Pentaammines aquation

IS further extended by the replacement of X by other anionic or neutral ligands. The inertness of the compounds makes such substitution reactions slow (taking hours or days to attain equilibrium) and, being therefore amenable to examination by conventional analytical techniques, they have provided a continuing focus for kinetic studies. The forward (aquation) and backward (anation) reactions of the pentaammines ... 

PEN polymers, 20 38, 44. See also Poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) properties of, 20 70 Penta-4-tolylantimony, 3 78 Pentaammine complexes, aquation of, 13 442... 

Prepared as described in reference 3, and recrystallized from dilute HC10 three times as discussed in reference 1. An equivalent amount of [Co(NH ) C1]C12 which has been allowed to aquate for 4 days at room temperature can be used, but lower yields are obtained. The purity of the initial pentaammine complex is important. Hexaammine impurities are carried through the reaction sequence. 

A S oh = + 38 + 6JK mol .5SS For both the spontaneous and base-catalyzed paths, the rates of isomerization for the three pentaammines follow the order Rh > Co > Ir for the base-catalyzed path the reaction rates are of the order 278 217 1, while for the spontaneous path (ks), the relative rate constants are 32 2.3 1. The greater reactivity of the Rh complex, attributed to a lower AH for both the spontaneous and base-catalyzed paths, contrasts sharply with the order generally observed for the aquation of d6 pentaammines. For example, for M = Co, Rh and Ir, the aquation of [M(NFI3)5Br]2+ in acidic solution occurs with relative rate constants of 4 1 0.001.587 The ordering is not always Co > Rh P Ir, however, as the rates of water exchange in aquapentaammine complexes in the Rh triad have relative rates of 0.53 1 0.0059,559 and the relative rates of acid aquation of the [M(NH3)5(C03)r ions are 1 1 0.02.518... 

The hexaamine complex can only be prepared in sealed tube reactions (equation 34), unless the incorporation of the sixth ammine ligand is catalyzed by ethanol. In contrast, the pentaammine complexes are very accessible, and their anation and aquation reactions (Scheme 11) have been exhaustively... 

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. 

Pentaammine(trifluoromethanesulfonato-0)iridium(III) trifluoromethanesul-fonate is a white powder that is air-stable provided it is not subjected to prolonged exposure to atmospheric moisture. The complex may be kept for many months in a desiccator over a suitable drying agent (silica gel, CaCL) without any noticeable decomposition. The aquation rate constant for the complex is 2.6 x 10 sec" at 25° (0.1 M CF3SO3H), and at elevated temperatures (60-80°) the rate of substitution is quite rapid in both aqueous and nonaqueous solvents. When dissolved in poorly coordinating solvents such as acetone or tetrahydrothiophene 1,1-dioxide (sulfolane), the solvent-substituted species are themselves comparatively labile, and many substitution reactions may be performed in these solvents. " ... 

Ruthenium(II) [Ru(NH3)50H2] can be most efficiently prepared by zinc amalgam reduction of [RuCl(NH3)5]Cl2 in aqueous solution. More recently, an alternative route avoiding Zn + and Cl" ion has been developed based on the aquation of electrochemically reduced [Ru(03SCF3)(NH3)5] " . Alternative routes include the photolysis and acid-catalysed hydrolysis of [Ru(NH3)g] and the reduction of [Ru(NH3)5(OH2)]. The lability of the aqua ligand in these systems makes [Ru(NH3)jOH2] an excellent starting material for the synthesis of substituted pentaammine complexes, and for the study of their kinetics of formation. 

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