Metal Ligand Preferences

The various metals show differing complexation tendencies and preferences for donor atoms. Ions of the alkali metals (Li, Na, K, Rb, Cs) have little electron pair acceptor tendency, although in aqueous solution they are hydrated by predominantly ion-dipole interactions. This tendency is greater for the smaller ions, which also have the lowest energy orbitals and greatest covalency in their interactions. Complexing is weak for this family. The alkaline earth ions, especially Mg(II) and Ca(II), are very common in the environment. Owing largely to their higher charge densities, they show a larger tendency to attract the ligand electrons than do the alkali metals, and complex-ing is important in their behavior. Much of the interaction can be ascribed to Coulombic forces, and since the metals are quite electropositive, buildup of electron density on them is unfavorable. Their strongest complexes are formed with strongly electronegative donors such as F— and oxygen ligands, which form highly polar bonds. Much more covalent in its bonding with ligands than the other alkaline earths, Be(II) forms stronger complexes than the other members of its family, and in biological systems it may act as a poison because it preferentially ties up coordination groups that are normally involved more weakly with Mg(II) or Ca(II). Chelation is very important for the formation of stable complexes with this family. The greater stability of chelate complexes in comparison to those with analogous unidentate ligands lies not so much with stronger metal-ligand bonding, but rather with an entropy effect. The number of degrees of translational freedom, and therefore the entropy, is increased by replacing several unidentate ligands (e.g., water molecules) by a polydentate one, and so increasing the number of independent species (see Section 9.5.5).

Ions of charge greater than 2+, and transition metal ions generally, are strongly complexed by a variety of ligands. The fluoride ion, and ligands with oxygen donors, are favored by ions of electropositive metals such as A1 and those of first three or four transition element families and also most metal ions in high oxidation states. Nitrogen donors become favored by the mid-transition element families in their lower oxidation states [e.g., Co(II), Fe(II)] although higher states still prefer F4 and oxygen donors. Ions of some of the heaviest and the least electropositive metals [Pt(II), Au(III), Hg(II)] favor larger, more polarizable (more covalent) ligands such as I4 or CP in preference to F4, and sulfur or phosphorus ligands in preference to oxygen or nitrogen donors. For example, the gold in seawater is believed to be present as AuCl4. However, it may be noted that the large amount of chloride in seawater for the most part is not involved in complexing because the predominant cations prefer oxygen donors such as water or carbonate.

Metal-ligand interactions are examples of Lewis acid-base behavior, and the preference of a metal ion for a particular ligand can be correlated with the concept of hard and soft acids and bases. Lewis acids and bases can be ranked according to an empirical property of hardness or softness, which is determined by such factors as size, charge, polarizability, and the nature of the electrons available for interaction. A hard acid typically is small, with a high positive charge and tightly held, chemically inactive electrons, while a soft acid is large, with a small or no positive charge and electrons in orbitals that are readily influenced by other atoms. Hard bases are small, and of high electronegativity, while soft bases are large, with easily distorted electron clouds. (Primarily, these are properties of the acceptor or donor atoms in polyatomic acids and bases. However, the properties of an individual atom in a molecule are often strongly influenced by the rest of the molecule, especially when there is extensive electron delocalization.) The examples in Table 9-3 illustrate these classes. There is a continuous range of this property, but no numerical values can be given.

TABLE 9-3

Some Lewis Acids and Bases on the Hard-Soft Scale

TABLE 9-3

Some Lewis Acids and Bases on the Hard-Soft Scale

Hard acids

Intermediate acids

Soft acids

H+, Li+, Na+ ,K+ ,Be2+, Mg2+ ,Ca2+,

Fe2+,Co2+,Ni2+,Cu2+,

Cd2+,Pd2+,Pt2+,Hg2+ ,Cu+,

Al3+,Cr3+, Fe3+, high oxidation

Zn2+,Pb2+,Sn2+

Ag+,Tl+

state metal ions

Hard bases

Intermediate bases

Soft bases

F4, OH4 ,H20, NO44, C024, carboxylic acid anions (RCOO4), NH3, aliphatic amines (RNH2)

Br4,N04, aromatic amines (e.g., C6H5NH2), pyridine

I4, CN4, CO, H4, organic sulfur compounds (R2S), phosphorus and arsenic compounds (R3P, R3As), ^-electron donors such as C2H4 and QH^R4

The utility of the hard-soft concept is simply the rule that like prefers like; that is, interaction among species of similar hardness or softness is preferred over interactions involving a large difference in hard-soft properties. Thus, a metal such as Hg(II), which is soft, prefers to complex with sulfur rather than oxygen donors. This is not to say that complexing between hard and soft species will not take place, only that it is normally less favorable than if these properties are matched.

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