Synthesis, characterization and DFT studies of the cobalt(III) complex of a tetrapodal pentadentate N4S donor ligand

The synthesis of the pentadentate ligand 2,6-bis(3,3-dimethyl-2,4-dioxocyclohexanyl)-4-thiaheptane (N4Samp) is described. The synthetic pathway involves the coupling of two 1,3-(dimethylenedioxy)-2-methyl-2-(methylene-p-toluenesulfonyl)propane moieties with sodium sulfide and subsequent synthetic elaboration to prepare the final N4S donor system. The cobalt(III) complex [Co(N4Samp)Cl]2+ has been prepared and subsequently crystallized as the tetrachlorozincate salt. The X-ray structure analysis confirms the pentadentate nature of the ligand and shows the thioether donor occupying one apex with four equivalent amine donors effectively occupying the equatorial plane of the molecule. The sixth coordination site is occupied by a chloro ligand. The electronic absorption and 13C NMR spectra have been studied. DFT calculations have been employed to explore structural and mechanistic comparisons between [Co(N4Samp)Cl]2+ and an analogous pentaaamine complex.     

Structure and bonding in [M(6)O(19)](n-) isopolyanions

The structure and bonding in [M(6)O(19)](n-) isopolyanions of Nb, Ta, Mo, and W have been investigated using density-functional methods. The computational-experimental agreement is good for the geometrical parameters of Mo and W species but less satisfactory for Nb and Ta clusters. The electronic structure of the anions has been probed with molecular-orbital, Mulliken-Mayer, and bonding-energy approaches. The results have indicated that M-O interactions are largely M d-O p in character and that sigma and pi bonds link the metal centers to terminal and bridging (O(b)) oxygen atoms. Some M-O(b) bonds exhibit a [M(4)O(4)] closed-loop structure, but this orbital-interaction mode has not been found to make a particularly outstanding contribution to the bonding stability of the molecules. Mayer indexes correspond to (fractional) multiple, approximately single, and low-order character for terminal, bridging, and internal bonds, respectively, and the valency analysis has yielded similar bonding capacities for the different oxygen atoms. A distribution of the negative charge over all types of oxygen sites and metal charges considerably smaller than the formal oxidation states have been obtained from the Mulliken analysis. Minimal structural changes have been detected on reduction of molybdates and tungstates, in accord with the general properties of the orbitals occupied by the added electrons.     

Extensions of the Generalized Pólya Process

Methodology and Computing in Applied Probability - A new self-exciting counting process is here considered, which extends the generalized Pólya process introduced by Cha (Adv Appl Probab...