Trans effect and influenc

Theoretical explanation of the trans-effect (and /rans-influence) has centred on two theories, one based on cr-bonding the other on 7r-bonding. The tr-bonding argument considers two frans-ligands sharing a metal p orbital (Figure 3.83). 

Explanations of the trans-effect and trans-influence have considered a- and rr-bonding, often to the point of mutual exclusion. 

As already mentioned, a purely 7r-bonding mechanism cannot account for the position of hydride in trans-effect and trans-influence series. Overall, therefore, a major role (though not necessarily the only one) for <7-bonding is implied. 

Like the isoelectronic Pd2+ and Pt2+, Au3+ exhibits both trans-effects and trans-influence. Table 4.13 (above) lists structural data for a number of complexes AuL3L, showing how the disparity in Au-X distances between cis-and trans-X depends on the position of L in the trans-effect series for the compounds listed, the effect is least noticeable in AuC13NH3 as these two ligands are proximate in the series. 

Complexes Containing R2(0)P Ligands. In contrast to the relatively minor trans effect and trans influence of the wide range of neutral P-donor ligands discussed above, negative ligands based on trans influence as established in the X-ray structure of (3-benzyladenine)Co-... 

In square-planar, 16-electron complexes (i.e. coordinatively unsaturated) as found for many group 9 (Co, Rh, Ir) and 10 (Ni, Pd, Pt) metals, the associative process is most common. The trans effect and trans influence ligand series [19-23] are also useful measures in the study of homogeneous catalysis. Apart from very small ligands, such as CO, H2, and NO, steric repulsion between ligands, as well as complexes and incoming substrates, plays a dominant role in determining the kind of intermediates and complexes formed and their equilibria in solution. 

Tin compounds have been most studied,173 especially those of the SnCl3 ion, by 119Sn nmr and Mossbauer spectra. The SnClj ligand has a high trans effect and trans influence but has a rather low nucleophilicity. Thus in contrast to N, P, and As donors, SnCl2 will not cleave halide bridges in compounds with, for example, Pt(ju-Cl)2Pt bridges. 

Hydride is also one of the highest Trans Effect and Trans Influence ligands. Structures of hydride complexes often show rather long M L bond lengths trans to H. H2 is a low trans effect ligand, however, and a trans H M (H2) arrangement is found in many nonclassical hydrides. 

---