Metal Chlorides undergo hydrolysis in water. So we add acid to prevent that. Now if we add the ligands and if the solubility product is low, we may end up with the precipitate. But do we need to play with the pH?
Yes and no. When you dissolve metal halides you form the corresponding aqua complexes, e.g. FeCl2 + H2O = [Fe(H2O)4Cl2]. This complex might further hydrolysis to [Fe(H2O)6]2+ + 2 Cl-. Additionally this complex might undergo deprotonation: [Fe(H2O)6]2+ + H2O = [Fe(H2O)(OH)]+ + H3O+.
Some metal salts of highly charge metal ion, .e.g. Al3+ or Fe3+ form highly acidic aqua complexes which might undergo massive deprotonation and form the corresponding hydroxido complexes [M(H2O)3(OH)3]. These neutral complexes might precipitate from aqueous solutions forming the corresponding solid hydroxides M(OH)3. In the case of Fe3+ and Al3+ this accually happens and addition of an acid might be a good idea.
If you add a suitable ligand the aqua, chlorido or hydroxo ligands might be replaced provided the incoming ligand is stronger than these three. E.g. if you add 3 equivalents of bpy (2,2'-bipyridine) you will end up with [Fe(bpy)3]2+ + 2 Cl- (in solution). If you evaporate this solution you will be able to crystallise the compound [Fe(bpy)3]Cl2.
Thus, adding an acid to an aqua complex (=metal salt in aqueous solution) will only have the effect that the aqua complexes are stabilised over the deprotonated hydroxo complexes. For the exchange of ligands it depend whether H2O or OH- are more easiier to be replaced. This depends on the metal ion, there are examples in which OH- is easier to be replaced than H2O (see also: trans effect series). Thus addition of acid would be unbeneficial.
I hope this is satisfactory
Strange. Water is rarely used in the modern preparative chemistry. Addition of acid is not good recipe, although may work in some cases. Generally, addition of acid is leading to increase a rate of dynamic processes in the system and facilitates formation of anionic/protonated cationic complexes. Is your metal chloride soluble in the solvent other than water ? The solvolusis rate/degree is depending on the solvent. Choose the appropriate solvent and carry out the reactions there.
Hi Vasantha you should be very careful if you are using metals those form metal hydroxide easily. So if you increase the pH much higher then you may get metal hydroxide ppt. ,you must use dry solvent.
you can try buffers with different PH .To prevent hydrolysis in water chelating agent can be used also but that depends on what ligand you are using for complexation.
Yes and no. When you dissolve metal halides you form the corresponding aqua complexes, e.g. FeCl2 + H2O = [Fe(H2O)4Cl2]. This complex might further hydrolysis to [Fe(H2O)6]2+ + 2 Cl-. Additionally this complex might undergo deprotonation: [Fe(H2O)6]2+ + H2O = [Fe(H2O)(OH)]+ + H3O+.
Some metal salts of highly charge metal ion, .e.g. Al3+ or Fe3+ form highly acidic aqua complexes which might undergo massive deprotonation and form the corresponding hydroxido complexes [M(H2O)3(OH)3]. These neutral complexes might precipitate from aqueous solutions forming the corresponding solid hydroxides M(OH)3. In the case of Fe3+ and Al3+ this accually happens and addition of an acid might be a good idea.
If you add a suitable ligand the aqua, chlorido or hydroxo ligands might be replaced provided the incoming ligand is stronger than these three. E.g. if you add 3 equivalents of bpy (2,2'-bipyridine) you will end up with [Fe(bpy)3]2+ + 2 Cl- (in solution). If you evaporate this solution you will be able to crystallise the compound [Fe(bpy)3]Cl2.
Thus, adding an acid to an aqua complex (=metal salt in aqueous solution) will only have the effect that the aqua complexes are stabilised over the deprotonated hydroxo complexes. For the exchange of ligands it depend whether H2O or OH- are more easiier to be replaced. This depends on the metal ion, there are examples in which OH- is easier to be replaced than H2O (see also: trans effect series). Thus addition of acid would be unbeneficial.
