A group of chemical compounds in which all or a part of the molecular bonding is of the coordinate covalent type.
This article summarizes the different types of compounds that are known and discusses their nomenclature, structure, stereochemistry, and synthesis. For a discussion of the nature of the coordinate bond and the stability and reactivity of complex compounds See also: Chelation; Coordination chemistry
Complex compounds contain a central atom or ion and a group of ions or molecules bonded to it. Many simple hydrates, such as MgCl2 · 6H2O, are best formulated as [Mg(H2O)6]Cl2 because it is known that the six molecules of water are bound to the central magnesium ion. Therefore, [Mg(H2O)6]2+ is a complex ion, and [Mg(H2O)6]Cl2 is a complex compound. The charge on this complex ion is 2+, because this is the charge on the magnesium ion and the coordinated water molecules are neutral. However, if the coordinated groups are charged, then the charge on the complex is represented by the sum of the charge on the metal and that of the coordinated ions. See, for example, the progression of charges on the platinum(IV) complexes listed below. Thus the charge is 4+ for [Pt(NH3)6]4+ because Pt is 4+ and NH3 is neutral. But the charge is 2− for [PtCl6]2− because of the 6 Cl−, that is, +4 − 6 = −2 (that is, 2−).
The metal complex may be a cation, have zero charge, or be an anion, as is exemplified by the following series of complexes:
These compounds are named according to rules set up by the Nomenclature Committee of the International Union of Pure and Applied Chemistry. Some of the rules are the following: (1) Name the cation first as one word followed by the anion as one word. (2) For a cationic complex, name the negative ligands first with an ending of -o, then the neutral ligands, and finally the metal followed by a Roman numeral in parentheses to designate its oxidation state. Neutral ligands are named as the molecule, except that H2O is aqua and NH3 is ammine. (3) The prefixes such as di, tri, and tetra are used before simple names such as chloro, aqua, and oxalato. Prefixes such as bis, tris, and tetrakis are used before complex names such as ethylenediamine, 2,2′-bipyridine, and trialkylphosphine. (4) Neutral complexes are named in the same way except that only one word is required. (5) Anionic complexes are also named according to these same rules except that an -ate ending is used. Additional rules are available to name the more complicated compounds, and some of these will be used in the discussion that follows.
Coordination numbers of 2–10 have been observed for different complex compounds. The most common coordination numbers are 6 and 4. Complexes having coordination number 6 generally have an octahedral structure, but may also be trigonal prismatic. Complexes having coordination number 4 are either square planar or tetrahedral. Table 1 gives examples of complexes having coordination numbers other than 4 or 6.
Metal complexes exhibit various types of isomerism. In many ways, inorganic stereochemistry is similar to that observed with organic compounds. Geometrical isomers are common among the substitution inert complexes of coordination numbers 4 and 6. Square planar complexes of the type Pt(NH3)2Cl2 exist in two forms, cis and trans. Likewise, cis-trans isomers of the Pt(NH2CH2COO)2 type have been isolated (Fig. 1). It is apparent from the above examples that the isomer with the same ligands or ligand atoms in adjacent positions is called cis, whereas the trans isomer has its like groups in opposite positions. There are also examples of geometrical isomers of complexes containing four different ligands (Fig. 2). Although geometrical isomerism of square complexes is most common with platinum(II), it has also been observed with compounds of nickel(II), palladium(II), and gold(III). See also: Stereochemistry
There are many examples of geometrical isomers for 6-coordinated complexes of cobalt(III), chromium(III), and the platinum metals. Most of the examples are of the type [Co(NH3)4Cl2]+ (Fig. 3). If three of the ligands differ from the other three, then only two isomers are possible (Fig. 4). Similar isomers are obtained with an unsymmetrical bidentate ligand (Fig. 5). The complex with 6 different ligands, such as [Pt(NH3)(py)(NH2OH)(Cl)(Br)(NO2)], can exist theoretically in 15 different geometrical forms.
Optical isomerism is also fairly common among these compounds. For example, 4-coordinate tetrahedral complexes containing unsymmetrical bidentate ligands, such as bis(benzoylpyruvato)beryllate (II) ion, have been resolved (Fig. 6). Optically active complexes of this type are likewise reported for boron(III) and for zinc(II). Most of the examples of optical activity occur with 6-coordinated systems containing three bidentate ligands, for example, trioxalatorhodiate(III) ion (Fig. 7). The resolution of 6-coordinated complexes of this type has been reported for the metal ions Al(III), As(V), Cd(II), Co(III), Cr(III), Ga(III), Ge(IV), Ir(III), Fe(II),(III), Ni(II), Os(II),(III), Pt(IV), Rh(III), Ru(II),(III), Ti(IV), and Zn(II).
The cis isomer of a complex containing two bidentate ligands and two monodentate ligands is asymmetric, and therefore exists in the form of mirror-image isomers, whereas the trans form is symmetrical and cannot be optically active (Fig. 8). Since the cis isomer of this type is optically active and the trans isomer is not, the successful resolution of one of the isomers has historically been used as a proof of its cis structure. Other types of isomerism are known for metal complexes (Table 2).
The synthesis of metal complexes containing only one kind of ligand generally involves simply the reaction of the metal salt in aqueous solution with an excess of the ligand reagent, reaction (1).
The desired complex salt can then be isolated by removal of water until it crystallizes, or by addition of a water-miscible organic solvent to cause it to separate. Many reactions, such as the one cited above, occur readily at room temperature. For the substitution-inert complexes (those slow to react), prolonged treatment at more drastic conditions is often necessary.
The preparation of geometrical isomers is much more difficult, and in most cases, the approach used is rather empirical. Generally, reactions yield a mixture of cis-trans products, and these are separated on the basis of their differences in solubility. The trans isomers of platinum(IV) complexes are prepared by the oxidation of the appropriate platinum(II) compound (Fig. 9). The cis isomers of cobalt(III) complexes can sometimes be prepared by the reaction of a carbonato complex with the desired acid (Fig. 10).
The easiest geometric isomers to prepare are those of platinum(II). For example, the cis and the trans isomers of [Pt(NH3)2Cl2] are prepared as shown in reactions (2).
The second step in each of these reactions results in the replacement of the ligand trans to a chloro group (Fig. 11). This phenomenon, that a negative ligand often has a greater labilizing effect on a group in the trans position than does a neutral group, for example, Cl− > NH3, is called the trans effect. Extensive use has been made of this trans effect in the synthesis of desired platinum(II) complexes. The complex cis-[Pt(NH3)2Cl2] (Fig. 11) is used as an antitumor drug for certain types of cancer.
Finally, the separation of optical isomers of metal complexes involves techniques similar to those used for organic compounds. The usual procedure is to convert the racemic mixture into diastereoisomers by means of an optically active resolving agent and then to separate the diastereoisomers by fractional crystallization. Nonionic complexes have been resolved by preferential adsorption on optically active quartz or sugars. See also: Ammine; Hydrate