Substitution Reactions without using a Solvent

In some reactions the ligand to be introduced is a liquid, and where there are no issues relating to a need for stoichiometric amounts of reagents it is possible to use the ligand in excess as effectively its own solvent in the reaction. This is particularly appropriate where the ligand can be removed readily by evaporation or distillation, or where the complex product precipitates from the reaction mixture and can be separated directly by filtration. The best example of this type of reaction is with ammonia, which has a boiling point of only −33 ◦C. For example, (6.18) is a reaction that proceeds readily, and avoids the hydrolysis problems that lead to preferential formation of Cr(OH)3 in aqueous ammonia.

While this reaction works well with most anhydrous metal ion salts, it is not necessarily convenient because of the handling difficulties involving liquid ammonia, and some salts form isolable ammine complexes sufficiently well using aqueous ammonia, as is the case with Ni(II) and Cu(II) for example. Higher boiling point and also chelating amines can be reacted effectively with anhydrous metal salts in the same manner as described for ammonia. These reactions do generate heat, as expected in an acid/base (metal cation/ligand) reaction, so careful mixing of reagents is required. In principle, this technique also offers a route to a wide range of [M(solvent)6]n+ salts through reaction of an anhydrous salt with traditional molecular solvents, provided the solvent is an effective ligand. Nitriles, amides, sulfoxides and alcohols may be introduced in this manner in many cases. Another approach which permits introduction of anionic ligands of strong acids, is to react a complex containing coordinated chloride ion directly with an anhydrous strong acid, such as trifluoromethanesulfonic acid, in the total absence of any other solvent. One example, where HCl is released as a covalent gas and leaves the anhydrous reaction mixture and the CF₃SO³⁻ anion enters the coordination sphere as an O-bound monodentate ligand is reaction with chloropentaamminecobalt (III) chloride (6.19).

The complex product can be isolated as a solid by very slow and careful addition of cold diethyl ether to the stirring cooled reaction mixture; a great deal of heat is generated in this process, so great caution is required. However, the product in this example is very useful as a synthetic reagent. The coordinated trifluoro methane sulfonate anion is an extremely labile group when bound to almost any metal ion and thus readily replaced. Consequently, these complexes find use as reagents for the introduction of other ligands through simple substitution reactions in a poorly coordinating solvent, written in general as in (6.20).

Another approach is to employ the effect of heat on a solid complex. It has been known for a long time that metal complexes, if heated strongly undergo decomposition reactions that eventually take them through to usually simple salts or oxides. Generally, ligands are lost in a series of steps related in part to their volatility, and this can be probed using a technique called thermal analysis which effectively amounts to following weight change with a sensitive balance during heating of a small sample. Simple neutral ligands often depart the coordination sphere as molecular species over a reasonably small and well-defined characteristic temperature range so that heating to a controlled temperature can allow controlled conversion to occur. The simplest examples are the hydrated salts of metal ions, such as [M(OH₂)n](SO₄) which on heating lose water to form anhydrous M(SO₄) usually with a distinctive colour change (such as from blue to nearly colourless, as seen for copper ion). For complexes containing water as one of several ligands, its loss tends to occur ahead of other ligands such as amines, allowing partial change of the coordination sphere. The metal centre involved still seeks to retain its original coordination geometry so that loss of water is usually associated with replacement in the coordination sphere by an involatile anion of the original salt such as in (6.21).

This approach permits the insertion of a wide range of stable anions apart from chloride into the coordination sphere, simply by commencing with them present as the counter-ion. At higher temperature, amine ligands can be lost in the same manner that water is lost, and may occur in a stepwise process that permits isolation of useful intermediate complexes. Eventually at sufficiently high temperature, all ligands are lost, as in (6.22).

This reaction of Pt(II) is general for a range of coordinated amines apart from ammonia, and appears to yield exclusively the trans geometric isomer (which is called, because of this exclusivity, a stereospecific reaction). Even chelated diamines can be substituted as they are inevitably more volatile than any anions present; thus chelated 1.2-ethanediamine can be replaced by two chloride ions in [Co(en)₃] Cl₃ to form cis-[CoCl₂(en)₂] Cl and by two thiocyanate ions in [Cr(en)₃] (NCS)₃ to form trans-[Cr(en)₂(NCS)₂] (NCS) on heating. Anions such as thiocyanate are ambivalent ligands that can coordinate through either of two donors, in this case the S or N atoms. Because heating is involved in these syntheses, the thermodynamically stable form will always be isolated, which in the case of thiocyanate with chromium (III) is the N-bound form. This behaviour can be easily demonstrated by commencing with the less stable isomer, and observing behaviour on heating; for exam ple pink[Co(NH₃)₅(ONO)]Cl₂ on mild heating changes to the thermodynamically stable yellow [Co(NH₃)₅(NO₂)]Cl₂, where an O- to N-donor isomerization has occurred.

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مواضيع ذات صلة

Reactions Involving the Metal Oxidation State

Catalysed Reactions

Chiral Complexes

Substitution Reactions in Nonaqueous Solvents

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)

Block d: Transition Metals

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

A New Body - Stereochemical Change

Lability and Inertness in Octahedral Complexes