I hope this is satisfactory
If basic properties of your target ligand are greater than that of alcohol (MeOH or EtOH ?), it will substitute alcohol anyway and you will obtain a desired complex. The scale of acidity in water and alkohol are totally different. You can find it anywhere. Why you need to adjust pH ? Are you targeting specific anions / cations, which exist only at the certain pH ? The negative drawback of alcohol is that the coordinated alcohol might turn in to the alcoxide ligand - very common phenomena for the group III, IV, V transition metal complexes. Why not to play with the acetonitrile or THF or pyridine. Anyway the solvolusis of metal chlorides is well documented in these solvents. A simple search in SciFinder or Reaxys will give you an answer which solvent to use very quickly. the actual solution depend on the chloride used.
Dear Prof. Axel Klein your answer is very accurate, what is your suggestion if some one is using an acid as a ligand to form iron(III) complex, so here is it not necessary to add one equivalent of base so that the ligand (acid) get deprotonted and binds the metal easily.
If I understood correctly, your ligand will be added in the protonated form H-L.
My answer is then: If H-L is very acidic (pKA < 2-5) it might not be necessary to deprotonate him using an additional base. For H-L with higher pKA it will probably be necessary, since then the main form of the ligand is protonated and thus not coordinating.
Ok. One of the method by which you can decide a suitable pH is by solvent extraction.Prepare your complex at variable pH values i.e Literally from pH=1 to pH=14. So now consider you have 14 complexes(of same metal ion & ligand) in front of you of different pH values i.e from pH=1 to pH=14.Now ofcourse all your complexes will serve as aqueous medium of known equal volume (say 25 mL).To all above 14 complexes one by one add known equal volume (say 25 mL) of organic solvent. Only that organic solvent in which your complex is totally soluble. Equilibrate each complex in separating funnel.Your complex will get extracted in organic layer.Now measure the absorbance of each complex one by one at different wavelengths.You will get lambda max. Now your wavelength will get fixed.So now just check one last thing regarding those 14 complexes, that which complex will give you maximum absorbance for that lambda max & at what pH value.That particular pH value of complex will be the most suitable pH for your complex preparation.Suppose your lambda max is 500 nm & you get maximum absorbance for complex of pH= 9 at 500 nm then be happy for the fact that you actually established a suitable pH value i.e pH= 9 for your that particular metal ion (say Nickel) - ligand complex.Then plot a calibration curve, you will get more accurate pH value graphically.
it will depend mainly on three factors- 1)tendency of metal ion involved to get hydrolyzed 2) pK value of the ligand and the concentration range 3) expected stability constant of the complex
The pH of the solution to isolate the complex in solid form is depends on the nature of ligand and the metal ion. Most of the time if ligand is in weak acidic in nature, then the pH of the solution is in basic to form the complex.
You can estimate the pH value corresponding to the maximum percentage of a given complex formed by a metal ion with a given ligand provided a series of determinations have been carried out beforehand.
1- As you are supposed to work in water (as not specified otherwise), you must first determine the pKw, the ligand protonation constants and the metal hydrolysis constants under your experimental conditions (temperature, ionic strength, metal and ligand concentration ranges),
2- Then, once all these parameters have been determined, you must calculate the formation constants (and solubility products if any precipitate is formed) of the metal-ligand complexes under the same experimental conditions.
3- Finally, you will introduce all these parameters in the appropriate speciation computer program, together with the metal and ligand total concentrations you need to explore (preferably within the concentration ranges used above in 1- and 2-) and you will obtain the pH ranges at which the species you want to form will reach its maximum percentage for each couple of total metal and ligand concentrations.
All other techniques will amount to guess these pH values somewhat "out of your thumb".
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