Vanadium trihalides

The most widely studied transition metal is titanium. At various times, all oxidation states of titanium (II, III, IV) have been proposed for the active site of titanium-based initiators. Most of the evidence points to titanium (HI) as the most stereoselective oxidation state, although not necessarily the most active nor the only one [Chien et al., 1982]. (Data for vanadium systems indicate that trivalent vanadium sites are the syndioselective sites [Lehr, 1968].) Initiators based on the a-, y-, and 8-titanium trihalides are much more stereoselective (iso-selective) than those based on the tetrahalide or dihalide. By itself, TiCl2 is inactive as an initiator but is activated by ball milling due to disproportionation to TiCl3 and Ti [Werber et al., 1968]. The overall stereoselectivity is usually a-, y-, 8-TiCl , > TiCL > TiCLj P-TiCl3 [Natta et al., 1957b,c],... 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Imido-thiolato-complexes of vanadium of the type [V(Bu N)(SR)3] have been made by the reaction of [V(Bu N)Cl3] with Li[SR] (R = Bu , SiPhj) 18). Comproportionation reactions of the trithiolate with the trihalide precursor were then used to derive the related complexes... 

In the vanadium(III) systems no anionic complexes are found. While this is not surprising for a competitive ligand of low donor properties such as the bromide ion, the insolubilities of the trichloride and triazide seem to prevent the formation of chloro- and azido-complex anions . It may be assumed that the insoluble trihalides are polymeric and thus hexacoordinated. Similar considerations may be responsible for the absence of azidochromates—only the species [Cr(N3)(TMP)s]2+ and [Cr(N3)3]x being present —while a tetrachlorochro-mate(III) appears to exist in solutions of trimethyl phosphate containing chloride ions ll. 

The dihalides of Mn, Fe, Co and Ni are readily available. Trihalides of vanadium and chromium can be used, but an excess of NaCgHg is then required as a reducing agent. Alternatively MX, can be reduced in situ to MX, using zinc or magnesium before NaC5Hg is added. 

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