Trans Influence of the Hydride Ligand

Therefore two hydride ligands usually avoid to occupy mutually tram coordination sites. For example, three H ligands in [K(18-crown-6)] [RuH3(PPh3)3] lie at the facial positions of a highly distorted octahedron. It seems unusual that Ir(H)2(Cl)2 P(i-Pr)3 2 has three pairs of trans ligands. The Ir—H distance is quite large (Fig. 3).  

Paramagnetic transition-metal hydride complexes are very rare, and this 17-electron Ir(IV) complex is the first example of a stable paramagnetic hydride complex of the platinum-group metals. It was prepared by the reaction of (NH4)2 [IrClg] with an excess amount of P(i-Pr)3 in refluxing ethanol containing concentrated HCl.  

---

However, in this case the formulation as a diplatinum(I) complex is more reasonable and the long Pt —Pt bond probably results from the high trans-influence of the hydride ligand. 

The difference between these two reactions may be caused by the trans influences of the hydride and phosphine ligands. Usually, phosphines avoid becoming trans to hydrides when possible. Therefore, complex 7 resists the reaction with another molecule of H2, since the oxidative addition is cis, requiring at least one site trans to P. 

Table 3.18 shows how the position of the low-frequency hydride resonance is affected by the trans-ligand, while study of a series of complexes trans-[Pt(PMe2Ph)2(Me)L]+ and neutral trans-Pt(PMe2Ph)2(Me)X shows the trans-influence of the ligand on 2/(195Pt- H) with better donors tending to reduce the value of J (Table 3.20) [152],... 

The two compounds of Figure 2 represent another example for the sensitivity of the bridging hydride position to the trans influence of the ligands, and also illustrate how this small structural detail can dramatically affect the reactivity of the compounds. 

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. 

Values of i>(Pt-H) show that the Pt—H bond is weakest for X = [CN] and the trawi-influence of the X ligands follows the order [CN] > F > Br > Cl . The signal for the hydride in the H NMR spectrum moves to lower frequency (higher field) with a decrease in the tra 5-influence of X . The traw-influence is not unique to square planar complexes, and may be observed wherever ligands are mutually trans, e.g. in octahedral species. 

---