UNIVERSITY OF ZIMBABWE ProgrammeBscd (ІІ) CourseCH202 Practical Number 4 Title d-block elements- Manganese AIMS /OBJECTIVES * To understand the reactions of manganese * To prepare Mn(acac)3 and calculate percentage yield * To calculate the percentage of Mn and acetyl acetone and the empirical formula THEORY Manganese is a first row transition metal that has a tremendous variety of oxidation states that appear in its compounds. The oxidation numbers range from Mn (III) in compounds like Mn(NO)3CO to Mn (VII) in KMnO4.
Compounds of manganese range in oxidation number between theses two extremes. This experiment involves the preparation of a Mn (III) complex of actylacetone (also named 2,4-pentanedione) which is a useful starting material for the preparation of other Mn (III) compounds. Manganese (III) complexes are relatively stable and can be prepared directly by reactions of the hydrous manganese (III) oxide or by oxidation of the hydrous manganese (II) oxide with air or an oxidizing agent. Pass,1997) In aqueous solution Mn(III) is readily hydrolyzed Mn3+ + 2 H2O >Mn(OH)2 + H+ and is most stable in acid solutions. Manganese (III) is also slowly reduced by water. (Liptrot,1993) 4 Mn3+ + 2 H2O > 4 Mn2+ + 4H+ + O2 Metal acetylacetonates are coordination complexes derived from the acetylacetonate anion and metal ions, usually transition metals. The ligand acetylacetonate is often abbreviated acac. Typically both oxygen atoms bind to the metal to form a six-membered chelate ring. The simplest complexes have the formula M(acac)3 and M(acac)2.
Mixed-ligand complexes, e. g. VO(acac)2, are also numerous. Variations of acetylacetonate have also been developed with myriad substituents in place of methyl. Many such complexes are soluble in organic solvents, in contrast to the related metal halides. Because of these properties, acac complexes are sometimes used as catalyst precursors and reagents. Applications include their use as NMR “shift reagents” and as catalysts for organic synthesis, and precursors to industrial hydroformylation catalysts. C5H7O2? n some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III). (wollins,1994)A general method of synthesis is to treat a metal salt with acetylacetone, acacH: Mz+ + z (acacH) M(acac)z + z H+ Addition of base assists the removal of a proton from acetylacetone and shifts the equilibrium in favour of the complex. Both oxygen centres bind to the metal to form a six-membered chelate ring. In some cases the chelate effect is so strong that no added base is needed to form the complex. (wilkmson,1995)
In this experiment a solution of manganese (II) chloride is oxidized with potassium permanganate in the presence of acetylacetone giving the brown acetylacetonemanganese (III), Mn (acac)3. Because the ground state for octahedral complexes like that of Mn(acac)3 is a 5Eg (t2g3eg1) there exists considerable Jahn-Teller distortion. Therefore, the complexes are not “pure” octahedral. Two forms of Mn(acac)3 are known: one with substantial tetrahedral elongation (two Mn-O bonds at 212 pm, and four at 193 pm), the other with moderate tetragonal compression (two Mn-O bonds at 195 pm and four at 200 pm).
The electronic spectrum of Mn(acac)3 shows a broad band at approximately 20,000 cm-1 (500 nm). 1 The complex forms lustrous crystals which are black to dark brown by reflected light and green by transmitted light. The Mn(acac)3 complex can be reversibly oxidized to Mn(acac)3 or reduced to Mn(acac)3 in acetonitrile solution (0. 1 M tetraethylamonium perchlorate). It has been shown that many electron transfer reactions like those above are ligand centered rather than metal-centered.
