Reactions Involving Aqua-complexes

Reactions Involving Aqua-complexes.—Kinetic studies of various aqua-complexes [PtCln(H20)4 ] ( =0—4) with a series of nucleophiles including halides, thiocyanate, DMSO, and ethene - show that in all cases the solvated species are at least ten times more reactive than the corresponding parent complexes. The entering-ligand efficiency order is as follows  

Nitro-complexes of platinum and palladium react with amidosulphuric acid according to reaction (3). 

The mechanism is proposed to be a rate-determining aquation of nitro-ligands followed by a redox process. The rate constants obtained from volumetric measurements of initial rates differ only by a factor of 100 between platinum (3 X 10 s ), and palladium (4 x 10 s ). 

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The palladium chloride process for oxidizing olefins to aldehydes in aqueous solution (Wacker process) apparendy involves an intermediate anionic complex such as dichloro(ethylene)hydroxopalladate(II) or else a neutral aqua complex PdCl2 (CH2=CH2)(H2 0). The coordinated PdCl2 is reduced to Pd during the olefin oxidation and is reoxidized by the cupric—cuprous chloride couple, which in turn is reoxidized by oxygen, and the net reaction for any olefin (RCH=CH2) is then... 

Decarboxylation of carbonate complexes is usually effected by acid hydrolysis with the formation of a C02 free oxide or hydroxide complex.128 All such reactions involve a protonated (bicarbonate) intermediate but there are some useful deferences which, in many instances, may be reconciled with the three main structural types of carbonate complexes. Both unidentate and chelate carbonates readily yield C02 on acidification, while there is a greater resistance to C02 loss when the carbonate is a bridging ligand. Unidentate carbonate complexes decarboxylate with the initial formation of a bicarbonate intermediate and subsequent loss of C02 without rupture of the M—O bond, viz. structure (3). By contrast, in chelate carbonate complexes, cleavage of the M—O bond occurs (with ring opening) with the formation of a bicarbonate aqua ion before the loss of C02, viz. equation (5).29... 

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. 

A detailed mechanism of the process shown in Scheme 1.3 is unknown. Two general mechanistic pathways, dissociative and associative, have been proposed for the ligand exchange reactions of X -iodanes (Scheme 1.4) [26, 127]. The dissociative pathway seems to be less likely to occur, because of the low stability of the dico-ordinated iodonium ion [PhIL]+ involved in this mechanism [127]. Such iodonium 8-1-2 species, however, have been frequently observed in the gas phase, for example, in mass spectrometry studies of protonated iodosylbenzene, [PhIOH]+ [101], or in the mass spectra of all known iodonium salts, [ArIR]+. The presence of cationic iodonium species in aqueous solution has been confirmed by spectroscopic measurements and potentiometric titrations of PhI(OH)OTs and PhI(OH)OMs [198] however, all available experimental data show that the iodonium species in solution are coordinated with solvent molecules or with available counteranions. X-Ray diffraction analysis of the protonated iodosylbenzene aqua complexes [PhI(H20)0H]+ isolated from aqueous solutions revealed a T-shaped stmcture, ligated with one water molecule at the apical site of the iodine(III) atom of hydroxy(phenyl)iodonium ion, with a near-linear O-I-O triad (173.96°), which is in agreement with a regular X -iodane structure [178]. 

Product distributions have been accurately determined or redetermined for the nitrosation reaction of [Co(NH3)5N3] " in acidic solution at 25 C and / = l.O In chloride media, [Co(NH3)5Cl] is formed along with the aqua complex and the product ratio [Co(NH3)5Cl] V[Co(NH3)50H2] is accurately linear in [d ] even when the coanion is the strongly ionpresent work strongly suggests that the product distribution and the nitrosation rate law are independent, that is, the products arise by a process subsequent to the ratedetermining step, and an intermediate is involved. 

The kinetics and mechanism of the decomposition and anation of trans-[Co(py)4(H20)2] in aqueous acidic chloride solutions has been studied in some detail. Two reactions occur the faster reaction involves replacement of coordinated pyridine by water or chloride ion in parallel pathways, while the slower reaction involves spontaneous reduction of [Co(py)3(H20)2Cl] and [Co(py)3(H20)3] with the chloro complex decomposing at a faster rate than the aqua complex. 

Reaction (14) describes the formation and back-dissociation of adducts in a reversible process. Reaction (15) usually involves a bond-reorganization within the adduct, with oxidation ofB and reduction ofNO, formation of the M(II) aqua-complex, and, in most instances, the evolution of gases (N2 and/or N2O), or of some other nitrosated product, depending on the... 

J0rgensen prepared (6) bl compound called anhydrobasic tetraammine diaquodiammine cobalt chloride, which he believed to be a dimer involving an oxo bridge. Werner found that OH could not be bonded to Co in the usual way since it did not react with dilute mineral acid to give an aqua complex. Decomposition of the complex by reaction with mineral acid gave three moles of cw-[Co(H20)2(NH3)4]X3,... 

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