Three-coordinate species

Silver tricyanomethide, AgC(CN)3, has been prepared by precipitation from a mixture of solutions of AgN03 and KC(CN)3. Flat needles could be obtained by recrystallization from dilute ammonia solution. The X-ray structure was undertaken to determine whether a planar three-coordinate species was formed. The results suggested that a distorted layer arrangement existed with three non-equivalent Ag—N bond lengths of between 211 and 226 pm. 

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. 

Different geometrical arrangements of the atoms attached to the central atom are possible for all coordination numbers greater than one. Thus, two-coordinate species may involve a linear or a bent disposition of the ligands and central atom. Similarly, three-coordinate species may be trigonal planar or trigonal pyramidal, and four-coordinate species may be... 

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... 

The same trend is found for the relative energies of 22 and 23 with respect to 18. Complexes 22 and 23 are formally two-coordinate and three-coordinate species, respectively. However, the acetylene ligand may serve as a four-electron donor in electron-deficient species (83). Therefore, 22 should be considered as three-coordinate and 23 as four-coordinate. The relative energies given in Table 16 show that CCSD(T)/n and B3LYP/II-IV give similar values for 23, while 22 is predicted too stable at the B3LYP level compared with CCSD(T)/II. 

The isotropic shifts of the three-coordinate [Hg(SR)3] anions agree quite well with the solution chemical shifts, indicating the lack of multiple species in solution (Table XIII). The isotropic chemical shifts of the three-coordinate species are deshielded from the two-coordinate compounds by... 

Burger and co-workers 24, 25, 26) prepared a number of binary metal compounds with the very bulky silazane ligand N(SiMe3)2, e,g, MLa (M = Al, Ga, Cr, and Fe), ML2 (M = Be, Mn, Co, Ni, Zn, Cd, and Hg), ML (M = Li, Na, and Cu). It is reasonably certain that the ML2 compounds contain two-coordinated metals and that the metals are three-coordinated in ML3 compounds. Recently Bradley and co-workers isolated some additional three-coordinated species of the transition metals ML3 (M = Sc, Ti, and V) 27, 28, 29) and the lanthanides 30, 31), X-ray single-crystal diflFraction analysis revealed that the three-coordinated compounds exhibit either trigonal planar MN3 units (Ti, V, Cr, Fe, Al, and Ga) or pyramidal MN3 (Sc, Eu, and Yb) 32, 33, 34, 35), The reason for this structural difference is not yet clear, but it does appear that the more covalently bonded compounds have the trigonal planar configuration. 

Detailed kinetic studies provide evidence for the stepwise mechanism shown in Scheme 6.3. In this mechanism, dissociation of phosphine precedes oxidative addition. The dissociation of ligand provides a site for coordination of the C-H bond in the square plane prior to oxidative addition and follows the trend noted earlier in tiiis chapter for Wilkinson s catalyst that addition and elimination from three-coordinate species is often faster than addition and elimination from four-coordinate species. 

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