The thermal cis-trans isomerization of cis-transoidal polyphenylacetylene (PPA) synthesized with Noyori's catalyst [Rh(C'CPh)(norbornadiene)(PPh 3 ) 2 ] has been investigated under both ambient and inert atmospheres in solution and in bulk. In all cases, an intramolecular cyclization results in cis-trans isomerization, and subsequent chain cleavage produces 1,3,5-triphenylbenzene. This reaction is accelerated by the presence of air, particularly when the reaction takes place in solution. Decreases in the cis content and molecular weight show that the intramolecular cyclization process takes place at 23°C in solution. The mechanism of this reaction is identical to that reported previously for cis-cisoidal and cis-transoidal PPA synthesized with Ziegler-Natta catalysts.

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A new photoswitchable half-sandwich nitro Ni II complex with the N-heterocyclic carbene ligand was synthesized and successfully crystallized. The compound constitutes an analogue of a known nitrate nickel(II) compound, [Ni(η 5 -Cp)(IMes)(NO 3 )] (Cp = cyclopentadienyl, C 5 H 5 ; IMes = 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene), and crystallizes with two symmetry-independent molecules in the asymmetric unit. When irradiated with 470 nm lightemitting diode light, the complex molecules undergo a photoisomerization reaction in the solid state. Formation of the respective NO 2 group linkage isomers was studied photocrystallographically at different temperatures. After irradiation of the single-crystal sample for ca. 3 h at 100 K approximately 20% conversion of the η 1nitro (Ni−NO 2 ) ligand to its exo-nitrito (Ni−ONO) form was observed. At ∼130 K the exo-nitrito binding mode transforms further to the more stable endo-nitrito conformation, whereas at temperatures higher than 175 K the crystal converts back to the ground state. It appears that the photoisomerization reaction can be to some extent triggered, or additionally stimulated, thermally. Consequently, the highest conversion level (ca. 90%) was achieved after sample irradiation at 145 K. Despite similar molecular energies and intermolecular interactions, the two symmetry-independent molecules behave somewhat differently upon irradiation and temperature changes. The photocrystallographic findings and molecular aspects were supplemented by theoretical computations, including the quantum-mechanics/molecular-mechanics approach.

The reactions of [Ru-(=CHR)Cl2(PCy3)2] (1: R = Ph; 1a: R = -CH=CPh2) with silver salts of carboxylic acids afforded new dimeric complexes of the general formula [Ru2(=CHR)2-(R'CO2)2(mu-R'CO2)2(PCy3)2(mu-H2O)] (2: R = Ph, R' = CF3; 3: R = Ph, R' = C2F5; 4: R = -CH=CPh2, R' = CF3; 5: R = Ph, R' = C6F5; 6: R = -CH=CPh2, R' = C6F5; 7: R = -CH=CPh2, R'=CCl3) in good yields. With R' = CF3, C2F5 or CCl3 these complexes are active catalysts for metathesis of acyclic alkenes, including unsaturated fatty acid esters, as well as for ring closing metathesis. The reactivity of these complexes with bases and weak donor solvents has been studied and their half-life times in several media were determined.