Lone pair ions

While the majority of the ions of elements can be considered to be spherical, a group of ions, the lone pair ions, found in at the lower part of groups 13, 14 and 15 of the Periodic Table, are definitely not so. These ions all take two ionic states. The high charge state, Mn+ can be considered as spherical, but the lower valence state, Mouter electron configuration [Kr] 4d 5s 5p2. Loss of the two p electrons will leave the ion with a series of closed shells that is moderately stable. This is the Sn2+ state, with a configuration of [Kr] 4d105s2. The pair of s electrons - the lone... 

XXVII. Localization—Bonds, Lone Pairs, Ion Cores.387... 

Co-ordinate bonds are formed by the sharing of electrons, both electrons being donated by the same atom. Thus the hydrogen ion, has no outer electrons whilst ammonia has eight, six shared with hydrogen atoms and one lone-pair. This lone-pair is donated to the hydrogen ion and the ammonium ion is formed ... 

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

When the ammonium ion NH is formed the lone pair becomes a bonding pair and the shape becomes a regular tetrahedron. 

As an example of a really strong base, the hydride ion H (for example in NaH) is unique it has one lone pair, a negative charge and a very small size. Like O , it is too strong a base to exist in water ... 

We have seen that a base can be defined as combining with a proton and, therefore, requires at least one lone pair of electrons. A more general definition of acids and bases, due to G. N. Lewis, describes a base as any species (atom, ion or molecule) which can donate an electron pair, and an acid as any species which can accept an electron pair— more simply, a base is an electron-pair donor, an acid an electron-pair acceptor. Some examples of Lewis acids and bases are ... 

Ammonia as a donor molecule. Because of the presence of the lone pair of electrons on the nitrogen atom, ammonia can behave as an electron pair donor. For example, ammonia abstracts a proton from a water molecule producing the tetrahedral ammonium, NH4, ion and forms the compounds HjN- AlClj and HjN- BClj. 

The. AMBKR force field expects lone pairs to be added to all sulfur atom s an d computes th e in teraction s as if these Ion e pairs were atoms with a specific type just like any oth er atom. I h e templates automatically add the expected lone pairs to sulfur atoms when using the. AMHhIR force field. 

This is due to a resonance effect. Aniline is stabilized by sharing its nitrogen lone-pair electrons with the aromatic ring. In the anilinium ion, the resonance stabilization is dismpted by the proton bound to the lone pair. 

Photoelectron spectroscopic studies show that the first ionization potential (lone pair electrons) for cyclic amines falls in the order aziridine (9.85 eV) > azetidine (9.04) > pyrrolidine (8.77) >piperidine (8.64), reflecting a decrease in lone pair 5-character in the series. This correlates well with the relative vapour phase basicities determined by ion cyclotron resonance, but not with basicity in aqueous solution, where azetidine (p/iTa 11.29) appears more basic than pyrrolidine (11.27) or piperidine (11.22). Clearly, solvation effects influence basicity (74JA288). 

These examples serve to illustrate the fact that, in reactions in which carbenium ions are formed in proximity to the acetal lone pairs, unexpected rearrangements may occur. 

The CK" ion can act either as a monodentate or bidentate ligand. Because of the similarity of electron density at C and N it is not usually possible to decide from X-ray data whether C or N is the donor atom in monodentate complexes, but in those cases where the matter has been established by neutron diffraction C is always found to be the donor atom (as with CO). Very frequently CK acts as a bridging ligand - CN- as in AgCN, and AuCN (both of which are infinite linear chain polymers), and in Prussian-blue type compounds (p. 1094). The same tendency for a coordinated M CN group to form a further donor-aceeptor bond using the lone-pair of electrons on the N atom is illustrated by the mononuclear BF3 complexes... 

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