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Enthalpies of addition were determined calorii~~etrically for the crystalline complexes MX, .2L where M = Zr or Hf, X = C1 or Br, and L = tetrahydrofuran or tetrahydrotliiophene. With oxygen as the donor atom of L, the two metalhalides exhibited nearly identical acceptor power but with sulfur as donor, HfGI, proved to be the better acceptor. Hafnium(1V) shows more class b character than zirconium(1V).

We report the synthesis of the ion V(DMS0)63+ and kinetics of its reactions with SCN-, sulfosalicylic acid, and 2,2-bipyridine. The complex formation rates show some dependence on the ligand. The value of AS* found for the reaction with bipyridine is quite negative. A definite mechanistic assignment cannot yet be made.Canadian Journal of Chemistry, 48, 2969Chemistry, 48, (1970 Introduction Since Taube's suggestion in 1952 (1) that octahedral V(II1) complexes might undergo ligand substitution by an associative mechanism because of the vacancy in the t 2 , subshell, the chemistry of this system has appeared to be a critical test of theories of substitution. It is interesting that crystal field models also suggest associative reactions of V(II1) complexes (2). In contrast, there is no evidence that do ions of small radius are subject to nucleophilic attack (3). Recently, the reaction of V(0H2)?+ with NCShas been discussed (4, 5) and the substantial negative entropy of activation taken as evidence of associative activation. This contention is, apparently, not supported by results of Espenson (6) who has studied reactions of V(OH2)63+ with azide and failed to find a substantial nucleophilicity difference between the two ligands,

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