You have to remove the (somewhat acidic) hydrogen on the central carbon in the acetylacetone before it will chelate to the chromium(III). That's what the urea is for. The urea reacts with water to yield CO2, ammonium ion, and hydroxide ion. The hydroxide ion then reacts with the acetylacetone, abstracting the hydrogen on the central carbon, producing the acac- ion.
The reactions involved are
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Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/moles/faq/print-yield-cracac3.shtml