what is the method of preparation of Co/Cr(acac) ?

Dear Shayma,

Acetylacetone's official name is 2,4-pentanedione but us old guys like the sound of chromium acac. Formula: CH3C(O)CH2C(O)CH3 now you may recall that it is easy to pull a H+ of a diketone like this to give the anion where the 1-ve charge is delocalized over both O atoms in the anion. Let me try to draw: H3C-C(-Oδ-)-CH-C(-Oδ-)-CH3 (internal C-C bonds should also have dotted lines over them). Mr Wikipedia does a much better job of drawing it than I do. [1] acac- is a surperb chelate (bidentate) ligand (results in 6-membered ring). So we use the OH- in NH3(aq) to pull the H+ off acacH:
acacH +OH- → acac- + H2O
Can't use strong base otherwise Cr(OH)3 will ppt.
The acac- then readily cmplxs with the Cr(III):
[Cr(H2O)6]3+ + 3acac- → Cr(acac)3 + 6H2O
Because Cr(acac)3 is uncharged it falls out of H2O solution as red/purple crystals (well it did when I last made it). Because its outer sheath consists of Me and CH groups Cr(acac)3 is soluble in organic solvents. Furthermore, Cr(III) is 3d3 with three unpaired electrons and Cr(acac)3 use to be used as a shiftless NMR relaxation agent (got to look out for shiftless agents). Nowadays, the NMR machines are so powerful that there is no need to use Cr(acac)3 to relax the C-13 nuclei in order that you can pulse them more rapidly.

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