THE MOLECULAR AGGREGATION OF ANHYDROUS bis-(2,4-PENTANEDIONO)-NICKEL(II) IN CCl4 SOLUTION

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Abstract
Although the trimeric nature of bis-(2, 4-pentanediono)-nickel(II), Ni(AA)2, in the solid state has been conclusively established by X-ray crystallography1 the molecular aggregation of Ni(AA)2 in solution is less certain. It had been concluded from earlier ebullioscopic measurements2 on benzene and CCl4 solutions of anhydrous Ni(AA)2 and cryoscopic measurements3 performed on benzene solutions that the trimeric aggregation was maintained in solution. These techniques give the number-average molecular weight, M n. However, a recent determination of the weight-average molecular weight, M w , of Ni(AA)2 in CCl4 solution, employing the Brillouin light-scattering technique, gave a value for M w of 2220, whereas the molecular weight of the trimer is only 771.4 From these data Ni(AA)2 was presumed to exist in CCl4 solution as a broad distribution of oligomers such that the value of the number-average molecular weight, M n , would be near the value for the trimer but population of higher molecular weight species would be such as to raise the value of M w to such a large value. Mathematically, such an argument is tenable since the value of M n is most sensitive to the lower molecular weight species present while the value of M w is sensitive to the presence of species of higher molecular weight.