Manganese Experiment

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  • Topic: Ligand, Manganese, Redox
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  • Published : November 7, 2012
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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− in 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...
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