This implies that in many transition metal complexes electron transfer reactions are facilitated by stabilization of the ligand-radical product via covalent bond formation with an unpaired d electron of the transition metal center. The covalent bond energy is proportional to the negative shift in the potential for the ligand oxidation relative to that for the free ligands anion. 3 The Mn(acac)3 prepared here may be used in a later magnetic susceptibility experiment. RESULTS Fig1: rections of manganese Test | Observation | Inference |
To a 5% solution of manganese (II)4M NaOH was added and the solution was allowed to stand for 10minutes| Cream precipitate were formed, black particles were observed at the surface after 10minutes| [Mn(H2O)6]2+ (aq) + 2OH-(aq) >[Mn(OH)2(H2O)4] (s) + 2H2O (l)It reacts in air to give manganese (III) oxide4Mn(OH)2 (s) + O2(g)>2Mn2O3 (s) + 4H2O (aq)| 4M NaOH was added and the solution was allowed to stand for 10minutesAnd hydrogen peroxide was addedThe precipitate was heated with dilute HCl | Cream precipitate were formed, black particles were observed at the surface after 10minutesEffervescence occurred and the solution turned into a black precipitate in a black solutionIt bleached a dump red litmus paper| [Mn(H2O)6]2+ (aq) + 2OH-(aq) >[Mn(OH)2(H2O)4] (s) + 2H2O (l)It reacts in air to give manganese (III) oxide4Mn(OH)2 (s) + O2(g)>2Mn2O3 (s) + 4H2O (aq) MnO2(s) + 4H+(aq) + 6Cl-(aq) > [MnCl6]2-(aq) + 2H2O(l)When heated[MnCl6]2-(aq) > MnCl2(aq) + 2Cl-(aq) + Cl2(g)Chlorine was evolved| KOH was dissolved in water and manganese (IV) oxide was added and the solution heated| Solution turned from black to grey when heated| 3MnO2 + 6OH- + ==> 3MnO42- + 3H2O| KOH was dissolved in water and potassium permanganate was added and the solution heated| Remained purple| | Freshly prepared iron (II) sulphate was added to: a) 1cm3 of acidified potassium permanganate solution b) 1cm3 of potassium permanganate to which one drop of 2M NaOH was added| The purple solution was decolourisedGreen precipitate was formed| MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) > Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)MnO4-(aq) + [Fe(H2O)6]2+(aq)+ 2OH-(aq)>MnO2 (s) + Fe(OH)2(H2O)4 (s)+ 2H2O (l)| Fig2: weighing of Mn(acac)3 Mass of filter paper + Mn(acac)3 /g| 1. 9| Mass of filter paper /g| 0. 2| Mass of Mn(acac) /g| 1. 7| Fig3: titration of Mn(acac)3 Final volume /cm3| 22. 2|
Initial volume /cm3| 5. 9| Volume used /cm| 16. 3| CALCULATIONS MnCl (aq) + 2Hacac(l) >Mn(acac)2 + 2HCl(aq) 5Mn(acac)2 +MnO4-(aq) + 8H+(aq)+ acac-(aq) > Mn2+(aq) + 5Mn(acac)3(s) + 4H2O(l) n(MnCl. 4H2O) = 2. 5/125. 84 =0. 01987moles acetylacetone : 1 ml contains 0. 98g therefore 10. 5mls contains (10. 5/1)? 0. 98g = 10. 29g n(acac)= 10. 29/100. 13 = 0. 103moles limiting factor is Mn2+ yield = n(MnCl. 4H2O)? mr(Mn(acac)3) = 0. 01987? 355. 33= 7. 06g % yield =(1. 6/7. 06)? 100%= 22. 7% Titration of Mn(acac)3 Reaction of manganese (III) with iodide ions 2Mn3+ + 2I–2Mn2+ + I2 Reaction of iodine with sodium thiosulphate I2 + 2S2O32- > 2I- + S4O62-
Volume of sodium thiosulphate used 6. 3 cm3 n(S2O32-) =(6. 3/1000)? 0. 1M =0. 00063moles n(S2O32-)= 1/2? n(I2) =n(Mn3+) therefore n(Mn3+) =0. 00063moles mass of Mn in the compound is 0. 00063moles? 59. 94 =0. 038g % of Mn in the complex =(0. 038/0. 4)? 100% = 9. 44% % of acac in complex= 100%-9. 44%=90. 56% Mn:acac 9. 4490. 56 9. 44/54. 9490. 56/100. 13 0. 170. 90 0. 17/0. 170. 9/0. 17 15. 3 ? 15 Empirical formula Mn(acac)5 DISCUSSION Manganese acetylacetonates is the chemical compound with the formula Mn(acac)3. On the synthesis of Mn(acac)3, 2. 5g of MnCl2 and 10. 5ml of potassium acetyl acetone was used, which would result in an expected yield of 7. 06 g of Mn(acac)3.
But the yield was 1. 6g which means a short fall of 5. 46g, this maybe because the reaction may reach equilibrium before going to completion, the reactants may undergo reactions other than the one leading to the desired product, the desired product may undergo further reactions, product may be lost in the isolation steps, the product may be contaminated and as a result of loses due to transfers which has been done. Answers to questions 3) potassium permanganate is used to oxidise Mn (II) to Mn(III) 4) it is necessary to add sodium acetate so that the acetate will form dative bonds with the empty d-orbital created by the oxidation of Mn from 2+ to 3+. )the structure is octahedral and the type of isomerisation that it can form is geometric isomerism CONCLUSION Mass of Mn(acac)3 obtained 1. 6g % yield of Mn(acac)3= 22. 7%
REFERENCE 1. Shriver, D. F. and Atkins, P. , (1994), Inorganic Chemistry, 2nd edition, W. H. Freeman and Company, New York, pg229. 2. Liptrot, G. F. ,(1993),Modern Inorganic Chemistry, 4th edition, Mills and Boon Ltd, Britain, pg290. 3. Wilkmson,G. and Gaus,P. L. ,(1995),basic inorganic chemistry,3RDEdition,John wiley and sons INC,Canada,pg561. 4. Woolins,J. D. ,(1994),Inorganic experiments,3RDEdition,VCH publisher INC,NY,pg28. 5. Pass,G and Sutcliffe,H. ,(1997),Practical inorganic chemistry,2NDEdition,Chapman and hall,London,pg43.
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