Halide dimers

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

Synthetic routes leading to two series of (COT)lanthanide(III) scorpio-nate mixed sandwich complexes were reported. The early lanthanide derivatives (COT)Ln(Tp) and (COT)Ln(TpMe2) (Ln = Ce, Pr, Nd, Sm) were obtained by reacting the dimeric halide precursors [(COT)Ln(p-Cl)(THF)]2 with KTp or KTpMe2. For small lanthanide ions (COT)Ln(Tp) (Ln = Er,... 

In the mechanistic study of metal-catalyzed living polymerization, this method has thus far been utilized primarily for analysis of model reactions to uncover the interaction between a metal catalyst and a carbon—halogen dormant end.170 176 Typical models for the dormant end include a-haloesters, such as alkyl haloisobutyrate and MMA dimer halides 1-25 (Figure 8) (for methacrylate), alkyl 2-halopropionate (for acrylate), and a-phenylethyl halide (for styrene). 

The tetracarbonyl derivatives rapidly decompose at RT to the dimeric halide-bridged tricarbonyl complexes with loss of CO ... 

As mentioned previously, the infrared spectrum in the carbonyl stretching region of the reaction mixtures is very useful in determining the extent of reaction. The presence of dimeric halide impurities is also easily detected (Table 1). 

Hauser et were able to isolate the intermediate dimeric halide when 9-chlorofluorene was allowed to react with only one equivalent of sodium amide in liquid ammonia. In t-butyl alcohol containing its potassium salt or a dilute aqueous solution of benzyltrimethylammonium hydroxide, the rate of formation of bifluorenylidene is second-order in 9-bromofluorene and first-order in the basicity of the reaction medium as measured by the ionisation of nitroaniline indicators . Under the same reaction conditions, protium exchange of 9-deutero-9-bromofluorene and elimination from 9-bromo-9,9 -bifluorenyl are much faster reactions than the conversion of 9-bromofluorene into bifluorenylidene. These facts are consistent with the displacement mechanism. 

With sodamide in liquid ammonia, dimeric halides (20b) were isolated in good yield. Experiments with optically active a-phenylethyl chloride indicated that the displacement reaction (20b) proceeds with Walden inversion. Clearly, the bimolecular displacement mechanism operates in the benzyl and benzhydryl series at low temperatures. 

At higher temperatures, and with aqueous or alcoholic base, isolation of dimeric halides is not possible. Provided that proton transfers occur rapidly, the slow steps for the two mechanisms would be (19b) and (20b), steps which are respectively uni- and bimolecular. Determination of the kinetic order thus provides a method of distinguishing the operative mechanism. 

Tris-cyclopentadienyls of the lanthanides have been prepared from the trichlorides and sodium cyclopentadienide. They are isolated from tetrahydrofuran as adducts, Cp3Ln(THF), in which the metal atom is formally ten-coordinate (Cp formally occupies 3 coordination positions). These high coordination numbers (8-12) are characteristic of lanthanide chemistry. Cp3Ln(THF) are converted into dimeric halides by reacting with more LnCl3. These species are useful starting materials for preparing other derivatives. 

The photolysis of Os(CO)5 in benzene has led to (n -C6H6)Os(CO)4 which is said to be similar to, but more reactive than, the iron analogue.The dimeric halide complexes [(arene)MX2]2 (M= Ru, Os) remain a useful entry into arene complex chemistry. The development of the ligand chemistry of these complexes continues.These substrates have also been used to make metalloborane complexes, used as catalysts for the coupling of furans and thiophenes and simple derivatives such as [ (p-cymene)20s2(0H)3] used for the catalytic oxidation of aldehydes to carboxylic acids. ... 

All the other aluminium halides are covalently bonded with aluminium showing a coordination number of four towards these larger halogen atoms. The four halogen atoms arrange themselves approximately tetrahedrally around the aluminium and dimeric molecules are produced with the configuration given below ... 

Halides. Gold(III) chloride [13453-07-1] can be prepared directiy from the elements at 200°C (167). It exists as the chlotine-bridged dimer, Au2Clg ia both the soHd and gas phases under an atmospheric pressure of chlorine at temperatures below 254°C. Above this temperature ia a chlorine atmosphere or at lower temperatures ia an iaert atmosphere, it decomposes first to AuCl [10294-29-8] and then to gold. The monochloride is only metastable at room temperature and slowly disproportionates to gold(0) and gold(III) chloride. The disproportionation is much more rapid ia water both for AuCl and the complex chloride, [AuCy, formed by iateraction with metal chlorides ia solution. 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

Coupling reactions and related fluoroalkylations with polytTuoioalkyl halides are induced by vanous reagents, among them metals such as copper and zinc, or by an electrochemical cell. More recently, examples of carbon-carbon bond forma tion by coupling of unsaturated fluorides have been reported Both acyclic and cyclic fluoroolefins of the type (Rp)2C=CFRp undergo reducUve dimerization on treatment with phosphines [42] (equation 33) The reaction shown in equation 33 IS also accompbshed electrocheimcally but less cleanly [43]... 

Solvent plays a significant role in these reactions In contrast to the formation of organocadmium compounds in the duect reaction of perfluoroalkyl iodides and cadmium powder in DMF [729], dimerization of the perfluoroalkyl halide was observed in acetonitrile [777] (equation 100)... 

In view of the facile oxidation of 10.13a-c it is not surprising that some metathetical reactions with metal halides result in redox behaviour. Interestingly, lithium halides disrupt the dimeric structures of 10.13a or 10.13c to give distorted cubes of the type 10.14, in which a molecule of the lithium halide is entrapped by a Ei2[E(N Bu)3] monomer. Similar structures are found for the MeEi, EiN3 and EiOCH=CH2 adducts of 10.13a. In the EiN3 adduct, the terminal... 

In the reactions of 10.13a with alkali metal terr-butoxides cage expansion occurs to give the sixteen-atom cluster 10.15, in which two molecules of MO Bu (M = Na, K) are inserted into the dimeric structure. The cluster 10.13a also undergoes transmetallation reactions with coinage metals. For example, the reactions with silver(I) or copper(I) halides produces complexes in which three of the ions are replaced by Ag" or Cu" ions and a molecule of lithium halide is incorporated in the cluster. ... 

---