Structural studies transition metal alkyls

Insertion of sulfur dioxide (SO2) into the metal-carbon bond of transition metal alkyl and aryl complexes has also been studied extensively. SO2 shows several binding modes to transition metals as shown in Scheme 7.15 because it is amphoteric, behaving as a Lewis acid and a Lewis base. The Lewis base character of SO2 provides the structural types r/ -planar (3) or (S,0) (4) where SO2 donates a pair of electrons to the metal accompanied by rr back-bonding from filled d orbitals of the metal atom. The Lewis acid behavior of SO2 as a ligand affords an 17 -pyramidal bonding mode (5) where SO2 accepts a pair of electrons from the metal. As ligands tike olefins or carbon dioxide generally tend to prefer... 

The properties of these complexes have been reviewed in great detail [2,21]. A representative structural study [22], illustrating the pseudo-tetrahedral M(C5Hs)3X coordination geometry, is shown in Fig. 23.2. Theoretical studies on the fictitious molecule CpallCH [9] or on CpjUCH j [10] reveal rather small U-C(Cp) overlap populations, but large U-CHj overlap populations (comparable to those in typical transition-metal alkyls). 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

How does the anionic alkyl of the original trialkylaluminum or of the dialkylaiuminum chloride, which has sufficient anionic character to undergo anionic hydride exchange or CH3OT reaction, form a catalyst which becomes cationic under certain polymerization conditions No studies of this have been reported. One possibility is an internal oxidation-reduction reaction that converts an anionic alkyltitanium trichloride to a cationic alkyltitanium trichloride (Equation 10). Basic and electrophilic catalyst components would determine the relative contributions of the anionic and cationic forms. This type of equilibrium or resonance structures could also explain the color in transition metal compounds such as methyltitanium trichloride (73). 

Otsuka et al. (110, 112) studied the polymerization of butadiene in the presence of an aged Co2(CO)8/2 MoC15 catalyst. The product obtained was predominantly an atactic poly(l,2-butadiene), the 1,2-structure being favored by low reaction temperature (e.g., at 40° C, 97% 1,2 at 30° C, > 99% 1,2). Similar experiments with a Ni(CO)4/MoCl5 catalyst yielded a polymer with 85% cis- 1,4-structure. The results of Otsuka et al. have been confirmed by Babitski and co-workers (8), who studied the polymerization of butadiene by a large number of binary catalysts, based on transition metal halide, transition metal carbonyl combinations. These systems are of interest as further examples of alkyl-free coordination polymerization catalysts for dienes (9, 15a, 109). Little is known of the origins of stereospecificity of these reactions. 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

Products isolated from the insertion of SO2 into transition metal-carbon bonds have been shown to adopt structures (I), (III), and possibly, (IV). The first mode of bonding, expected of class b (la) or soft metals (105), is by far the most common one encountered. In fact, oxygen-bonded insertion products have been isolated only for titanium and zirconium (131, 132). However, recent spectroscopic studies have demonstrated that (III) is the kinetic product of the insertion with a number of metal carbonyl alkyls and aryls it then isomerizes to (I) (72, 73) ... 

By the time the alkylation studies were started, the reactivity of the dicarbollide anions toward electrophilic agents had been studied mainly on insertion reactions of boron, or heteroatom, or transition metal into the place of the missing vertex of the icosahedron, restoring its structure. A broad area of metallocarboranes was developed as a result of these studies, which was the subject of many articles and reviews. 

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