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The mass spectra of a series of hexaalkyldimetals are described and discussed. The fragmentation rules available i n the literature for organornetallic compounds of group 1V B are also valid here. The PMR spectra of these derivatives have been recorded and analyzed.The mass spectra of several dimetals of group IV B, including hexaphenyldisilicium (9, digermanium derivatives R,Ge, [R' = iPr (2)

Iodine cleavage of the metal-metal bond in hexasubstituted dimetals of the fourth group has been studied kinetically. The reactivities of disiliciums, digermaniums, ditins and mixed dimetals are discussed in function of the substituents and of the metals involved in the metal-metal bond.Only a few kinetic studies have been devoted to the cleavage of metal-metal bonds M-M' by electrophiles (M and M' being metal atoms of the IV B group:The determination of the reactivity of a larger series of such dimetals with respect to a given halogen in similar experimental conditions might be usefull to throw some light on the mechanism of these reactions.The sto'ichiometry of these reactions R,M-M'R; + X, = R3MX + RiM'X is known, but we have checked [6] that this is the only reaction and that no trace of alkyl halide RX or R'X is found in the reaction medium. IODODEMATALLATION OF HEXAAKYLDISILICIUMS IN CHLOROBENZENEHexamethyl-and hexaethyldisilicium react with iodine in chlorobenzene following a clean second-order reaction [6]

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SUMMARY.Recehred : 05/03/1992 -Accepted : 14/09/1992The acidity function H. of tetrabutylammonium hydroxide in aqueous acetonitrile has been evaluated by use of H a m e t t indicators in the range of molar fractions in water lying between 0.05 and 0.65. The results are correlated with the available data concerning the structure of these mixtures.

It is shown that one ortho methyl group has a weaker accelerating effect than a para methyl group in the iododestannylation of phenyltrialkyltins, whatever the size of the alkyl chains bound to the tin atom. Two ortho methyl groups lead to a rate which is exactly that calculated assuming additivity of the effects, when the leaving group is Sn(CH3)3. If Sn(nPr)3 is the leaving group, 2, 6‐disubstitution leads to a small but significant increase in rate. The results are interpreted in terms of overcrowding in the reactant, and of relief of strain in the transition state; the observed effects are however smaller here than in the case of similar silicon derivatives, and the difference is attributed to the difference in carbon‐metal bond lengths.