Activation of C-C and C-H bonds by the Rh(I) and Ir(I) complexes (PCP) MCl (M) Rh, Ir; PCP=C6H3(CH3)(CH2PH2)2) has been studied by density functional methodology. C-H activation from either of the three-coordinate intermediates 1a and 1b has a high barrier (>25 kcal/mol). Direct C-C activation does not occur from either 1a or 1b because the C-C bond is sterically inaccessible. Plausible C-C and C-H activation mechanisms under mild conditions are related to four-coordinate

Density functional calculations for copper clusters Cun and their monocarbonyls CunCO (n≤13) have been performed using the relativistic ECP plus DZ basis set augmented by an f polarization function for copper atom. Equilibrium geometries, harmonic frequencies, and static mean polarizabilities of Cun and CunCO are determined. The feature of CO adsorption on the copper cluster and the effect of CO adsorption on stability and polarizability of the cluster are investigated. Calculations show that CO adsorption on copper clusters is selective in terminal coordination, and the favored adsorption sites are dominated by the local orientation of relevant frontier orbitals and the distribution of overall electrostatic potential surfaces of copper clusters. The interaction of Cun with CO in the copper cluster carbonyls leads to significant odd-even variations of the static polarizability differences between CunCO and the separated components Cun and CO. Size dependences of cohesive energies, CO binding energies, and static mean polarizabilities have been explored.

The structure and stabilities of linear and cyclic isomers of C6-and N2C4 + were investigated by DFT, MP2, CISD, and CCSD methods. The linear isomers of C6-and NC4N+ are predicted to be the most stable forms. Multireference configuration interaction methodology was used for the calculation of the doublet and quartet excited states. Assignments to observed transitions in matrix spectroscopy of these species are made. The first X 2п→ 2п transitions for C6-, NC4N+, and CNC3N+ occur at 1.98, 2.14, and 2.65 eV, respectively, showing similar features with large oscillator strengths. On the other hand, a significant difference exists in the X 2пu→1 2Σg + band system between the C6 - and NC4N+. Correlation between the relative molecular orbital energy and spectroscopic properties is discussed. The predicted electronic spectra agree well with available experimental data.

Mechanisms of N2 dissociative adsorption on small ruthenium clusters are studied by density functional calculations. The calculations indicate that the step of a ruthenium cluster has high activity for N2 activation, where an ensemble of five Ru atoms on the stepped surface of clusters is responsible for the active site. Such high activity arises from a strong charge-transfer interaction due to local phase adaptation between the p* orbital of N2 and the filled cluster valence orbital over the step region. Results from cluster models with different size show that the activation mechanism and the barrier are sensitive to the structural environment of the step. N2 dissociation over the step of the 11-atom cluster is a two-step process, where the rate-determining step has a barrier of 22 kcal mol21. N2 dissociative adsorption on the stepped surface of 15-atom and 21-atom clusters is a one-step process, and the barrier is～7-10 kcal mol21. Theoretical calculations on the 11-atom Os and Fe cluster models reveal a general activity of the stepped sites for N2 activation.

The geometries and stabilities of the FeFe cofactor at different oxidation states and its complexes with N2 have been determined by density functional calculations. These calculations support an EPR-inactive resting state of the FeFe cofactor with four Fe2+ and four Fe3+ sites (4Fe2+4Fe3+). FeFeco(μ6-N2) with a central dinitrogen ligand is predicted to be the most stable complex of the FeFe cofactor with N2. It is easily formed by penetration of N2 into the trigonal Fe6 prism of the FeFe cofactor with an approximate barrier of 4 kcal mol-1. The present DFT results suggest that an FeFeco-(μ6-N2) entity is a plausible intermediate in dinitrogen fixation by nitrogenase. CO is calculated to bind even more strongly than N2 to the FeFe cofactor so that CO may inhibit the reduction of nitrogen by Fe-only nitrogenase.