Preferences– Do You Like What I Like?

 Ligand preference for and affinities of metal ions present themselves as experimentally observable behaviour that is not easily reconciled. As a general rule, when a metal (M) is mixed with equimolar amounts of ligands A and B, the result is not usually equimolar amounts of MA and MB. Both metal ions (Lewis acids) and ligands (Lewis bases) show preferences. In seeking an understanding of this phenomenon, we can sort ligands according to their preference for forming coordinate bonds to metal ions that exhibit more ionic rather than purely covalent character. Every coordinate covalent bond between a metal ion and a donor atom will display some polarity, since the two atoms joined are not equivalent. The extreme case of a polar bond is the ionic bond, where formal electron separation rather than electron sharing occurs; coordinate bonds show differing amounts of ionic character. Small, highly charged entities will have a high surface charge density and a tendency towards ionic character. In a sense we can think of these ions or atoms as ‘hard’ spheres since their electron clouds tend to be drawn inward more towards the core and thus are more compressed and less efficient at orbital overlap. A large, low-charged entity tends to have more diffuse and expanded electron clouds, making it less dense or ‘soft’, and better suited to orbital overlap. This concept, applied to donor atoms and groups, leads to us defining ‘hard’ donors as those with a preference for ionic bonding, and ‘soft’ donors as those preferring covalent bonding. An important observation is that the ‘hard’ donor atoms are also the most electronegative. Therefore, arranging donor ions or groups in terms of increasing electronegativity provides us with a trend from ‘soft’ to ‘hard’ ligands:

The ligands at each ‘end’ show distinct preferences for different types of metal ions. The F− ligand binds strongly to Ti4+ whereas CN− binds strongly to Au+, and it is clear these metal ions differ in terms of both ionic radius and charge. Any individual metal ion will display preferential binding when presented with a range of different ligands. This means that metal ions can be graded and assigned ‘hard–soft’ character like ligands; however, by convention, we define the least electronegative as ‘hardest’ and the most electronegative as ‘softest’. For the metals, they grow increasingly ‘soft’ from left to right across the Periodic Table, and also increase in softness down any column of the table. This definition for metals allows us to apply a simple ‘like prefers like’ concept– ‘hard’ ligands (bases) prefer ‘hard’ metals (acids)‘soft’ ligands (bases) prefer ‘soft’ metals (acids). This principle of hard and soft acids and bases (HSAB), developed by Pearson is a simple but surprisingly effective way of look in gat experimentally observable metal–ligand preference. To see the concept in action, we should look at a few examples of how it allows ‘interpretation’ of experimental observations; yet, without a strong theoretical basis the concept is somehow unsatisfying. One example involves the long-established reaction of Co²⁺ and Hg²⁺ together in solution with SCN−.The ligand has both a 'soft’ donor (S)and a‘hard’donor(N)available; although like-charged, Hg²⁺ is larger than Co²⁺ and further across and down the Periodic Table, and thus ‘softer’. This reaction results in a crystalline solid [(NCS)₂Hg(-SCN)2Co (NCS)₂Hg (SCN)₂] with each metal in a square plane of thiocyanate donors, with four S atoms bound to each Hg2+ and four N atoms bound to the central Co²⁺. Only this thermodynamically stable product where the softer S bonds to softer Hg²⁺ and harder N bondstoharderCo2+is known. If the ‘wrong’ end of a ligand like thiocyanate binds initially to a mismatched metal ion, it will usually undergo a rearrangement reaction to reach the stable, preferred form. Where SCN− is for cedinitially to bond to the ‘hard’Co3+ through the ‘soft’ S atom, it undergoes rearrangement to the form with the ‘hard’ N atom bound readily. It is possible to apply the concepts exemplified above to reaction outcomes generally. In all cases it is a comparative issue; for example, a ligand defined as ‘harder’ versus one other donor type (such as OH₂ versus Cl−) may be considered ‘softer’ if compared with another type of ligand (such as NH3 versus OH₂); this aspect will also be true for metal ions. Definition of hard/soft character is the result of empirical observations and trends in measured stability of complexes. For example, hard acids (such as Fe²⁺) tend to bind the halides in the order of complex strength of F−>Cl−>Br−>I− and soft acids (such as Hg²⁺) in the reverse order of stability. However, as with any model with just two categories there will be a ‘grey’ area in the middle where borderline character is exhibited. This is the case for both Lewis acids and Lewis bases. Selected examples are collected in Table 3.2 below; a more complete table appears later in Chapter 5. The ‘hard’–‘soft’ concept is, from a perspective of the metal ion, often recast in terms of two classes of metal ions as follows:

Class A’ Metal Ions (‘hard’)– These are small, compact and not very polarizable; this group includes alkali metal ions, alkaline earth metal ions, and lighter and more highly charged metal ions such as Ti4+.Fe3+. Co3+.Al3+. They show a preference for ligands (bases) also small and less polarizable.

Class B’ Metal Ions(‘soft’)–These are larger and more polarizable ;this group in cludes heavier transition metal ion such as Hg2+.Pt2.Ag+ as well as low-valent metal ions including formally M(0) centres in organometallic compounds. They exhibit a preference for larger, polarizable ligands. This leads to a preference pattern outlined in Figure 3.27. There is also a changing preference order seen across the rows of the Periodic Table, with Class A showing a trend from weaker to stronger from left to right, and Class B showing the opposite trend of stronger to weaker, defined in terms of the measured stability constant of ML complexes formed.

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دراسة: تناول القهوة يوميا يخفض خطر الإصابة بالاضطرابات النفسية

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مواضيع ذات صلة

Coupling– Polymetallic Complexes

A Separate Race - Organometallic Species

Supersized - Binding to Macromolecules

Vive la Difference– Mixed-metal Complexation

Polynucleating Species - Molecular Bigamists When One is Not Enough

Many-Armed Monsters - Introducing Ligand Shape

Meeting More Metals - Bridging Ligands

More Teeth Stronger Bite - Polydentates

Greed is Good– Polydentate Ligands The Simple Chelate

Monodentate Ligands– The Simple Type Basic Binders

Ligands with More Bite– Denticity

Different Tribes– Donor Group Variation