T-shaped intermediates

In the case of the very bulky chelate complex (21-1), the reduction process generates a T-shaped intermediate whose ligand binding enthalpy decreases in the order L = H2 > N2 > C2H4, i.e., a trend which is the inverse of what would normally be expected.4... 

Figure 3 Inversion of ASF3 trough edge inversion via a T-shaped intermediate...

Ligand exchange by dissociative activation now appears to be well established for some 16-electron square-planar compounds, though examples are not common. The detail of the intimate mechanism is crude by comparison with associative reactions, but all the evidence indicates the involvement of T-shaped intermediates. 

It is interesting to note that bond forming reactions by reductive elimination between atoms of similar or the same electronegativities such as C—C and C—H smoothly take place by virtue erf metal reduction, which are usually difficult processes in the sense of organic chonistry. In addition the processes also proceed under neutral mild conditions, where undesired side reactions may be prevented. The mechanism of reductive elimination has also been studied extensively on Ni, Pd, and Au complexes as well as theoretically. The processes sometimes require prior ligand dissociation to give unstable T-shaped intermediates from which facile cis reductive elimination takes... 

Scheme 13.3). Dissociation of the PPh ligand from 7 forms a T-shaped intermediate 9, which undergoes relatively rapid interconversion with other T-shaped intermediates 10 and 11 via transient Y-shaped species. The reductive elimination proceeds from the Y-shaped species. This study provided a basis for subsequent studies on the thermolysis of a variety of organotransition metal complexes. 

The mechanism of coupling between aryl iodides and aryl palladium complexes has been investigated. The formation of a T-shaped intermediate [LPd(Ar)-(Ar )] is necessary to achieve a mutual cis arrangement of the aryl groups before reductive elimination can take place. Crossover experiments show that bimetallic... 

However, when very bulky Ugands are used, a dissociative mechanism probably operates through T-shaped intermediates such as 50 (Scheme 1.31) [149, 219] these intermediates may then evolve by Sg2 (cyclic) transmetaUation with the organostannane via 59. 

Formation of T-shaped intermediates from square-planar complexes greatly accelerates the reductive elimination of [Pd(L)2RR j complexes [61]. 

The reductive elimination of a series of [Pd(L-L)Me2] complexes revealed that only complex 76a with Cy2PCH2PCy2, with the smallest bite angle, leads to a smooth elimination of ethane, while complexes 76b,d, with more stable chelates, do not eliminate ethane under mild conditions (Scheme 1.44) [61-63]. The reductive elimination from these complexes is most probably preceded by dissociation of one of the diphosphine arms to form a T-shaped intermediate [356]. The resulting Pd(0) complex [Pd(L-L)] undergoes dimerization to form complex 77. 

For Pt(II) complexes of the type IH(R)2(L)2 and Pt(Ar)20->2 in noncoordinating solvents, a dissociative mechanism involving a 14-electron T-shaped intermediate has been suggested. This pathway typically has a positive AS and the rate is retarded by free L in solution. 

Me2phen)] shows exchange of two sites of Me and PPh3 ligands via a T-shaped intermediate with monodentate coordination of the phenanthroline ligand. 

One of the most interesting but controversial mechanisms for isomerization proposed in recent years is the three-coordinate route spontaneous ligand loss to form a T-shaped intermediate which changes geometry. In the period under review more such species have been proposed and new ideas on the geometry change step have helped to explain previously difficult areas of interpretation. 

More examples of )8- and y-eliminations from organoplatinum complexes which proceed by spontaneous ligand loss to form three-coordinate intermediates have been reported. Loss of Et3P from c/s-[PtEt2(PEt3)2] is followed by -hydrogen transfer, but deuteration studies indicate that the two ethyl groups of the T-shaped intermediate are equally likely to... 

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