As Lewis Base

Phosphanes are characterized, among other things, by their electron lone pair. This electron lone pair can be expected to be utilized in a cr-donor interaction toward a Lewis acid, making the phosphane a Lewis base. In fact, that is the reason for the popularity of phosphanes in transition metal chemistry. Of course, the Lewis basicity not only makes them good ligands, but lets phosphanes develop a rich and diverse main group chemistry as well. 

The most obvious choices for a Lewis acid to exploit the Lewis basicity of phosphanes are group 13 elements with their intrinsic electron deficiency. Looking at BH3 as the Lewis acid component, we can easily discern the trends in the Lewis basicity of phosphanes. In the top part of Table 6.1, the phosphanes experience a pronounced coordination chemical shift of A y=60-135ppm from a well-shielded 

Phosphorus-31 NMR Spectroscopy, Springer-Verlag Berlin Heidelberg 2008 

We wonld expect a considerable downfield shift npon coordination of the phosphane, and we are therefore not snrprised to observe it in the ensning adducts. However, why do we observe an npfield shift upon coordination to the borane with the phosphanes in the lower part of the table The difference mnst lie in the behavior of the snbstituents on phosphorns, as this is the one parameter that changes as we look down the list. In the top part, the snbstituents are H, methyl, and phenyl, whereas in the lower part, the snbstitnents are flnoride, amide, and methoxide. The latter three (F, NMe and MeO) are capable of a 7r-bonding interaction toward phosphorus that increases as the electron density on phosphorus diminishes upon coordination. Since the P-NMR chemical shifts are more sensitive toward. ir-interactions than cr-interactions, the net result can very well be an upfield shift upon coordination of the phosphane, if substituents capable of . r-backbonding are present on phosphorus. 

The same intramolecular Lewis base - Lewis acid interaction can be observed when a chlorophosphane is used instead of a fluorophosphane. However, the chloride is less strongly bonded than fluoride, resulting in the displacement of chloride by the phosphane without the use of an auxiliary Lewis acid. The chemical shift of the tricoordinate phosphorus atom is sensitive to the steric bulk of its carbon substituent. Evidently, sterically demanding substituents like tert-butyl hinder the 2T-bonding interaction from nitrogen, resulting in the observed downfield shift. 

---

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

Prereactive dihalogen complexes with O- and S-heterocycles as Lewis bases in the gas phase 99AG(E)2686. 

Using curved arrows, show how the species in part (a) can act as Lewis bases in their reactions with HCI, and show how the species in part (b) can act as Lewis acids in their reaction with OH-. 

Which of the following are likely to act as Lewis acids and which as Lewis bases ... 

The tendency of the halogens to form chain-like polyanions that are stabilized by delocalization of the negative charge [15,34] is a basic chemical principle. Donor-acceptor interactions between Lewis-acidic Br2 and halide anions, but also with polyhalides acting as Lewis bases, give rise to the formation of a variety of homo and heteroatomic adducts. The maximum number of atoms in these chains increases with the atomic weights... 

Trialkyls are only known as Lewis base adducts. Reaction of Au2Br6 with methyllithium at 70°C affords an unstable AuMe3 (which is probably AuMe3Br ), but stable phosphine adducts AuMe3PR3 (R, e.g. Me, Ph) can be made [169]. 

The protons come from the water molecules that hydrate these metal cations in solution (Fig. 10.19). The water molecules act as Lewis bases and share electrons with the metal cations. This partial loss of electrons weakens the O -H bonds and allows one or more hydrogen ions to be lost from the water molecules. Small, highly charged cations exert the greatest pull on the electrons and so form the most acidic solutions. 

Many of the d-block elements form characteristically colored solutions in water. For example, although solid copper(II) chloride is brown and copper(II) bromide is black, their aqueous solutions are both light blue. The blue color is due to the hydrated copper(II) ions, [Cu(H20)fJ2+, that form when the solids dissolve. As the formula suggests, these hydrated ions have a specific composition they also have definite shapes and properties. They can be regarded as the outcome of a reaction in which the water molecules act as Lewis bases (electron pair donors, Section 10.2) and the Cu2+ ion acts as a Lewis acid (an electron pair acceptor). This type of Lewis acid-base reaction is characteristic of many cations of d-block elements. 

