Cations with lone pair

In general, the behavior of cations with lone pairs of electrons is not well reproduced by the lattice strain model. For example, if we consider lead, then partition coefficients reported by B indeman et al. (1998) may be represented as follows ... 

Substituent effects on intermediates can also be analyzed by MO methods. Take, for example, methyl cations where adjacent substituents with lone pairs of electrons can form 71 bonds, as can be expressed in either valence bond or MO terminology. 

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

The high electrophilicity of the positively charged element can be modified by intramolecular donation from remote donor substituents. This interaction leads to solvent-free cations with coordination numbers for the positively charged element > 3 and to a considerable electron transfer from the donor group to the element. Frequently used donor substituents utilize heteroatoms with lone pairs (e.g. amino, hydrazino, methoxy, carboxy, phosphino, etc.), in many cases in combination with pincer-type topology of the ligand, for the stabilization of the cationic center. These strongly stabilized cations are beyond the scope of this review and instead we will concentrate on few examples where we have weak donors such as CC multiple bonds, which stabilize the electron-deficient element atom. 

The structures of these ions normally conform to those predicted by the VSEPR theory, as shown in Fig. 17.2.2. Since the anion XY has two more electrons than the cation XY+, they have very different shapes. The anion IFj- is planar with lone pairs occupying the axial positions of a pentagonal bipyramid. In [Me4N](IF6), IFg is a distorted octahedron (C3V symmetry) with a sterically active lone pair, whereas both BrFg and ClFg are octahedral. The anion IF " has the expected square antiprismatic structure. 

Shape Valence State Electron Pair Repulsion Theory - Application to Molecules, Anions and Cations of Main Group Elements - Molecules with Lone Pairs - Oxyanions. Hybridisation -sp, sp, sp, sp d and sp d Hybrid Orbitals - Multiple Choice Questions. 

A dissection of the influence of solvent and base should be possible from studies in dipolar aprotic solvents containing various bases in which the two reactants can be varied independently. Although bases whose conjugate acids have pATa values of less than 11 rarely induce elimination in protic media , halide ions have been used successfully in this connection in dipolar aprotic solvents . Of course in the latter solvents, which lack exchangeable hydrogens but possess atoms with lone pairs capable of solvating cations, pATa values may differ appreciably from those already derived in protic sol-vents . There is obviously ample scope for future research in this field. 

By stabilizing the cyclohexadienyl cation intermediate, lone-pair donation from fluorine counteracts the inductive effect to the extent that the rate of electrophilic aromatic substitution in fluorobenzene is, in most cases, only slightly less than that of benzene. With the other halogens, lone-pair donation is sufficient to make them ortho, para directors, but is less than that of fluorine. 

Strategy In Lewis acid-base reactions, the acid is usnaUy a cation or an electron-deficient molecule, whereas the base is an anion or a molecule containing an atom with lone pairs, (a) Draw the molecular structure for C2H5OC2H5. What is the hybridization state of A1 in AICI3 (b) Which ion is likely to be an electron acceptor An electron donor ... 

Lewis acid-base theory (Jensen, 1980) is an outgrowth of the Lewis model of chemical bonds. A Lewis acid is a chemical species that can accept an electron pair. Lewis acids can be cations like Fe or Cu or they can be species with empty or partially empty valence orbitals such as CO2 or SO2. Lewis bases can donate an electron pair. Lewis bases are anions like OH" or S or they can be species with lone pairs such as HjO or NHj. The transfer of cations from a solid, such as szomolnokite (FeS04 H20), to form a hydrated ferrous ion in solution is a typical Lewis acid-base reaction. 

Think About It In Lewis acid-base reactions, the acid is usually a cation or an electron-deficient molecule, whereas the base is an anion or a molecule containing an atom with lone pairs. 

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