ABSTRACT:The trans influence of ligands T in trans-[Ni(Cl) 2 It is shown that the trans influence is related to the contribution (C 1 ) to Ni from T . Thus, the smaller the contribution C 1 to Ni the larger the Ͻ Ni ͉ NH3 Ͼ overlap and the stronger the TCl 2 NiONH 3 bond. The contribution C 1 is in turn related to the orbital energy ( T ) of T. Thus the lower ( T ) is, the smaller is C 1 . There is thus an inverse relation between the trans influence of a ligand T and its electronegativity.

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Investigations of the ground and excited states absorption are very important for the development of advanced optical limiters. Linear response time-dependent density-functional theory (LR-TDDFT) has become popular for calculating absorption spectra of molecules in their ground state. However, calculation for the excited state turns out to be much more complicated. In the present paper, it is shown that the transition dipole moments between two excited states can be wellestimated based on auxiliary excited-state wavefunctions extracted from the LR-TDDFT calculation. For application, the absorption spectra in zinc phthalocyanine (ZnPc), distyrylbenzene (DSB), and 3-methylthiophenes heptamer (3MT heptamer) are investigated in detail along with different density functional models, and results are compared with experimental data and other theoretical methods. The computational cost of the present method is much cheaper than other theoretical methods, such as quadratic response TDDFT.

We have explored the ability of square-planar Pt(II) complexes with different trans (T) and leaving (L) ligands to activate methane. Here methane activation is a two-step process consisting of (1) CH4 uptake via associative substitution of L with CH4 followed by (2) C−H bond activation. It was found that strongly trans-directing ligands such as phosphines give rise to high C−H activation barriers that make the corresponding complexes unsuitable as alkane activation catalysts, although the barriers for alkane uptake are low. On the other hand, weakly trans-directing oxygen-coordinating ligands afford poor alkane activation catalysts since alkane uptake has a high barrier, although C−H activation is barrierless. Moderately trans-directing amines and imines have acceptable barriers for C−H activation and methane uptake, with the latter process being slightly more facile. Thus, these ligands are the better choice for alkane-activating catalysts. It was finally found that the overall activity of the nitrogen-based catalysts could be increased by making use of weakly coordinating molecules, such as methanol, acetone, or THF as both leaving ligands and solvents.

We have studied the influence of different ligands X (X ) F, Cl, Br, I, NO 2 , and CN) on the C-H bond activation of CH 4 in trans-PtCl 2 X(CH 4 ) -, 1, and trans-PtClX 2 (CH 4 ) -, 2, where X is either trans (1) or cis (2) to methane. For 1 with X in the trans position, the trans-PtCl 2 Xfragment interacts with CH 4 through the overlap between the empty d σ -based orbital 2a 1 pointing along the Pt-X direction and σ CH on CH 4 . An interaction also takes place between an occupied d σ -based orbital 1b 1 and the empty σ* CH orbital on CH 4 , where the d π metal orbital is positioned perpendicular to the PtCl 2 Xplane. The d σ metal orbital contribution in 2a 1 is antibonding with respect to σ x on X, whereas d π in 1b 1 is antibonding with respect to π x . Through the series F, Cl, Br, I, NO 2 , and CN, the energies of σ x and π x increase. This is mostly an electronegativity effect. The increase in energy causes an increase in the contribution from σ x and π x to 2a 1 and 1b 1 , respectively. As a consequence, the bonding overlaps 〈σ CH |2a 1 〉 and 〈σ CH / |1b 1 〉 will diminish, as only the d-component in 2a 1 and 1b 1 contributes to the overlap. As a result of the decreasing bonding overlaps, the Pt-CH 4 bond strength will decline. It is thus shown that the experimentally established order of trans-labilizing power for the series of ligands X studied here, F < Cl < Br < I < NO 2 < CN, can be related to the orbital energies of σ x and π x and the electronegativity of the elements that are involved in these orbitals. The labilization of the Pt-CH 4 bond in the C-H activation transition state is even larger than in the adduct 1, leading to an increase in the C-H activation barrier along the series F < Cl < Br < I < NO 2 < CN. For 2 with X in the cis position, solvation has the largest influence on trends in the Pt-CH 4 bond for both 2 and the transition state. However C-H activation barriers are quite similar for different X.