Iridium trans influence

The second contribution from Luis A. Oro and co-workers Binuclear Iridium Complexes in Catalysis deals with the demonstration that the trans-influence of the ligands operates through the bridging ligands or intermetalhc bonds and governs the reactivity of the diiridium framework. 

Thus, in the dinuclear d -d iridium (III) complex, bridged by two pyrazolyl ligands, the intermetallic trans-influence of the terminal hydride ligand on the bridging hydride is crucial for decoordination of acetonitrile and activation of diphenylacetylene. The transmission of the trans-effect along the binuclear backbone results in the selectivity of the hydrogenation into cis-stilbene. 

We can verify that statement by looking at a pair of iridium complexes, trans-[Ir(CO)L2Cl] with an Ir(I)-center, and mer- h CO)L C with an Ir(III)-center. The two complexes differ in that the latter is derived by oxidizing the square planar Ir(I)-complex adding chloride ligands to the axial positions (see Fig. 7.1). Whereas the A value is virtually unaffected by this oxidation (-0.357 and -0.377, respectively), the B value decreases from 29.11 to 0.70 upon going from Ir(I) to Ir(III). In other words, the influence of oxidation state of the central metal is increased. 

Although the mechanism takes place exclusively at one of the metal sites, the ligand at the spectator iridium center occupying the position trans to the bridging hydride (L) effectively influences the catalytic activity of the binuclear system. Therefore, the intermetallic communication and, consequently, the binuclear nature of the complex play a crucial role on the performance of the catalyst even when the second metal is not directly involved in the catalytic cycle (Fig. 4). 

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