Solvento-complexes

Davies, J. A. Hockensmith, M. Kukushkin, V. Y. Kukushkin, Y. N., Homoleptic Solvento-Complexes Starting Materials for Preparative Coordination Chemistry, in Synthetic Coordination Chemistry Principles and Practice World Scientific River Edge, NJ, 1996, p 95. 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly... 

The relative reaction rates and the stability of the aquo complex make it possible to identify the aquo complex as an intermediate and study the individual acts separately. However, if the solvento complex were less stable and the anation rate much faster than the solvolysis, it would not be possible to observe this intermediate, and the process would be kinetically indistinguishable from a unimolecular dissociative process. Both processes would exhibit overall first-order kinetics and the usual mass-law retardation and other competitive phenomena characteristic of an extremely reactive intermediate. 

This problem requires a modified approach which Gray 16) has solved in the case of substitution in square planar complexes. He uses the fact that bases, like hydroxide, substitute very slowly but will immediately deprotonate, and hence stabilize, a protonic solvento intermediate. This elegant approach cannot be applied to the octahedral cobaltammines whose reaction rate with such bases is very high. 

Our alternative approach has been to synthesize the solvento intermediate and then study its reactions in isolation. We thereby hope to show that its reactivity and steric course is inconsistent with the postulate stating that it is an intermediate in the substitution reactions. Complexes of the type cis- and trans-[Co en2 CH3OH q]+2 (7)f and cis-[Co en2 (CH3)2SO Cl]+2 32) have been prepared. We have shown in the first case that the lability of the coordinated methanol does not sufficiently explain the nonappearance of the solvento complex in the reactions of cis- and trans-[Co en2 Cb]"1" in methanol unless it is not an intermediate in the reaction. In dimethyl sulfoxide solution, cis- and trans-[Co en2 Cb] have been shown to isom-erize to an equilibrium mixture that also contains the solvento intermediate 32). 

Detailed kinetic studies indicate that about 80% of the time isomerization goes via the solvento intermediate. But this is not a rate-determining solvolysis, rather a temporary diversion of the intermediate of a dissociative reaction. Watts 33) has recently prepared the dimethylformamide complex, [Co en2 DMF Cl]+2 and has shown that it cannot be an intermediate in the isomerization of cis- and trans-[Co en2 Cl2]+ in dimethylformamide. 

Romeo, R., and Alibrandi, G. (1997), Structure-reactivity correlations for the dissociative uncatalyzed isomerization of monoalkylbis(phosphine)platinum(II) solvento complexes, Inorg. Chem., 36,4822—4830. 

At first glance the k, term, first ol der with respect to complex and independent of Y, would suggest a dissociative pathway. Strong evidence, however, supports the view that this pathway also is associative. It must be recognized tbat, in general, solvent (S) molecules will be nucleophiles and will therefore compete with Y for ML2TX to form ML2TS (sometimes called the solvento complex). Thus the two-term rate law could be written as ... 

Occasionally the amount of solvento complex present is too great for the stationary state approximation to be valid and the analysis of the rate constants will be more complicated.441 A rapidly established equilibrium between the substrate and the solvento complex is not uncommon in labile systems with low concentrations of the nucleophile and leads to a rate law of the type... 

Acid-catalyzed elimination of RH has again proven a valuable tool, applied most extensively to Tp Pt(H)2Me (152). Thus, protonation of 152 with [H(OEt2)2]BArf4 affords the anticipated solvento complex... 

Bis(arene) complexes of ruthenium (99), first prepared in relatively low yield by the Fischer-Hafiier synthesis (see Fischer-Hafner Synthesis) (MCb/AlCb/arene) are now readily available through solvento cations [(arene)RuS] + (S = acetone, nitromethane), which in turn are made from the dimeric half-sandwich halides [(arene)RuCl2]2 (Section 6.3) with Ag+ with the appropriate arene as solvent (Scheme 23). This method allows a whole range of functionalities to be incorporated to at least one of the arenes. The dications (99) + are of relatively high solvolytic stability and are decomposed by DMSO only to form [(arene)Ru(DMSO)3] +. 

In the earliest authentic halocarbon complex (1982), o-diiodobenzene was found to chelate to cationic Ir(III) as shown in diagram (5)." An earlier proposed example proved to be misidentified when the crystal structure was carried out. To be stable, any such complex must resist oxidative addition, hence the use of an oxidation state, Ir(III), that is only oxidized with difficulty. The normally rather weakly basic halocarbon lone pairs are often reluctant to bind, but chelation and involvement of the least electronegative hahde, iodine, favor binding as does the cationic character of the complex. A series of such complexes was soon found, including complexes of fr(I)" and a series of weakly bound dichloromethane complexes for certain systems." These solvento complexes can be very labile and so find use as precursors for binding of other weakly basic hgands. Even fluorocarbon complexes proved viable." A review of the area is available. It now seems... 

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