The trans Influence

Some examples (different electronic ground states) of experimentally determined structures and geometries obtained by strain energy minimization with the two models, i. e., with and without the harmonic sine term for the metal-ligand-metal interaction, are presented in Table 12.3. 

Complex a Parameter b) n X-ray Urey-Bradleyc Urey-Bradley and electronic term d) 

0 tetrahedral twist angle square plane 0 = 0°, tetrahedron 0 = 90°. 

While bond distances were generally well reproduced with the earlier, simpler force field[57 65-1, the addition of the harmonic sine term has led to a considerable improvement in terms of the reproduction of the angular distortions, which were previously overestimated in highly strained complexes. 

The trans influence is defined as the extent to which a coordinated group lengthens the bond trans to itself. The origin of the trans influence is thought to be electronic ligands that are trans to each other can compete for a stabilizing interac- 

Complex1 1 Parameterlb) (°) X-ray Urey-Bradley c) Urey-Bradley and electronic term1 11 

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Crystal structures have been determined (Figure 2.56) for RhCl3(PEt2Ph)3 and RhBr3(PMe2Ph)3 (in both cases wer-isomer) - in each case, the Rh-X bond length shows the trans-influence of a tertiary phosphine [110]. 

Its structure shows the trans-influence of hydride and pronounced distortion from square planar geometry (H—Rh-P 70.7°) owing to steric crowding. (RhH(PPh3)3 is rather less distorted (H-Rh-P 75.8°) [118a], This 16-electron complex shows no tendency to add an extra molecule of phosphine, unlike the less hindered RhH(PEt3)3. It is, however, an active... 

Mixed mono-complexes can be made (Figure 3.68) the trans-influence of tertiary phosphine on the Pt-S bond is noticeable [121]. 

The series m-Pt(PhSC2H4SPh)X2 (X = Cl, Br, I) have been studied structurally (Table 3.12) and show little difference in the trans-influence of the halide ions on the Pt-S bond [122],... 

They display the considerable trans-influence of hydride and aryl groups (the Pt-Cl bond lengths should be compared with the value of c. 2.31 A in PtCl4 ). The Pt-P bond lengths are more insensitive to the phosphine, but by synthesis of ds-Pt(PR3)(PR3)Cl2 [150] complexes, it has been possible to study the effect of electron-withdrawing substituents on the Pt-P bond, as well as on the trans-influence of the phosphine (Figure 3.94). 

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],... 

Table 3.20 The trans-influence of ligands on 27(l95Pt- H) in tram-A+ and tram-AXa...

A seminal paper [155] examined platinum-phosphorus NMR coupling constants in a series of cis- and trans-platinum(II and IV) complexes. The trans-influence had hitherto been explained in terms of d7r-p7r bonding, in other words, such a mechanism dominated with trans-effect... 

The structures of both these complexes demonstrate the trans-influence of phosphines in lengthening the bond to the trans-halogen (Table 4.13). 

In addition to routine spectroscopic characterization, transition metal silyls have been examined by a variety of physical methods, principally to determine (1) the definite presence of an Si—M bond, (2) the manner in which such a bond is influenced by other ligands, (3) whether such a bond possesses any w-component, and (4) the trans influence of the silyl ligand. 

The structures of the cis- and frans-isomers of Pt(l,4-thioxane)2Cl2 have been determined. The Pt—S distance (2.298 A) is longer in the /rans-isomer than in the CM-form (2.273 A) showing the trans-influence of thioxane to be greater than that of chloride [119]. 

The range in Mo-C(Cp) distances reflects the electronic asymmetry about the molybdenum caused by the nature and spatial arrangement of the other ligands bonded to the metal. In other words, these variations are a manifestation of the trans-influence. 

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