Three-coordinate species shape

The reductive elimination step (iv) is a three-centre mechanism, which creates the carbon-carbon bond, regenerates the catalyst and needs the R1 and R3 groups to be cis on the palladium. This may be the case when cis bidentate ligands are used188. On the other hand, a trans to cis isomerization may precede the reductive elimination, which operates through T-shaped Pd(II)189,190 or Pd(IV)191-193 intermediates. Finally, recent studies argued that a T-shaped three-coordinate species c -[PdR1R2L] may be formed directly by an associative transmetallation step. 

Complexes of zero-valent palladium (d ) are tetrahedral if the coordination number is four, trigonal if the coordination number is three, and hnear if the coordination number is two. For Pd (d ), the geometry is square planar, unlike nF, which is normally octahedral, but like PF. With a very bulky ligand, 14-electron three-coordinate T-shaped species can be stabilized and structurally characterized. For example, the complex Pd(Ar)l[P(/-Bu)3], where Ar = 2,4-xylyl, can be isolated and the aryl group is opposite the open coordination site. For the few Pd (d ) complexes that have been structurally characterized, octahedral geometry is found. 

The trialkylgold(III) complexes (4. 7, and 8) were employed in the earliest mechanistic study on reductive elimination of a-bonded organometallic complexes [12]. A detailed kinetic investigation, together with a molecular orbital calculation, indicated the following mechanism involving T- and Y-shaped three-coordinate species... 

Using 1,4,8,11-tetraazacyclotetradecane, the structure of complex (800) (distorted trigonal planar Cu-Cu 6.739 A) was determined. Reactivity with 02 was investigated to demonstrate the formation of trans-l,2-peroxo species.585 As part of their work with copper(I) complexes with 02, the structure of a dicopper(I) complex ((801) distorted tetrahedral 7.04 A), supported by macrocyclic ligand environment, was reported by Comba and co-workers. Tolman and co-workers structurally characterized a three-coordinate copper(I)-phenoxide complex (802) (planar T-shaped) that models the reduced form of GO.587 The copper(I) analogue [Cu(L)][CF3-SO3]-0.43MeOI I (803) of a copper(II) complex (534) was also reported to demonstrate the role of ligand framework conformability in CV /Cu1 redox potentials.434 Wilson and co-workers... 

Symmetry requirements for concerted reductive elimination of dialkyls have been considered (314), and for trialkylgold(III) species reductive elimination from a trigonal, three-coordinate intermediate was found to be symmetry forbidden. Solvent participation or the involvement of T-shaped species, however, was suggested as possible. Charge transfer to the high-oxidation state gold(III) center and reductive elimination from such a charge transfer state was proposed as an alternative reaction pathway. 

Available structural details for these silicates, and for the wide range of other known species containing Mn" and silicate, are summarized in Table 23. Coordination numbers four (tetrahedral), five (both square pyramid and trigonal bipyramid and in-between structures), six (mainly octahedral and distorted, but including one trigonal prism), seven (of various shapes) and eight (dodecahe-dra and distorted rhombohedra) all are defined and there is evidence for three-coplanar in a zeolite the hydrated form is five-coordinate (3 x 2.28 A and 2 x 2.05 A) but, on dehydration of this Mn-exchanged zeolite, the solid contains planar three-coordinate Mn with Mn—O = 2.11 A, ... 

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. 

Theoretical studies have also been made on nickelacyclopentane. In practice the product of decomposition of such a compound, [L Ni], depends on its coordination number, being, respectively, butene, cyclobutane, and ethene for (n + 2) = 3, 4, and 5. The theoretical studies show that for planar four- and three-coordinate systems viz. cis-[L2Ni] and [LNi]), the reductive elimination of cyclobutane is symmetry allowed, while the production of ethene is not. The reverse is true for tetrahedral [L2Ni]. Five-coordinate species, [LsNi], can give either cyclobutane or cyclobutane and ethene depending on their shape. 

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