Lewis acid-base definition complex ions

Perhaps the greatest area in which the Lewis acid-base approach is most useful is that of coordination chemistry. In the formation of coordination compounds, Lewis acids such as Cr3+, Co3+, Pt2+, or Ag+ bind to a certain number (usually 2, 4, or 6) of groups as a result of electron pair donation and acceptance. Typical electron pair donors include H20, NH3, F , CN , and many other molecules and ions. The products, known as coordination compounds or coordination complexes, have definite structures that are predictable in terms of principles of bonding. Because of the importance of this area of inorganic chemistry, Chapters 16 through 22 in this book are devoted to coordination chemistry. 

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

These definitions describe the Lowry-Bronsted concept of acidity and basicity. The Lewis concept of acids and b s is more general, and may be useful for reactions in which protons are not involved. A Lewis acid is any substance that can accept electrons, and a Lewis base is any substance that can donate electrons. Small, high-charge metal ions such as Al + and Fe " are strong Lewis acids because they tend to complex with functional groups on molecules (Lewis bases) by accepting electrons from them ... 

The choice of acid or base for solvent is simplified appreciably for melts containing complex ions (as a rule, they are anions), which are prone to the acid-base dissociation. Dissociation of this ion is assumed as the intrinsic acid-base equilibrium of a melt of such kind. In this case, the simpler eliminated anion will be considered as the base of the solvent and the coordinationally unsaturated residue will be the acidic particle of the solvent. Naturally, the division of particles formed by the auto-dissociation into acids and bases is made on the basis of the Lewis definition [13] an acid is the acceptor of an electron pair and a base is the donor of this electron pair. Ionic melts based on complex halides of gallium(III) [28], aluminium(III) [29] and boron(III) [30,31] may serve as examples of successful application of the above approach. The electron-deficient covalent halide (e.g. A1C13, BF3) in these melts is the solvent acid, and the corresponding halide ion is the base of the solvents ... 

On the other hand, the aluminum halides have a strong Lewis acid character, and because of this they can react strongly with the alkali halides and form complex ions of the type AlCU". In this way the nonconducting liquid AI2CI6 reacts with other halides, forming strongly ionic melts. According to the Lewis definition [387] of acids and bases, an acid is defined as a substance that can accept a pair of electrons to form a bond a base is a substance that can donate a pair of electrons to the formation of a covalent bond. 

The term species may mean a discrete molecule, a simple or complex ion or even a solid exhibiting non-molecularity in one or more dimensions (graphite as an example). Free atoms seldom act as Lewis acids and bases. They usually have one or more unpaired electrons and flieir reactions are more accurately classified as free radical. The donor orbital is usually die highest occupied molecular orbital HOMO, and the acceptor orbital is usually the lowest unoccupied molecular orbital or LUMO. The molecular orbital definitions have a number of important consequences ... 

One method used for the preparation of [Pt(en)2]Cl2 or [Pt(en)3]Cl4 is the direct reaction between ethylenediamine and PtCU or PtCLt, respectively. The technique is to add slowly the solid platinum salts to the liquid ethylenediamine. This addition is accompanied by a vigorous evolution of heat, which is to be expected whenever a strong acid is added to a strong base. Recall (Section 2.1) that in terms of the Lewis definition of acids and bases, the formation of coordination compounds involves an acid-base reaction. In this particular case, the platinum ions are the acids and ethylenediamine is the base. Metal dimethylsulfoxide complexes have been prepared and characterized. One method used to prepare some of these complexes is a direct reaction (12) in the absence of any added solvent. 

Here the generalized or Lewis definition of an acid and a base is applied. Thus an add is a substance that can accept a pair of electrons and a base is a substance that donates a pair of electrons. It follows then that a metal ion, like hydrogen ion, is an acid and a ligand with its pair of electrons is a base. The formation of metal complexes and the replacement of one ligand by another or one metal by another in these systems then are examples of acid-base reactions. 

The Lewis theory regards bases as non-bonding-pair donors and acids as nonbonding-pair acceptors. According to this definition, the formation of a complex from a metal ion and ligands can be envisaged as an acid-base reaction. 

---