Preparations-reactions of complexes, Chemistry tutorial


Many techniques are utilized in preparation of coordination complexes and in the transformation of one coordination compound into another. The preparation and reactions of complexes have produced huge research outputs in synthetic inorganic chemistry. Complexes preparation might involve; substitution reaction (replacing one or more ligands with others in a complex), direct reaction (involving only ligand and metal salt without solvent provided the ligand is a liquid or gas), decomposition reaction (where a complex is transformed to another by heating at specific temperature) and redox reaction (where change in oxidation state of the metal may lead to formation of a new complexes). Apart from the diverse techniques stated, a change in reaction conditions such as pH, temperature, solvent polarity can lead to formation of new compound. It is hence significant to learn the diverse ways that complexes can be synthesized.  

Preparation and reactions of complexes

For over 2 hundred years, coordination complexes have been generated via a variety of techniques. Among the first few complexes synthesized, Zeise's salt, K[Pt(C2H4)Cl3], known for decades, and Werner's cobalt complexes serve as template for synthesis of abundant complexes known today. Synthetic techniques employed to prepare coordination complexes range from simply mixing of reactants to variation of reaction's conditions and use of non-aqueous solvents. The techniques utilized in preparation of complexes are various and new methods keep emerging due to advancement in technology. Several of such methods will be explained in this Course

i. Direct reaction

This includes amalgamation reaction of metal salt and ligands in liquid or gaseous state. This reaction can as well be carried out in suitable solvent if both reactants are solids. Instances are 

The product of the second reaction involving chromium salt is a solid mass that might be hard to handle but through employ of inert solvent as toluene, the product can be simply filtered and dry in purer state. In the third reaction liquid ammonia is employed which can be permitted to evaporate to provide the product.

ii. Substitution reaction

The replacement of one ligand via another is the most common kind of reaction of coordination compounds, and the number of reactions of this kind is huge. Several are carried out in aqueous solutions, several in non-aqueous media, and others can be carried out in the gas phase. Abundant instances of such reactions are general and frequently carried out in qualitative test of cations using aqueous alkali solution or ammonia. 

[Ni(CO)4]  + 4PCl3   →   [Ni(PCl3)4]   + 4CO

[PtCl4]2- + NH3   →   [PtCl3NH3]- + Cl-

[Cr(CO)6]  + 3Py →  [CrCl3(Py)3]  + 3CO

[Co(NH3)5Cl]2+  + H2O    →    [Co(NH3)5H2O]3+  + Cl-

[Co(NH3)5H2O]3+   +  NCS-   →    [Co(NH3)5NCS]2+   + H2O

Substitution in square planar complexes of platinum

One observation from a large collection of experimental results is that ligands not undergoing substitution themselves can influence substitution at sites directly opposite them (trans) and, to a lesser extent, at adjacent sites (cis). Examples lie with Pt(II)  square planar complexes, where some ligands show strong  trans  effect, causing ligands directly opposite them to be more readily substituted than those in  cis  position. ligands opposite a chloro ligand in a square planar platinum complex, are substituted more readily than those opposite an ammine ligand. Experimental studies have produced an order of trans effects for various ligands that coordinate to Pt(II). 

CO ∼ CN- > PH3 > NO2- > I- > Br- > Cl- > NH3 > HO- > H2O

The significance of the order is that it can be employ to forecast the products of reactions including Pt square planar complexes and products of other related complexes. In a reaction where a ligand through stronger trans effect then Cl- is present, the chloro trans to this ligand will be substituted instead of chloro ligand that is in cis position to this ligand. The reason is because ligands with stronger trans effect form bonds that are stronger with shorter bond length hence making the ligand opposite them to be weak with longer bond length. This builds such trans ligand more susceptible to substitution. The subsequent instances illustrate the influence of the trans effect. As we know that NH3 has less trans effect there the products of the first 2 reactions are cis as imagined but CO has stronger trans consequence, the product of the last reaction established this.


iii. Reaction of metal salts

Two different metals salts, through appropriate anion that can act as ligand, combine mutually to form complex in these a way that the anion will behave as ligand. Another related reaction is one including a complex and metal salt to generate a new complex.

2AgI + HgI2   →   Ag[HgI4]

2[Ni(en)2Cl2]  + NiCl2      →    3[Ni(en)2Cl2

iv. Partial decomposition reactions

Such are reactions in that stable complexes are heated to derive out volatile ligands in order to form new complexes. The coordination number might transform and in several cases continue constant. The reactions happen in solid state.

v. Reduction and oxidation reaction

Many coordination complexes can be prepared when a compound of the metal is either diminished or oxidized in the presence of a ligand. The redox reaction can as well happen between 2 complexes where transfer of electron(s) can lead to new complexes. This technique is utilized to prepare complexes of metal ion in unstable oxidation state. For instance Co(III) solution cannot be utilized to prepare its complexes since it is extremely unstable due to its strong oxidizing ability. Complexes of the ion are prepared via oxidation of solution of Co(II) in the presence of ligand. The complexation of the Co(III) assists to prevent reduction of this extremely strong metal ion. Complexes of Cr(III) are as well prepared in similar manner.

[Fe(CN)6]4- + [IrCl6]2- →[Fe(CN)6]3- + [IrCl6]3-

4CoCl2  +  8en  +  4en.HCl  +  O2  →  4[Co(en)3]Cl3  +  2H2O

4CoCl2  +  8en  +  8HCl  +  O2  →  4 trans-[Co(en)3Cl2]Cl.HCl  +  2H2O

[Co(NH3)5Cl]2+ + [Cr(H2O)6]2+ + 5H2O → [Cr(H2O)5Cl]2+ + [Co(H2O)6]2+ + 5NH3 

[Co(NH3)5CN]2+ +[Cr(H2O)6]2+ + 5H2O → [Cr(H2O)5NC]2+ + [Co(H2O)6]2+ + 5NH3

[Cr(H2O)5NC]2+   →   [Cr(H2O)5CN]2+ (fast reaction)

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