Trans Effects

The sequence covers the rate effects of a range of 10 and is roughly applicable to all square-planar platinum complexes. Based on this sequence, the steric course of a substitution can be predicted, e.g.. 

On the other hand, the trans-complex was formed according to scheme (2.8.7), because the irons effect of Cl NH3. 

On the other hand, bulky ligands in c/s-position exhibit large steric effects. Thus, in the reaction (2.8.8), the replacement of trans-phenyl by trans-mesityl (mesitylene = 1,3,5-trimethylbenzene) leads to a rate decrease by a factor of 30, but if phenyl is in ds-position the rate decrease factor is about 10  

Extensive studies have shown that the trans effect of a variety of ligands decrease in the order  

This iirformation can now be used to design preparations of platinum(n) compounds or to predict their kinetic behavior. Investigations have demonstrated that the trans effect phenomenon may also be important in complexes of other metals. 

Cattalini, G. Marangoni, and A. Cassol, Inorg. Chim. Acta, 1969, 3, 74. 

Reactions in [PtCl4] -ethylene-[SnCl3] systems are dominated by the large 7r-acceptor and trans effect properties of the [SnClg] ligand. This is also true for analogous systems containing other alkenes. A preliminary 

Irradiation of [Pt(Ns) dien]+ at its charge-transfer maximum produces azide radicals, and presumably transient platinum(m). Similarly, pulse radiolysis experiments on [PtCU] and [Pt(CN)4] generate transient platinum(i) and platinum(iii) species, whose kinetic properties were investigated.  

The following chelate ring-closure reaction is an example of intramolecular substitution in a square-planar complex  

Belluco, B. Crociani, R. Pietropaolo, and P. Uguagliati, Inorg. Chim. Acta Rev., 1969 3 19. 

Finally, ab initio MO calculations on the products of H2 and [RhCPHa) found two energy minima. One corresponds to a structure of C2 symmetry, resulting from addition of two H atoms (17). The other is based on a pyramidal structure, where an occupies an axial site (18). 

The trans directing ability of MeCN at Pt(II) has been compared with its trans influence, by examining replacement reactions of [PtCl3(NCMe)] by amines for the former, and the X-ray crystal structure for the latter. The directing influence (dynamic trans effect) is high while the bond-weakening trans influence is low. 

The kinetics of the reactions of [PtCl3L] (L = dmso, Et2S, PMc3, PEt3, PPh3, or AsEt3) with many neutral or anionic nucleophiles, L, to produce trans-[PtCl2LL ] have been followed in 95% methanol and the second-order rate constants 2 (the reactions follow the usual two-term rate law) used to make comparisons 

The hydrolysis reactions of cis- and trans-[AuMe2Et2] in the presence of PPh3 produces cis-[AuEt2Me(PPh3)] and trans-[AuEtMe2(PPh3)], respectively. This would seem to indicate that the trans effect of Et was greater than that of Me in these Au—C protonolysis reactions. 

Another dissociative process leads to a different sort of isomerization in the bis-N 0 bridged head to head a-pyridonate complex, (19). It isomerizes in 

MeCN and the cis series dmso MeCN PhCN py 2-Mepy  

A crystal structure of [PtL(cod)] [PtL(SnCl3)3] (where L, 2-methyl-allyl, is in both cation and anion) indicates that the trans influence of 2-methylallyl is similar to that of olefins.  

Preparations have been described of pure samples of cis- and trans-[PtCl2(NCMe)2], based on classical methods dependent on the trans effect order of Cl MeCN. It appears that bis-acetonitrile-platinum dichloride made by the standard procedure is a mixture of isomers. The reaction of the rhodium compound (13) with triphenylphophine resulted in the CO trans to 

Ab initio calculations on the trans effect at platinum(II) and rhodium(I) rationalize its operation in terms of the x and n effects on the stability of trigonal-bipyramidal reaction intermediates. Theoretical studies on some palladium(II) ethene compounds (all of them Wacker process intermediates) suggested a trans influence series OH C1 OH2 at this metal.  

The first example of a head-to-head to a head-to-tail isomerization in a palladium complex has been reported for [Pd2(en)2(biL)2](N03)2. The ligands biL are a-pyridonato, which bridge the palladium atoms as shown in 

---

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

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. 

The preparation of the isomeric forms of Pt(NH3)2Cl2 is discussed in terms of the trans-effect in section 3.8.9 [67]. 

In the case of the trans-complex, only the two chloride ions are substituted, the trans-effect of ammonia being too low to give substitution with the result that white needle crystals of trans-[Pt(NH3)2(tu)2]Cl2 are formed [73],... 

Kinetic study [141] of complexes of the type trans-Pt(PEt3)2XCl was of great value in establishing the strong trans-effect of hydride (Table 3.13) examination of the data for a wide range of reactions gives rise to a series... 

The trans-effect is, therefore, a kinetic labilizing effect rather than a thermodynamic one. An approximate series is ... 

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). 

One or two ligands such as CO and CN that have high observed trans-effects (and therefore are out of place in the above series) do possess empty orbitals that can act as 7r-acceptors to remove electron density from the metal ion, making the region trans to the ligand electron deficient and able to be attacked by the nucleophile in the transition state (Figure 3.84). 

This would give rise to an order of the kind R2C=CR2, CO > CN- > N02 > SCN- > 1 > Br > NH3 > OH-. Therefore, a combination of a-and 7r-effects can be considered to give rise to the observed trans-effect series. 

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

A 7r-bonding explanation notes that several ligands high in the trans-effect series are good -acceptors and thus siphon off 7r-density, making the region trans to it electron deficient and thus attractive to ligands that are electron rich. 

The classic application of the trans-effect lies in the synthesis of the cis- and trans-isomers of Pt(NH3)2Cl2, known as Peyrone s salt and Reiset s salt after their respective discoverers in 1844. 

Because Cl- has a stronger trans-effect than NH3, a group opposite to Cl-is replaced in the second substitution. Similarly, in the synthesis of the transisomer by heating Pt(NH3)4+ with Cl-(Figure 3.86), it is the ligand trans to chloride that is again replaced in the second step. 

The cis- and trans-isomers of [Pt(NH3)(N02)Cl2]- have been synthesized from PtCl - merely by choice of the order of ligand substitution (Figure 3.87). (In the second step, chloride trans to chloride is more labile.) The second substitution is dictated by N02 having a higher position in the trans-effect series than chloride [144],... 

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. 

Application of the trans-effect to synthesis ofplatinum( IV) complexes... 

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. 

The trans-effect can be used synthetically. In the reaction of Br- with Au(NH3)4+, the introduction of the first bromine weakens the Au—N bond trans to it so that the introduction of a second bromine is both sterospecifically trans and rapid. (A similar effect occurs in the corresponding chloride.) The third and fourth ammonia molecules are replaced with difficulty, permitting the isolation of AuBr2(NH3)2 (second-order rate constants at 25°C are k] = 3.40, k2 = 6.5, k2 = 9.3 x 10-5 and k4 — 2.68 x 10 2lmor s l at 25°C) [141]. 

Factors responsible for this order include the trans-effect, charge neutralization, and statistical effects. 

---