The thermal dissociation of 1-phenyl-3-methyl-4-(X-phenylhydrazo)-5-pyrazolone metal chelates [M(XPhHyPy)](X=m-OH (I),m-OCH3(II),m-COOH (III),p-CH3 (IV),p-OCH3 (V) orp-COCH3 (VI) was studied by TG, DTG and differential thermal analysis (DTA). A rough sequence of thermal stability, obtained from
the peak maximum temperatures, for the various metal chelates was Hg(II)<Cu(II)<Fe(III)<UO2(II). The bonding of the ligands to metal ions was investigated by elemental analysis and infrared spectroscopy. The number
and relative energies of nitrate combiantion frequencies are discussed in terms of the complexation of para-substituted hydrazopyrazolone
with Th(IV) and UO2(II) metal ions.