Organo-ligands which can pick up a proton should also be able to act as Lewis bases towards metal ions, but only two cases have so far been reported. Ethinyl- and vinylcobalamin both show reversible equilibria with Ag(I) ions, which have been ascribed to equilibria such as... 

A Lewis base must have valence electrons available for bond formation. Any molecule whose Lewis stmcture shows nonbonding electrons can act as a Lewis base. Ammonia, phosphorus trichloride, and dimethyl ether, each of which contains lone pairs, are Lewis bases. Anions can also act as Lewis bases. In the first example of adduct formation above, the fluoride ion, with eight valence electrons in its 2 s and 2 p orbitals, acts as a Lewis base. 

Removing electrons from a metal atom always generates vacant valence orbitals. As described in Chapter 20, many transition metal cations form complexes with ligands in aqueous solution, hi these complexes, the ligands act as Lewis bases, donating pairs of electrons to form metal-ligand bonds. The metal cation accepts these electrons, so it acts as a Lewis acid. Metal cations from the p block also act as Lewis acids. For example, Pb ((2 g) forms a Lewis acid-base adduct with four CN anions, each of which donates a pair of electrons Pb ((2 ( ) + 4 CN ((2 q) -> [Pb (CN)4] (a g)... 

The first use of chiral sulfoxides as Lewis-base catalysts in the allylation of aldehydes with allyltrichlorosilane was reported in 2003. The formation of the... 

The products are, in turn, starting materials for a rich chemistry, only superficially explored to date. The phosphorus atoms serve as Lewis bases toward metals. Coordination complexes with one or two tungsten atoms have been isolated (Eq. 34). They also readily undergo insertion reactions... 

Two stannenes have been synthesized by the reaction of a stannylene with a boranediylborirane (Eq. 34).85 The boranediylborirane has been shown to react toward suitable reagents as though it were the carbene,101 which is only slightly higher in energy than the boranediylborirane.102 The reaction occurs at room temperature in pentane solution. The resulting stannene has a considerable contribution from the ylide resonance structures. The carbene arising from the boranediylborirane is extremely electrophilic, and therefore the stannenes can be considered formally to be adducts of the stannylene as Lewis base and the carbene. 

Stannylenes are in the first place Lewis acids (electron acceptors) as can be easily derived from the structures of the solids (Chapter 3). When no Lewis bases (electron donors) are present, they may also act as Lewis bases via their non-bonding electron pair (see polymerization of organic stannylenes). 

MolSurf parameters [33] are descriptors derived from quantum mechanical calculations. These descriptors are computed at a surface of constant electron density, with which a very fine description of the properties of a molecule at the Van der Waals surface can be obtained. They describe various electrostatic properties such as hydrogen-bonding strengths and polarizability, as well as Lewis base and acid strengths. MolSurf parameters are computed using the following protocol. 

Some of their halogens appear to have been introduced as electrophiles rather than as Lewis bases or nucleophiles, which is their character when they are solutes in seawater. 

Large cations give a favorable match of cation and anion characteristics, so in accord with the hard-soft interaction principle, the salts that have been isolated contain ions such as R4P+. Because of having an unshared pair of electrons, the SnX3" complexes can function as Lewis bases. 

As already was observed for hypercoordinated adducts MX3(ER 3)2, no stibine and bismuthine adducts of low-valent alanes, gallanes or indanes have been prepared, to date. According to the lability of low-valent group 13 compounds toward disproportionation into M(III) and elemental M, stibines and bismuthines are expected to be too weak as Lewis bases, preventing them from the stabilization of such compounds. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Mixed-valence compounds. The ground state of monomeric species RM (M = B, Al, Ga, In), which have an electron lone pair at the metal center, is singlet and the singlet-triplet energy gap increases with increasing atomic number. Consequently, these compounds are able to act as Lewis bases. Reactions with Lewis-acidic... 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

---