Rhodium complexes aquation

Z>) Reactions of [Rh(NH ),X. The aquation and anation of [Rh(NH3X]" in acidic aqueous solution (equation 105 was first studied by Lamb who used conductance changes to monitor the interconversion of acido and aqua rhodium complexes , for the chloro and bromopentaanimine-rhodium(III) cations. He found that both aquation and anation are first-order in [Rh ], and that the hydrolysis is accelerated by the presence of OH . The forward form of equation (105) (aquation) has since been studied for X = Cl-, Br , l-, >.=56 oHj, SO, NO3" and Nj . Representative results for the aquation of various pentaammine salts are given in Table 25. For aquations of halides or nitrate ions, rate expressions which are first-order in [Rh] and zero-order in [H+] are found. Poe gives a detailed discussion of the thermodynamic, kinetic and intrinsic-kinetic trans effects, and compares the aquation and anation of [Rh(NH3)5X]-" (X = Cl, Br and I) with the [RhfenjjXY]" system. When X = S04, a two-term rate expression was observed... 

Complexes of trifluoromethanesulfonate anion with cobalt(III) are labile oward substitution under mild conditions, and they have proved to be useful synthetic precursors to a variety of aminecobalt(III) complexes. The pentaammine-(trifluoromethanesulfonato-O)rhodium(III) ion, which is readily prepared from [Rh(NH3)5Cl]Cl2 in hot CF3SO3H, is also versatile as a synthetic precursor. " Its synthesis and solvolysis to give essentially quantitative yields of the penta-ammineaqua- and hexaamminerhodium(III) ions are described below. The aqua complex has previously been prepared by the base hydrolysis or Ag -induced aquation of [Rh(NH3)5Cl]Cl2 in water, but the present method presents a cleaner and more rapid alternative. The methods for preparation of the [RhCNHj) ] ion have evolved from the procedure of J0rgensen. They involve prolonged reaction of [Rh(NH3)5Cl]Cl2 with ammonia in a pressure vessel at elevated temperature. The solvolysis of [Rh(NH3)5(0S02CF3)](CF3S0j)2 in liquid ammonia is a simple, high-yield, and rapid alternative. 

Rate data are reported for the acid-catalysed aquation of the [M(NH3)5COal+ ions (M = Rh or Ir ), as well as the rates of formation of these carbonato-com-plexes from [M(NH3)50H] + and carbon dioxide. At 298.1 K and p = 0.5 mol 1 , the aquation rate constants are 1.13 and 1.45 s and the formation rate constants 470 and 5901 mol S for the rhodium(m) and iridium(m) complexes respectively. The close similarity of these rate constants for both metal ions indicates carbon-oxygen bond fission for the aquation reactions. 

Hydrolysis of [Rh(NH3)5(N02)] and of cis-[Rh(NH3)4(N02)2] results in replacement of ammonia, but hydrolysis of trans-[Rh(NH3)4(N02)2] results in replacement of a nitro ligand. This pattern parallels that established earlier for the analogous cobalt(III) complexes, and derives from the dominance of the trans effect of NO2. Activation parameters for the cobalt(III) and rhodium(III) complexes are compared. It is concluded that ammonia loss occurs by an Id mechanism at both metals, but that loss of nitrite has more associative character in the case of rhodium(III). " Evidence from aquation, base hydrolysis, and ligand replacement studies gives an indication of chemically nonequivalent coordination sites in [Rh(tren)Cl2]. ... 

Kinetics of aquation of [Ru(LLLL)X2], with LLLL = cyclam, 2,3,2-tet, en2, or (NH3)4, and X = Cl or Br, have been followed by cyclic voltammetry. Rate constants, and activation parameters (A// and A5 ) have been evaluated, and compared with kinetic parameters for reactions of analogous compounds of ruthenium(III) and cobalt(III). Similar trends obtain for all three sets of complexes. There is retention of stereochemistry, rates decrease as the extent of chelation in LLLL increases, and trans complexes are less labile than cis analogs. Reactivities are determined by solvation of the initial and transition states, by nephelauxetic effects, and by a-trans effects. A limiting dissociative (D) mechanism is proposed for the ruthenium complexes, with square-pyramidal geometry for the transient intermediate [cf. rhodium(III) photochemistry below. Section 5.8.10]. Differences in isomer lability have also been described for... 

Redox catalysis of substitution is well established for chromium(ii) catalysis of substitution at chromium(m), and for platinum(ii) catalysis of substitution at platinum(iv) it has also recently been demonstrated for rhodium(i) catalysis of substitution in a particular series of rhodium(ra) complexes (see later. Section 4). Redox catalysis of substitution at cobalt(iii) is less common, cobalt(n) being generally ineffective, but chromium(ii) is an effective catalyst for aquation of [Co(NHg)5(NCO)] +. ... 

These latter observations are consistent with significant associative character for the aquations of these rhodium(m) and chromium(ra) complexes, with greater associative character for the latter series. 

Whereas the assignment of mechanism to spontaneous thermal aquations may at times be uncertain, the mechanism of metal-ion-catalysed aquation of halide complexes of cobalt(iii), chromium(ni), and similar cations is unlikely to be other than dissociative as far as the metal(m) centre is concerned. In Volume 2 of this Report it was mentioned that the catalytic effect of metal ions on solvolysis rates of t-butyl halides could be correlated with the stability constants of the respective metal-halide complex formed. Such a correlation is now reported for metal-ion catalysis of aquation of halide complexes of cobalt(m), chromium(m), and rhodium(m). Indeed this correlation is sufficiently general as to embrace such catalysts as H+ and HgCl+ as well as metal ions such as Hg + and A linear free-energy (AG vs. AG°) correlation... 

Catalysed Aquation.— In 1971 Rudakov and Kozhevnikov reported a correlation between the rate constants for metal-ion (M +) catalysed solvolysis of t-butyl halides and the stability constants of the respective complexes Now these authors have shown that a similar correlation applies, albeit rather approximately, to metal-ion catalysis of aquation of halogeno-cobalt(ra), -chromium(ni), and -rhodium(in) complexes. In fact the catalysts mentioned include not only metal ions such as Hg , Tl +, and Ag+, but also complexes such as HgCl+ and T1C1 +. This correlation can be improved by making an allowance for the different coulombic repulsions in systems of different charge products. If the rate constant for the catalysed aquation is ki and that for uncatalysed aquation Ato, the stability constant of the metal-ion complex produced K, the product of the charges on the reactants zazb, and C is a coulombic interaction constant, then the correlation conforms to the equation... 

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