Bridged compounds 2 , axial substitution

Attempts to isolate Os2piv4Br2 were not successful. However, adjustment of a 2 X 10 M solution of Os2piv4Cl2 to 2 x 10" M in tetraethylammonium bromide resulted in rapid (< 5 min) and complete conversion to a complex with a spectrum attributable to the dibromo derivative. No attempt was made rigorously to exclude moisture in these experiments, and it is likely that the axial substitution reaction would otherwise be much slower [12].Onalongertime scale, > 15 min, an additional slow reaction became evident. As a similar reaction was observed in the presence of excess tetraethylammonium chloride, substitution of halide for bridging carboxylate probably takes place. A partially substituted compound of this type has been structurally characterized [13j,and the Os2Xg" anions have been prepared from the acetate under forcing conditions [6]. 

The main difference between the structures of compound (IV) and compounds (I), (II), and (III) is that in compound (IV) half of the bridging and terminal bromine atoms, and all the axial bromine atoms, are substituted by iodine atoms, with partial substitution being statistical [75]. Thus, according to the X-ray diffraction data, the effective Tc-Tc distances are intermediate between analogous distances in bromide and iodide complexes of technetium(IV) (Table 1) [104,105]. We observed that the substitution of I for Br has almost no effect on the Tc-Tc distances. 

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