Solvento complexes, formation

Kinetic results for solvent exchange at solvento-metal cations are summarized in the Table. In addition to these quantitative results, the rate of water exchange at zinc(ii) has been estimated from a variety of complex formation kinetic results to lie between 10 - and 10 - s at 25 Qualitative information relevant to DMSO exchange at ruthenium(ii) and at dioxouranium(vi) is available from n.m.r. spectra and fluorescence studies respectively. 

The authors were able to determine that the rate constant for the second step is 1.4x10 times smaller than that for reaction with PPhj to form the reactant. There is a parallel pathway in which the initial dinuclear species is formed from a solvento complex. Furthermore, there is another pathway that is catalysed by the reactant but is not inhibited by PPhj. This pathway was assigned to associative formation of a dinuclear complex, without loss of PPhj, followed by intramolecular rearrangement to yield the product. 

Complex formation between edta and the [Cr(H20)6- (DMF) ] + ions has been studied and the relative reactivities of the solvento-complexes are Cr +ordinated water molecules are omitted. ... 

Various approaches can be adopted to perturb the equilibrium shown in Scheme 3.3 and to favor formation of [Ru(CD3CN)(PR3)2Cp]+. For example, the interaction of [RuCl(PR3)2Cp] and chloride ion acceptors, such as silver, sodium, or ammonium hexafluorophosphates, results in precipitation of AgCl, NaCl, or NH4CI and 100% conversion of the starting compxmnd into the solvento-complex. The reaction with NaPFg is illustrated in Scheme 3.4. 

In these reactions, the less soluble silver halide is precipitated and a solvent molecule occupies the free coordination site. Halide abstraction from a related, mixed, chloride-iodide complex with silver perchlorate results in precipitation of the less-soluble Agl and formation of the corresponding solvento-complex. Scheme 3.9 ... 

Solvento-complexes also may be generated through redox transformations of other types of N-donor ligands. 2,63 poj. example, the reduction at an electrode of the arenediazenate ligand in the ruthenium(II) complex, [RuCl(bipy)2(N2C6H4Me)]2+, in MeCN solution leads to formation of [RuCl(bipy)2(MeCN)]+ in addition to toluene and dinitrogen. 

The formation of solvento-complexes as a result of redox transformations of coordinated ligands is not r tricted to N-donor ligands reactions with other ligands are well known. For example, oxidation of the dimethyl sulfoxide ligand in [Co(NH3)5(DMSO)]3+ can be effected by treatment with KMn04 or CI2 in aqueous solution. The sulfoxide oxidizes to the sulfone which exhibits a much lower donor ability than Me2SO and, as a result of hydrolysis, is expelled from the complex generating the aqua-complex. 

Sargeson and Taube z have provided an interesting example of the formation of a solvento-complex. Nitrosylation of [Co(OC(0)NH2)(NH3)5]2+ is known to lead to a redox-induced elimination and coordination of a water molecule at the vacant site, Eq. 3.13 ... 

NMR spectroscopy was used to monitor ethane liberated in the synthesis of [Zr(MeCN)Cp2Me][BPh4] by this method. Oxidation of the methyl ligand apparently proceeds via intermediate formation of AgCH3. The X-ray crystal structure of the acetonitrile solvento-complex, [Zr(NCMe)Cp2(CH2Ph)]+, has been determined. Figure 3.3. 

In some cases, a metal ion exhibits different coordination numbers in different oxidation states. It may be possible, then, to take advantage of the change in coordination number to form solvento-complexes. For example, platinum(II) complexes typically exhibit a coordination number of four, whereas, the coordination number is usually six for platinum(IV) complexes. Electrochemical oxidation of [PtCl2(PR3)2] (PR3 = various phosphines) in acetonitrile was once thought to lead to the formation of... 

Formation of solvento-complexes through reactions of coordinated ligands (non-redox)... 

An additional pathway for the formation of solvento-complexes is the decomposition by probe acids of coordinated carbonate or oxalate ligands. There are several examples of such reactions reported in the literature. Thus, the ruthenium(II) bis(aqua)-complex has been prepared by the reaction shown in Eq. 3.20 ... 

Cleavage is followed by extrusion of molecular hydrogen and formation of a solvento-complex. 

Elimination of CO by photolysis is frequently employed in the preparation of solvento-complexes. For example, UV-irradiation of [Re(CO)3Cp] in etherll or THpUS produces [Re(CO)2(solv)Cp]. Interaction between [Re(CO)2(solv)Cp] and aryldiazonium tetrafluoroborate leads to the replacement of the solvento-ligand and formation of the rhenium(I) complex, [Re(CO)2(N2Ar)CpHBF4].il8 Irradiation of [Mn(CO)3Cp] in THF is used to prepare [Mn(CO)2( IW)Cp] which can be reacted with KCNH to form K[Mn(CN)(CO)2Cp]. 

The complexes, [Fe(py>4]X2 (X = Br, 1), have been prepared by oxidation of metallic iron with bromine or iodine in methanol. Pyridine is added to the resultant solution and the pyridine complexes precipitated.22 Interaction between metallic zinc or cadmium and an excess of arsenic pentafluoride in liquid sulfur dioxide leads to the formation of solvento-complexes containing two or four SO2 ligands depending on the experimental conditions.23/24 The reactions studied are summarized as follows, Eq.4.4 ... 

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