Bridged Dicobalt Complexes

Bridged Dicobalt Complexes.— There is evidence for intramolecular substitution in amido-bridged binuclear cobalt(m) complexes of formula (1) (X = Cl, Br n = 4). The reaction (1) - (2) occurs much more rapidly than would be expected from knowledge of reactivities of analogous [CoX(NH3)s] complexes. The simplest explanation of this is that the favourable geometry of the binuclear complex constrains the potentially bridging oxygen atom to be close to the cobalt(iii) centre from which X  

Bridged Dicobalt Complexes.—A study of the acid-catalysed bridge-cleavage reactions of the [(nta)Co-p-(OH)a-Co(nta)] ion has revealed the presence of two isomers in solution [probably (21) and (22)] with differing reactivities. The faster-reacting species is characterized by an observed rate constant, / =0.35 0.05 s at 

Finta and Cs. Virhelyi, Acta Chim. Acad. Sci. Hung., 1974, 83, 281. 

2 X 10 mol H+ (298.1 K, 1= 1.0 mol dm ), the reaction being first-order in [H+]. The slower-reacting complex ion shows a more complex dependence on [H+] as expected for a mechanism of the following type  

Two groups have reported the formation of dinuclear complexes between cobalt-(m) and iron(ii) and iron(iii) complexes. Discussion of these will be found in the sections concerned with iron complexes. 

Bridged Dicobalt Complexes.—Rate constants for the forward and reverse reactions of aquation and chloride anation for the equilibrium between (7) and (8) have been determined. The amido-bridged chloro-aquo-complex (8) can Equate further to give the amido-hydroxo-bridged species (9), which 

Two studies have been made of the hydroxo-bridge cleavage of the di-/u--hydroxobis[bis ethylenediamine cobalt(m)] complex (14). Certain differences in the interpretation of the results are apparent. In perchloric acid, aquation and bridge cleavage take place the same seems to happen in nitric acid, but in hydrochloric or sulphuric acids, whose anions are potential ligands, there is some anation as well as aquation and bridge 

Acid aquation of the bisoxalato-di-/x-hydroxo-complex (15) involves bridge fission to give dioxalatodiaquocobaltate(m) previous to electron transfer, en route to the final cobalt(n) product. Another reaction involving substitution and redox is that of (16) with nitrous acid. The active 

Kinetic studies of photochemical reactions of bridged dicobalt complexes are discussed in the appropriate section later. Thermal deaquation of solid cw-[Co(NH3)4(OH2)2]2(S04)8,3H20, though not of m-[Co(NHa)4(OH2)-(804)] + salts, leads to the formation of the sulphate-bridged species cis,-c/j -[(S04)(NH3)4Co(jLt-S04)Co(NHa)4(S04)], which aquates rapidly in aqueous solution to produce c/j-[Co(NH3)4(OH2)(S04)]+. 

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 

Kinetic parameters for the two aquation pathways are reported in Table 6. The kinetic characteristics of aquation of this complex are compared with those for other bridged dicobalt complexes containing various combinations of hydroxo- and amido-bridges. There is also some comparison with aquation of bridged complexes of other metals thus, for instance, aquation of [(NH3)5Cr OH Cr(NH3)5] + follows a rate law of type (4) above but with no measurable contribution from the term.  

Varhelyi, J. Zsako, and Z. Finta, J. Inorg. Nuclear Ghent., 1972, 34, 2583. 

The kinetic pattern for aquation of the newly characterized iU-amido-At-selenato-dicobalt complex (11) is similar to that established for its / -amido-A -sulphato-analogue. In both cases the product is (12), and there is strong kinetic evidence for transient intermediates of the type (13). A rate law of the form shown in equation (4) above operates kinetic parameters for aquation of the Ai-amido-jM-selenato-complex are included in Table 6. A preliminary report on the kinetics of aquation of the closely related acetato-complex (14) indicates that the rate law of equation (4) also applies here, but the rate law for aquation of the yU-amido-/ -phosphato-complex (15) is more complicated. 

Some qualitative information on the lability of several A -peroxo-dicobalt ammine or amine compounds can be gleaned from a primarily spectroscopic paper dealing with these species. Further mention of kinetic studies of bridged dicobalt complexes will be found later in this chapter, in the sections on metal-ion-catalysed aquation and on formation reactions. 

In the presence of excess H+ and Br ions, pseudo-first-order rate constants (Jc) for the equilibration of (9) and (11) are given by 

In equation (15) and are rate constants for the cleavage of the hydroxo-bridged species (9) with and without acid catalysis respectively, and k is the rate constant for the back reaction. At 298.1 K, = 4.33 0.171 mol S = 

This study is typical of several hydroxo-bridge cleavage reactions made in recent years in the presence of co-ordinating anions (X ) for which the rate equation (15) applies. In a recent communication values of ki [equation (15), Br- = X ] and associated activation parameters have been summarized for various anions (X ) and these results are collected in Table 8. Although these data are believed to refer to 

A further study of the acid-catalysed cleavage of the singly bridged dicobalt(m) ion [(NH3)6Co-iU-(OH)-Co(NH3)5] + has been made in an attempt to find evidence for the elusive five-co-ordinate intermediate ion [Co(NH3)6] +. The reaction was investigated as a function of [H+] (10- —1.0 mol 1 ) and added anion concentration ([Y ], Y = Cl or MeSOs), and the observed rate constant (A ) was fitted to an equation of the type 

Equation (16) is interpreted as involving ion aggregation as well as protonation of the 

lliewalt, M. Zehnder, and S. Fallab, Helv. Chim. Acta, 1977, 60, 867. 

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Reaction of Co2(CO)g with Ph2GeH2 proceeds smoothly at room temperature to afford a diphenylgermylene-bridged dicobalt complex in high yield [Eq. (40)].75 In this reaction, the A-type complex Co(CO)4 2(/u.-GePh2) can be ruled out as an intermediate, because under the same conditions, it does not lose CO to give the product. 

Amido transition-metal complexes are hydrolyzed similarly . Complex species, such as the amido-bridged dicobalt complex, undergo reversible hydrolysis in aq acid ... 

Co(S03)en2]+ as an intermediate.Kinetic investigations of reactions of bridged dicobalt complexes have included two v/hich involve replacement of water by other ligands (see above).23.24 a preliminary report of the reaction of [CoR(DMG)2(OH2)] with ligands has appeared, and the formation of [CoCI(DMG)2(PR3)] from cobalt(ii) chloride in ethanolic solution has been described. 

A -benzylidene-2-hydroxyaniline (Bha) and its related derivatives (a-f. Figure 47) can be catalytically oxidized by Oj in the presence of the hydroxo-bridged dicobalt complex [Co2L( i-OH)] (Figure 46) H3L =2,6-bis[(2-hydroxy phenyl)imino-methyll-4-methylphenol in DMF at... 

Cobalt(m).— Anation of [Co(NH3)6(OH2>] + by sulphate or chloride proceeds by a dissociative interchange mechanism, in which the reactive species are ion-pairs and ion-triplets, The activation enthalpy for anation of [Co(NH3)6(OHa)] + by glycine is 29 kcal mol again a dissociative mechanism is operative. An earlier study of oxalate anation of [Co(en)a(OH2)2] in acidic solution is complemented by a study of oxalate anation of [Co(en)a(OH2)(OH)] + in basic solution. Several examples of anation reactions of bridged dicobalt complexes have been mentioned in the section on aquation of these complexes. ... 

Table 6 Kinetic parameters for the aquation of bridged dicobalt complexes [subscripts correspond with those in equation (4) of the text]... 

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