A theoretical study of ethene trimerization at a cationic (C6H5CH2C5H4)Ti fragment generally supports the metallacycle mechanism proposed earlier for this reaction. However, the crucial formation of the 1-hexene complex from a titanacycloheptane intermediate occurs by direct Cβ → Cα ‘ hydrogen transfer rather than by the more traditional β-elimination/reductive elimination sequence. The pendant arene moiety “breathes” during the reaction, being more strongly bound at the TiII stage than at the TiIV stage of the reaction. Its main role is to make the olefin complex formation more endothermic, thus increasing the barriers for formation of titanacyclopentane and titanacycloheptane intermediates. For the “naked” (C5H5)Ti system, which lacks this effect, further ring growth wins over hexene formation. But even for the bridged (C6H5CH2C5H4)Ti system, we find that the various reactions are very delicately balanced.

A series of complexes ML2(x+) (M = Mn-Zn, L = 2,6-bis(iminomethyl)pyridine) was investigated by theoretical methods. Electron transfer from the metal "t(2g)" orbitals to the ligand pi orbitals is reflected in the elongation of ligand C-N bonds and shortening of the C(py)-C(imine) bonds. Using zinc complexes as references, these deformations could be used to quantify the number of electrons transferred. Strong transfer is found in low-spin MnL2(+) (ca. 2 e) and in high-spin MnL2(+) and low-spin MnL2(2+), FeL2(2+), and CoL2(+) (ca. 1 e each). Smaller transfer is found in CoL(2)(2+), and the transfer is insignificant in high-spin MnL2(2+), NiL2(2+), and CuL2(2+). Analysis of the unpaired electron density on the metal (using the Staroverov-Davidson method) shows that the contribution of a biradical description, in which ligand radical anions are antiferromagnetically coupled to the metal center, is significant in most cases. In the case of CoL2(+) and high-spin MnL2(+), where the metal-ligand bond is weakened, it amounts to over 50% of the total transfer.

No abstract

No abstract

No abstract

No abstract

Reaction of LLi (L = ArNC(Me)CHC(Me)NAr, Ar = 2,6-Me2C6H3) with [Rh(COE)2Cl]2 (COE = cyclooctene) produces stable, three-coordinate LRh(COE) (1). At room temperature in solution, the LRh fragment moves rapidly over one face of the COE ligand via reversible allyl hydride formation.

Oxygenation of C-H and C=C bonds of hydrocarbons with H2O2 and O2 is an important industrial method to convert mineral oil into useful chemicals. Despite their enormous economic impact, these reactions are still not fully understood. In the early 1970s, the potential of Rh and Ir complexes for olefin oxygenation was investigated intensively. Simple inorganic salts of these metals proved to be rather useless for industrial application when compared with the traditional Wacker system. However, the appropriate choice of ligands allows the stepwise oxidation of olefins at Rh and Ir. These systems are therefore useful to study mechanistic details of substrate binding and C-O bond formation at the catalytic metal center. Insight from these model studies helps in understanding the catalytic reactions at these (and possibly other) metal centers. Further insight into the differences between the Rh system and traditional Wacker-type oxidation at Pd may lead to useful applications.