Mercury, olefin complexes

Mercury(II) complexes of two t5T>es have been prepared (6) from pent-4-enyl diphenyl phosphine and mercuric halides. These complexes are of the formulae [LHgX2]2(X = Cl, Br, I) and LaHgXg (X = Br, I). In no case is the olefin coordinated to the metal. 

Kouwenhoven (6) sjmthesised two bis-olefinic ligands related to the simple pent-4-enyl compounds mentioned above. These were of the general formula PhD[(CH2)aCH =CH2]2 (D = P, As) and formed platinum(II) and mercury(II) complexes. The platinum complexes LPtCla (L = PhD[(CH2)3 CH=CH2]2) were found to be monomeric species containing, as shown by their i. r. spectra, one coordinated and one free double bond. The two mercury complexes [LHgClaJs did not contain coordinated olefinic groups. 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

The 13C spectra of some (3-methoxyalkyl mercuric chlorides have been reported.175 The results indicate that (Hg-C) ( 1600—1800 Hz) varies in direct proportion to the increase in electron density at the mercury-carbon bond, and that an increase in mercury-carbon alkyl substitution, an effect which is in direct contrast to that observed for platinum 7r-olefin complexes. 

A non-dependence of the thermodynamic equilibrium constant on the solvent for two different types of diols was found 34>, which indicated that Ag+ as well as undissociated AgN03 formed complexes with olefins, comparable with mercury salt-olefin complexes 35>. Further formation constant investigations 36> by gas chromatography of silver complexes of cyclo-olefins had shown that methyl substitution at the double bond markedly reduced the stability and... 

Fleischmann, M., Pletcher, D. and Race, G.M. (1970) The electrochemical oxidation of mercury(It)-olefin complexes. Journal of the Chemical Society B, 1746. 

Olefins often form a-bonded organomercury compounds via a mercury TT-complex intermediate. Since this reaction proceeds quite readily and quantitatively, it may be used as an analytical method for the detection of olefins. 

The reaction between Hg(II) and olefins has been examined from several angles and work prior to 1950 has been summarised by Chatt . Several types of complex and product are formed, depending on the olefin, which involve no change in the oxidation state of the mercury atom. Propenyl ethers have long been known to produce the corresponding glycol plus metallic mercury but no kinetics are available . 

Chelate complexes could only be prepared in the case of platinum(II) as the metal ion, while the group V atom alone acted as a donor toward palladium(II) and mercury(II). The coordinated olefin in the chelate complexes was found to be readily displaced by monodentate ligands such as tertiary arsines, -toluidine and the thiocyanate ion. It was suggested by these workers that chelation would take place more readily if the olefinic phosphine or arsine were subject to greater steric restrictions than was the pentenyl ligand. 

Zn(R-dtp)2 complexes have been characterized and their thermal stabilities investigated 173,184,190,297-299,301-305) Zn(R-dtp)2 compounds are thermally degraded to volatile olefins and non-volatile residues and this serves as the basis for gas chromatographic determination of the compounds 304,30s) Several papers describing pyrolyses of Zn(R-dtp>2 complexes have discussed mechanisms for formation of olefins, sulfides, and other products 173,184,190,298,299, 304) Dakternieks and Graddon i8s,283)35 mentioned earlier, have reported thermodynamic measurements for depolymerization and adduct formation reactions of zinc, cadmium and mercury R-dtp compounds. 

Mercury(II) forms either coordination complexes or organometallic complexes with olefins, depending on the reaction conditions 19, 34). In solutions of suitable pH and purity of reagents the following equilibria are established ... 

For the hydroxymercuration of olefins in aqueous perchloric acid, Halpem and Tinker (273) have obtained a rate expression first-order in Hg + and in olefin and have observed an excellent correlation between the logarithm of the rate constant and Taft s a value for substituted olefins. This they interpret as suggestive of a transition state with considerable carbonium ion character. Although they have found no evidence for mercury(II)-olefin 7r-complex intermediates, they do not exclude the possibility of their existence. 

The authors suggest this method as a preparative procedure for allyl-mercury compounds starting from olefins and dienes via the 7r-allylpalla-dium complexes. 

---