollowing the ideas of Mulliken the donation towards an acceptor can be viewed as sketched in Scheme 2. 

In practice one can differentiate between two kinds of donors, the resulting donor-acceptor bonds can be largely ionic (class I) or covalent (class II) [16]. Donors of the first type (class I) stem from the first row of the periodic table of elements, such as amines, ethers, in detail structures in which the Lewis basis centre possessing the non-bonding lone pair is strong electronegative. Donors of the second type are constituted from elements of the second row of the periodic table of elements, such as phosphines, thioethers, etc. (class II). These Lewis donors are... 

In this progress report we have reviewed the latest developments in the large area of cationic low-coordinated species and their coordination with Lewis donors. It is clear that these species are of a broad interest, in particular for catalysis. In some cases, e. g. the methylene phosphenium cation, the donor adducts also open new routes for synthesis. Regarding the mechanism for the diverse donor-addition reactions, the structural details are only poorly understood and need a better classification. In particular the variation of the Lewis-donor has to be established. Hitherto in most cases iV-donation is studied. It includes amines or pyridines. Obviously the effect of other donors, such as phosphines, thioethers needs to be studied as well. The siliconium cation for which these effects are better known could provide an understanding for further investigations within this field. 

Phosphines are classical Lewis bases or ligands in transition-metal complexes, but the cationic species shown in Fig. 15.4.l(i) are likely to exhibit Lewis acidity by virtue of the positive charge. Despite their electron-rich nature, an extensive coordination chemistry has been developed for Lewis acidic phosphorus. For example, the compound shown below has a coordi-natively unsaturated Ga(I) ligand bonded to a phosphenium cation it can be considered as a counter-example of the traditional coordinate bond since the metal center (Ga) behaves as a Lewis donor (ligand) and the non-metal center (P) behaves as a Lewis acceptor. 

Nitrile derivatives of the metal carbonyls have been discussed together with other nitrogen donor molecules in a number of contexts. Much of the early work has been reviewed by Manuel 337) in his article on Lewis base-metal carbonyl complexes in Volume 3 of this series, and by Stroh-meier 436) in his review of photochemical substitution reactions. In general, nitriles are weaker Lewis donors than phosphorus and nitrogen bases 436), but compared to carbon monoxide, better electron donors but poorer acceptors 427). Force constants and assignments for a series of complexes [(MeCN)jjM(CO)g J (M = Cr, Mo, W) were studied 165, 228, 296) and... 

Gutman chose the reaction enthalpy of solvent with the reference aeeeptor (antimony pentachloride) to quantify Lewis-donor properties. The donor number, DN, is a dimensionless parameter obtained from negative values of reaction enthalpy. The data obtained from electrochemical and NMR studies were combined into one scale in whieh data... 

A review paper examines the nucleophilic properties of solvents. It is based on accumulated data derived from calorimetric measurements, equilibrium constants, Gibbs free energy, nuclear magnetic resonance, and vibrational and electronic spectra. Parameters characterizing Lewis-donor properties are critically evaluated and tabulated for a large number of solvents. The explanation of the physical meaning of polarity and discussion of solvatochromic dyes as the empirical indicators of solvent polarity are discussed (see more on this subject in Chapter 10). ... 

Study of Lewis donor-acceptor interactions and related problems... 

As was seen from the discussion in the preceding sections, the predominant part of the interaction between a solute and a donor or acceptor solvent (solvation) is a Lewis donor-acceptor interaction. The stronger Lewis acid and Lewis base the two reactants are, the more negligible are the effects of other factors influencing the solvation, and the more this process can be regarded as complex formation, in the classical sense, where the stability of the solvate is determined by the stability of the coordinate bond formed between the electron-pair donor and electron-pair acceptor reactants. 

As even this brief account shows, the Lewis donor-acceptor equilibrium and its thermodynamic data are strongly influenced by the inert solvent, the experimental method and especially the nature of the reaction partner. 

Fig. 20 Generic Weiss-motifs of lower order (left) and higher order (right) lithium magnesiates (R = alkyl anion, LD = neutral Lewis Donor)...

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