HALOGEN EXCHANGE AT RE3-CLUSTERS - A NEW SYNTHETIC ROUTE TO BINARY AND TERNARY RHENIUM(III) BROMIDES - CRYSTAL-STRUCTURES OF CS2[RE3BR11] AND CS3[RE3BR3CL9]

The substitution of ''inner'' ligands in transition metal clusters in aqueous HX solutions is hitherto unknown. For the first time the substitution of bridging and terminal chloride for bromide ions was observed at rhenium clusters, [Re3(mu-Cl(i,b))3(Cl(o,t))6(Cl(i,t))(3-x)(H2O(i,t))x](3-x)- (x = 0 - 3), via the reaction of ''ReCl3 . 2H2O'' in hot hydrobromic acid solution under an inert gas atmosphere. This establishes a new synthetic route to ternary Re(III) bromides as well as to ReBr3. However, ternary Re(IV) bromides, A2ReBr6 (A = Rb, Cs), are dominating in the presence of oxygen, rhenium(III) bromides are only by-products. Dark brown rods of Cs2[Re3Br11] are obtained from argon saturated, hot hydrobromic acid solutions of ''ReCl3 . 2H2O'' and CsBr. The crystal structure (orthorhombic, Pnma (Nr. 62); a = 955.51(5); b = 1 610.29(10); c = 1372.70(9); Z = 4; V(m) = 318.0(2) cm3 mol-1; R = 0.084, R(w) = 0.058) consists of defect clusters [Re3Br3i,bBr6o,tBr2i,tsquarei,t]2- in which one in plane, terminal position is not occupied. The substitution of ''inner'' ligands has been observed in the case of chloride for bromide only, the Br(i,b) and I(i,b) ligands in ReBr3 and ReI3, respectively, are not substituted in hydrochloric acid even at temperatures as high as 100-degrees-C. Bordeaux red square pyramids of CsReBrCl3 = CS3[Re3(mu-Bri,b)3Cl6o,tCl3i,t] are obtained from hot hydrochloric acid solutions of ReBr3 . 2/3 H2O upon evaporation. CsReBrCl3 (orthorhombic, C2cm (Nr. 40); a = 1 419.0(1); b = 1 419.2(1); c = 1 080.30(8) pm; Z = 4; V(m) = 327.6(3) cm3mol-1; R = 0.033, R(w) = 0.028) is isostructural to the corresponding chloride CsReCl4.