Vol. 81 the dissociative path is probably very rapid even in alkaline solution a t 0" and, further, that a bimolecular path is expected to be important because of the possibility of forming a transition state by attachment of an ethylenediamine molecule in an axial position. Thus, bimolecular exchange may occur through rapid formation and dissociation of the tris complex, in spite of its low thermodynamic stability constant . I 3 Bjerrum and Nielsen suggested that the third ethylenediamine molecule in the tris complex is monodentate and that copper(I1) has coordination number 5 in the complex. Although such a structure may be involved as an initial step in the reaction, it cannot be the only form of the tris complex. If proton exchange results reported in this paper are indeed explained by the biinolecular exchange of ethylenediamine, it is necessary that the ethylenediamine ligands be equivalent in soiiie intermediate or transition state, (13) J. Rjerrum and R J. Xielsen, Acto Chem. Scand., 2, 297 (1948). most probably one having nearly octahedral symmetry. It should be noted that the bimolecular exchange rate constant expressed in terms of ethylenediamine exchange is because there are four exchangeable protons in each ethylenediamine molecule.The observed activation energy, 4.5 kcal./mole, is abnormally low for unimolecular (or pseudo-unimolecular) dissociation of a transition metalethylenediamine chelate. However, it is quite reasonable for an "associative" reaction Cu(en)2(H20)2++ + en + Cu(en)s++ + 2H20 If that reaction involves the rate-determining step in proton exchange, the ensuing "dissociation" Cu(en)d*+ 4-2H20 ---t Cu(en)z(H?0)2++ + en (15) iiiust be rapid, in spite of an anticipated high activation energy, because of a large entropy of activation..Ivs~rv, TEXAS kenf =i ka'/4 (13)

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edge the generous support of the Corn Industries Research Foundation, Washington, D. C., during the course of this work.

Examines the important properties of flammable materials and the health hazards associated with toxic materials.

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