Description: This article discusses carbon-hydrogen bond activation by Titanium imido complexes. Abstract: This paper reports calculations that probe the role of R (hydrocarbon) and R' (ligand substituent) effects on the reaction coordinate for C-H activation: Ti(OR')₂(=NR') + RH → adduct → transition state → (OR')₂Ti(N(H)R')(R). Compounds with R = H, Me, Et, Vy, cPr, Ph, Cy, Bz, and cubyl are studied using quantum (R' = H, SiH₃, SiMe₃) and classical (R' = SiᵗBu₃) techniques. Calculated geometries are in excellent agreement with data for experimental models. There is little variability in the calculated molecular structure of the reactants, products, and most interestingly, transition states as R and R' are changed. Structural flexibility is greatest in the adducts Ti(OR')₂(=NR')•••HR. Despite the small structural changes observed for Ti(OR')₂(=NR') with different R', significant changes are manifested in calculated electronic properties (the Mulliken charge on Ti becomes more positive and the Ti=N bond order decreases with larger R'), changes that should facilitate C-H activation. Substantial steric modification of the alkane complex is expected from R-R' interactions, given the magnitude of ∆Gadd and the conformational flexibility of the adduct. Molecular mechanics simulations of Ti(OSiᵗBu₃)₂(=NSiᵗBu₃)•••isopentane adducts yield an energy ordering as a function of the rank of the ...

Description: This article discusses electronically unsaturated three-coordinate chloride and methyl complexes of iron, cobalt, and nickel. Abstract: Three-coordinate organometallic complexes are rare, especially with the prototypical methyl ligand. Using a hindered, rigid bidentate ligand (L), it is possible to create 12-electron methyliron (II) and 13-electron methylcobalt (II) complexes. These complexes are thermally stable, and ¹H NMR spectra suggest that the low coordination number is maintained in solution. Attempts to create the 14-electron LNiCH₃ led instead to the three-coordinate nickel(I) complex LNi(THF). Single crystals of LMCH₃ are isomorphous with the new three-coordinate chloride complexes LNiCl and LCoCl. Along with the recently reported LFeCl (Smith, J.M.; Lachicotte, R.J.; Holland, P.L. Chem Commun. 2001, 1542), these are the only examples of three-coordinate iron(II), cobalt(II), and nickel(II) complexes with terminal chloride ligands, enabling the systematic evaluation of the effect of coordination number and metal identity on M-Cl bond lenghts. Electronic structure calculations predict the ground states of the trigonal complexes.

Description: In this article, the authors describe two types of memory and illustrate each using artificial and actual heartbeat data sets. The first type of memory, yielding anomalous diffusion, implies the inverse power-law nature of the waiting time distribution and the second the correlation among distinct times, and consequently also the occurrence of many pseudoevents, namely, not genuinely random events. Using the method of diffusion entropy analysis, the authors establish the scaling that would be determined by the real events alone. The authors prove that the heart beating of healthy patients reveals the existence of many more pseudoevents than in the patients with congestive heart failure.

Description: Article describing the discovery of the element gallium. The authors travel to sites pertinent to the history of gallium, including the home and laboratory of its discoverer and the location from which it was procured.

Description: Article describing the history of the element strontium from its discovery in Strontian, Scotland, to its early uses in English medicine. Maps with directions and photographs of pertinent locations are included.