Here again, diadsorbed seems preferable to complex,6 but good evidence supports surface intermediates which resemble -bonded diene and allyl.3-6We suggest that bicyclononane-dio and -d\% result from exchange of h,h and g,g. Adamantane (bicyclononane with carbon atoms 3 and 7 bridged by methylene) exchanges only one hydrogen atom per period of adsorption. Since all of its vicinal hydrogen atoms are rigidly staggered, this accords with predictions.2 Accordingly, the atoms at carbon atom 9 in bicyclononane will not exchange in conjunction with others barring some transannular reaction. In exchange of 1,1,3,3tetramethylcyclohexane on palladium,7 there is a maximum at da in addition to one at d¡ which results from alternation between A and B. Formation of bicyclononane-dio and -du is consistent with the proposal7 that dw in cyclopentane and d-a in tetramethylcyclohexane result from alternation between -bonded olefin and -bonded allyl. This cannot exchange h' and g'.These experiments were run in a flow apparatus with 5% palladium-on-alumina catalyst2 previously reduced in hydrogen at 350°. The partial pressures were cyclopentane, 93 mm.; bicyclononane, 8 mm.; adamantane, 5 mm.; and deuterium to 1 atm.Acknowledgment. This work was supported by the Petroleum Research Fund of the American Chemical Society, Grant 1579-C. We thank Dr.

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 unsolvated 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°v apor pressures range from ca. 5 mm for the ytterbium(III) complex down to 0.2 mm for the lanthanum(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.Although it has been known for many years that X*. ß-diketonates of metals in the first transition series are volatile, efforts to find similar volatile complexes of the lanthanides were always unsuccessful until very recently.2-4 Previously known chelates of the lanthanides were invariably either nonvolatile or thermally unstable. Indeed virtually all other reported lanthanide compounds are either nonvolatile or have very low volatility at temperatures below 700°. In part Is we recently described the synthesis and gas chromatographic behavior of the lanthanide complexes of 2,2,6,6-(1) (a)

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