Molecular Weight Studies

Kennedy, J. P. and Trivedi, P. D. Cationic Olefin Polymerization Using Alkyl Halide — Alkyl-Aluminium Initiator Systems. I. Reactivity Studies. II. Molecular Weight Studies. Vol. 28, pp. 83-151. 

Although a crystal structure has been reported for the hexamer,1 at least two distinctly different crystalline forms exist and molecular weight studies indicate that monomers, dimers, and trimers exist in equilibrium in solution. For these reasons, the complexes are referred to herein as in an indefinite state of aggregation, that is [CuH(PR3)]. ... 

Unfortunately such "effective molecular weight studies involve the major assumption that the whole deviation from ideal solutions is due to association and this assumption can only be controlled by some non-osmotic method such as the spectrophotoraetric one. Dolezalek s papers of the 19)0 era provide ample warning of the falsity of this assumption when indiscriminately applied however, where associative interaction is pronounced, comparison of spectroscopic and osmotic data shows that the careful evaluation of the latter leads to essentially correct results. 

A variety of tools address the stoichiometry and molecular weight of compounds. The necessary condition that at least two metals be present for multiple metal bond formation is a simple sorting method for initial studies the molecular unit as determined by any type of molecular weight study must correspond to that of at least two metals per molecule. Conductivity measurements supply similar data for ions, and mass spectral data can indicate the presence of at least two metals per molecule. Analytical data with nonintegral ligand-to-metal ratios require that some multiple number of metal centers be present in order to formulate a stoichiometric compound. This array of techniques only eliminates the possibility of metal—metal bonds for mononuclear metal complexes, and further studies are always necessary to confirm the presence of an attractive metal-metal interaction. 

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