Dihydroxo-bridged complexes

Dihydrogen complexes, osmium, 37 3(X)-301 Dihydroxo-bridged complexes acid-base equilibria, 32 108-110 quantitative considerations, 32 115-118 binuclear, 32 66-67 crystallographic data, 32 61, 63 stability complexes, 32 103-104... 

The yields of the dihydroxo-bridged complexes vary and are often relatively low, probably owing to the formation of different isomers and higher polynuclear complexes and to hydrolysis of the ligand L. For many of the complexes listed above, the preparative procedures involved the preparation of the mononuclear species in situ. 

With a few exceptions, all of the dihydroxo-bridged complexes listed above may exist in several, and often many, isomeric forms. The topic is discussed briefly in Section II, and a few additional comments seem relevant in the present context. 

Dinuclear dihydroxo-bridged complexes can often be obtained from the parent mononuclear complexes by the solid-state reaction Eq. (7). This was first reported by Werner (7, 11) and Dubsky (18), and it is generally the most convenient method for the preparation of dihydroxo-bridged complexes of Cr(III), Co(III), Rh(III), and Ir(III) with L4 = (NH3)4 or (en)2 [and (tn)2 in the case of chromium(III)] (67, 131, 133, 214 219). With the exception of the ammonia chromium(III) complex, these reactions are essentially quantitative and the rate of reaction follows the order chromium(III) > cobalt(III) > rhodium(III) >... 

Fig. 11. Intermolecular hydrogen-bonded pairs of aqua hydroxo complexes have been found in several crystal structures and are probably always present in those salts of the type cis-[ML4(H20)(0H)lX2, which yield dinuclear dihydroxo-bridged complexes upon heating in the solid state.

The heterobinuclear complex A,A-(H20)(en)2Cr(0H)Co(en)2(H20)5+ can be obtained similarly but has been characterized only in solution (215). (H20)5Cr(OH)Cr(H20)5+ has likewise been characterized only in solution, and can be made by acid hydrolysis of the dihydroxo-bridged complex (26). 

Dihydroxo-bridged complexes may deprotonate to form /(-hydroxo-p-oxo and di-/i-oxo species, as shown in Eqs. (35) and (36). 

Fig. 16. Intramolecular hydrogen bond stabilization of dihydroxo-bridged complexes.

Acid hydrolysis of the dihydroxo-bridged complex frans-(H20)(tacn)-Rh(0H)2Rh(tacn)(H20)4+ yields mononuclear triaqua complex (100). Only one reaction step was observed for [H+] = 0.2-1.0 M, the rate expression being of the form kobs = a + 6[H+], If it is assumed that the first bridge cleavage is faster than the second, as has been found for the corresponding tetraammine and bis(ethylenediamine) complexes, the observed [H+] dependence can be interpreted in terms of Scheme 4, which for large [H+] leads to the approximate expression kobs = 4 + ( 5/I a5)[H+]. Values for k4 and ksIKaS are listed in Table XXVII. 

XI. Equilibria between Tri- and Dihydroxo-Bridged Complexes A. Cobalt(III)... 

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