VOLATILE LANTHANIDE CHELATES .2. VAPOR PRESSURES HEATS OF VAPORIZATION AND HEATS OF SUBLIMATION

Thermodynamic measurements were undertaken on a series of remarkably volatile and thermally stable lanthanide chelates of 2,2,6,6-tetramethyl-3,5-heptanedione. The complexes are unusual examples of un-solvated monomeric lanthanide chelates in which the coordination number is six. Large differences were found in the vapor pressures, heats of vaporization, and heats of sublimation of the tetramethylheptanedionato complexes. The findings corroborate the trend detected by gas chromatography; complexes of the lanthanides with higher atomic number are considerably more volatile than those of the lighter, larger members of the lanthanide series. For example, at 200° vapor pressures range from ca. 5 mm for the ytterbium(III) complex down to 0.2 mm for the lan-thanum(III) complex. A plot of the heat of vaporization of the chelates vs. the atomic number of the metal in general reflects the effects of the lanthanide contraction, but there is an irregularity at gadolinium. © 1969, American Chemical Society. All rights reserved.