Associative ligand substitution trans Effect

In the kinetic trans effect, the departure of the trans ligand is probably aided by a stabilization of the transition state via the same mechanisms operative for the trans influence.114 Both associative and dissociative ligand substitution processes seem to be facilitated in this way.117... 

In addition to carbenes exerting a strong trans influence (thermodynamic), which may be manifest as a trans effect (kinetic) in ligand substitution reactions, heteroatom-functionalized carbene ligands (with their associated dipolar resonance contributor, Figure 1.16) may assist in the stabilization of transition states of reduced coordination number by electron donation to the metal. 

Square-planar Exchange in square-planar complexes is associative, and is influenced by the trans effect, complexes whereby some ligands facilitate substitution of trans ligands. 

Substitution reactions of platinum(II) complexes containing one or more metal carbon bond(s) have a long history.217 Interest in such complexes and their reactivity is associated with their function as catalysts in synthetic procedures and in environmental applications. One important characteristic is the kinetic trans effect this is ascribed to the labilisation of a ra/is-positioned group caused by a C-bonded ligand,218 typically alkyl or aryl. In a mechanistic study in which kinetics measurements at elevated pressures were employed, cyclometallated Pt(II) complexes were used.219 The complex shown in Scheme 4 is water-soluble permitting the kinetics of substitution by a wide range of nucleophiles to be studied in aqueous solution. 

Substitution reactions in square planar complexes take place via associative mechanisms. In the kinetic trans effect, a series can be constructed in which species are put in order of their ability to labilize ligands trans to themselves. Using this series, high-purity square planar isomers can be readily synthesized. The trans-directing ability of a ligand is also directly related to its polarizability. 

These ranges show that there is variation of substituent effect with the character of the substituted a-boron atom. There are generally also noticeable jS- and y-effects on the other cluster boron atoms. For example, small )S-shift effects of either sign (depending on the cluster type, and on whether or not the substituent exhibits non-(T electronic interaction with the cluster),are usually associated with the deshielding a effects. In addition, differential /S effects of up to several ppm in the same cluster are encountered when the a atom is a transition metal center with mixed ligands that have different trans effects. y effects are often less marked ... 

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