Trans- influences

There are, however, short Rh—H contacts (2.77-2.84A) to ortho-hydrogens in phenyl groups. The Rh—P bond trans to Cl is some 0.1 A shorter than the others, evidence of the weak trans-influence of chloride [46]. 

The long Rh—P bonds in the tertiary phosphine adducts show little dependence upon the tertiary phosphine and are interpreted in terms of a largely cr-component in the Rh-P bond they are also affected by the strong trans-influence of the Rh-Rh bond. 

Kinetic inertness, evidently caused by the electronic configuration, leads to a remarkable unreactivity of the Rh-H bond to hydrolysis. In the absence of air, it is unaffected by ammonia solution in dilute solution, the ammonia tram to hydride is reversibly replaced by water, showing that the hydride has a trans-effect parallel to its trans-influence. 

Both these hydrides insert alkenes and alkynes the crystal structure of [Rh(NH3)5(C2H5)]2+Br2 shows the ethyl group has a trans-influence comparable to that of the hydride [93]. 

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]. 

The structures of the two tri(n-butyl)phosphine complexes of this type have been determined (Figure 2.58), again showing the high trans-influence of PR3 compared with Cl (Figure 2.58). 

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... 

Additionally, MO calculations indicate the lowest energy orientation occurs with the three strongest trans-influence ligands (two hydrides and a PPh3) in a facial configuration. Calculations on compounds IrX(CO)(PR3)2 indicates that weak donors X and strong 7r-acceptors PR3 favour addition in the XIrCO plane [134, 135],... 

Mixed haloplatinate(IV) ions have been synthesized [34] by use of substitution reactions on PtCl - and PtBr using the stronger trans-influence of Br, the ci s-isomers can be made by treating PtBr with Cl- (in the presence of Br2) ... 

Since thiourea has a high trans-influence, the ammonia trans to it is replaced, repetition of the sequence causing the formation of yellow needles of Pt(tu)4Cl2. 

The greater trans-influence of the tertiary phosphine manifests itself in the Pt—Cl bond lengths. 

The ds-isomer has the shorter and stronger Pt-C bond, a reflection of the lower trans-influence of chloride [109]. 

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],... 

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

A considerable amount of X-ray data has now been accumulated to support the concept of trans-influence. In a series of ions, PtLCl3 (L = neutral ligand), the Pt—Cl bond trans to the neutral ligand displays much more sensitivity to L than do the c/s-chlorines (Table 3.15) [147],... 

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... 

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. 

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

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. 

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