Coordination compounds Lewis acid-base definitions

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. 

Metal carbonyl complexes are an interesting series of coordination compounds in which the ligands are CO molecules, and in many cases the metals are present in a zero oxidation state. In these complexes, both the metal and ligand are soft according to the Lewis acid-base definitions. Although the discussion at first will be limited to the binary compounds containing only metal and CO, many mixed complexes are known that contain both CO and other ligands. 

Some liquid covalent halides can act as nonaqueous solvents " based on Lewis acid-base behavior, according to the donor-acceptor definition. The self-dissociated ions consist of a cation formed by subtraction of a hahde ion from the neutral compound, while the anion is formed by its addition (equation 24). Salts derived from such covalent halides can be considered as titration products of the parent acidic and basic compounds (equations 25 and 26). In such cases, both the cation and the anion usually possess a stable coordination number with a high geometrical symmetry. 

In 1923, Gilbert Newton Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. This definition is even more inclusive than the previous one because it includes all Bronsted-Lowry acids and bases as a subset and provides the foundation for the field of coordination chemistry. A coordination compound is the product of a Lewis acid-base reaction, such as the one shown in Equation (14.11) and Figure 14.5, in which the metal ion (Lewis acid) and ligand (Lewis base) are held together by a coordinate covalent bond. 

CljB + NH3 CbBiNHj Metal ions in coordination compounds are also electron-pair acceptors and therefore Lewis acids. The definition includes the traditional Bronsted acids since H is an electron acceptor, but in common usage the terms Lewis acid and Lewis base are reserved for systems without acidic hydrogen atoms. 

Thus, Lewis s definition is a much broader definition that includes coordination compound formation as acid-base reactions, besides Arrhenius and Lowry-Bronsted acids and bases. Examples ... 

The Lewis definition thus encompasses all reactions entailing hydrogen ion. oxide ion. or solvent interactions, as well as the formation of acid-base adducts such as R,NBF, and all coordination compounds. Usage of the Lewis concept is extensive in both inorganic and organic chemistry, and so no further examples will be given here, but many will be encountered throughout the remainder of the book.11... 

One definition of a metal complex or coordination compound is "a compound formed from a Lewis acid and a Bronsted base a Lewis acid being an electron pair acceptor and a Bronsted base a proton acceptor. Thus the interaction of the Lewis acid metal centre in Ni(C104)2 with the Bronsted base ammonia to form a complex according to equation 4.1... 

The particular virtue of these definitions is that the Lewis acid and the Lewis base strengths are both estimates of the valence of the bond that links the cation with the anion. The most effective bonds will therefore occur between a cation whose Lewis acid strength (Sg) is close to the Lewis base strength (Sb) of the anion. This is known as the Valence Matching Principle. Compounds between badly matched ions, i.e. Be (Sa = 0.5 v.u.) and ClO j" (Sb = 0.08 v.u.), are difficult if not impossible to form as both the cation and the anion will be forced into unusual coordination. If they... 

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. 

In a further generalization, Lewis [4] advanced a definition that was no longer restricted to proton-exchange reactions. An acid was defined as any substance that can accept a pair of electrons from a donor substance, and a base as any substance able to donate an electron pair and form a "dative" covalent bond. In these more general terms, the acid-base interaction includes the formation of covalent bonds and applies to any chemical reaction in which an addition compound (adduct) is formed through a coordinative bond ... 

In the original Lewis definition (1923 [29], 1938 [30]), aeids are electron-pair acceptors and bases are electron-pair donors. The fundamental reaction between a Lewis acid A and a Lewis base B is the formation of a complex (or adduct or coordination compound or addition compound) A-B (reaction 1.17) ... 

First and foremost is the existence of isolable coordination compounds. By analogy with ammonium ions and the boron trifluoride adduct of ammonia (discovered by Gay-Lussac in 1809) it has been possible to isolate and characterize oxonium salts of alcohols, ethers, and ketones (49,176,314) and to prepare Lewis acid adducts of the ethers, sulfides, and phosphines (329), as well as many other compounds which fall under our definition of weak bases. 

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