Monohydroxo-bridged complexes

Monoglyme, 3 248 Monohydroxo-bridged complexes acid-base equilibria, 32 107-108... 

B. Kinetics of the Cleavage of Dinuclear Monohydroxo-Bridged Complexes... 

The monohydroxo-bridged complexes of the type L5M(0H)ML4X"+ are known for M = Cr(III) and may exist as cis and trans isomers cis and trans refer to the position of X relative to the bridge. Both isomers of the aqua erythro cation (NH3)5Cr(OH)Cr(NH3)4-(H20)5+ are known and crystal structures of their salts have been reported (cf. Table I). 

Condensation to monohydroxo-bridged complexes is often described by Eq. (28), for which the equilibrium constants Kd are related to those defined by Eq. (27) by Kd = Q2i/Qh, where is the first acid dissociation constant of the mononuclear aqua ion. 

Acid Strength of Water Ligands in Monohydroxo-Bridged Complexes Affected by Intramolecular Hydrogen Bonds at 25°C in 1.0 M (Na,H)C104... 

Fig. 15. Intramolecular hydrogen bond stabilization of monohydroxo-bridged complexes.

The Kh values in Table XXI show some obvious and very reasonable trends. For the monohydroxo-bridged complexes it is seen that Kh is very sensitive to changes in the first coordination sphere, but is practically insensitive to the nature of the metal centers, i.e., logffH(NH3) 2.3 and logf H(en) 3.1, independent of the metal ion. 

Cleavage of the studied qualitatively. In 4 M HC104, cleavage of the first bridge is much faster than that of the second bridge and the product /ac,/ac-(H20)2(NH3)3Cr(0H)Cr(NH3)3(H20)25+ has been isolated. It was shown that the monohydroxo-bridged complex at pH 2 reforms the dihydroxo-bridged species (87). 

The binuclear Co(n) cryptate (monohydroxo bridged) was also found to combine reversibly with dioxygen yielding a doubly-bridged cascade complex containing both hydroxo and peroxo bridges within the cryptate structure. 

Bridge cleavage of di- and trihydroxo-bridged complexes in acidic or basic solution provides a convenient method for the synthesis of the corresponding monohydroxo-bridged (32-36, 133, 214-218) and dihydroxo-bridged (50, 76, 100, 132, 174, 185, 186, 188) species, as exemplified by reaction Eqs. (18)-(20). 

Most of the kinetic studies made have been spectrophotometric and have employed acidic or neutral solutions. For the ammine and ethylenediamine complexes pseudo first-order rate constants were obtained for solutions initially containing either monohydroxo-bridged species [ obs(M)] or dihydroxo-bridged species [ obs(D)]. It was found that kobs(M) = obs(D), which is consistent with reversible first-order kinetics. 

Bridged Dicobalt Complexes.— Aquation of the monohydroxo-bridged cation [(NH3)5Co OH Co(NH3)6] + in aqueous HCIO4 (10 — 2M), at an ionic strength of 2.0M (LiClOJ, follows the rate law... 

---