Shape three coordination

HS — high spin LS = low spin. b In case of tetrahedral geometry. c In case of square-planar geometry. i In feet two Cu—N bonds are elongated. T-shape three-coordination. 

Figure 11.1. Schematic representation of typical silver (central atom) coordination environments with different coordination numbers and geometries (a) linear two-coordinate (b) triangular three-coordinate (c) T-shaped three-coordinate (d) square planar four-coordinate (e) tetrahedral four-coordinate (f) trigonal bipyramidal five-coordinate (g) tetragonal pyramidal five-coordinate (h) octahedral six-coordinate (i) trigonal prism six-coordinate (j) seven-coordinate (k) tetragonal prism eight-coordinate.

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. 

For the d8 low-spin trigonal bipyramid, we expect the stronger a donors to occupy the axial positions from our preceding arguments. The T-shape three-coordinate structure (VIII) is much more stable... 

Studies of arm-off (COD)Rh[2,2 -C6H3(CH2NMe2)2] (68) have also suggested interconversion proceeds via a T-shaped three-coordinate intermediate. Other apparent rotations observed in [(COD)Rh(bidentate)]+ complexes appear to via breaking of Pd-N or Pd-P bonds. ... 

A straight bond formalism for this compound would require the iridium atom to be related to a hypothetical T-shaped three-coordinate system (just as, on a straight bond model, the cobalt atom in solid Co2(CO)g is to a square pyramidal d system). No such systems are known or expected. 

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 reaction of the T-shaped three coordinate platinum boryl complex trans-[ Cy3P)2Pt B(Br)Fc ] [BAr 4] (21) (Fc = ferrocenyl) with 4-methylpyri-dine leads to the formation of trans-[(Cy3P)2Pt(Br) B(Fc)(NC5H4Me-4) ]... 

The complex exhibits unique properties originating from the Ni(I) center, such as a T-shaped three coordination structure, longer Ni-N (2.007(3) A) and Ni-C (2.205(3) A) bonds than those of organonickel(II) complexes, and paramagnetism confirmed by ESR measurement. 

Step/ Such geometry changes are, of course, central to isomerizations via T-shaped three-coordinate intermediates. A crystal structure determination on c/s-[Pt(CH2CMe3)2(PEt3)2] indicates that loss of PEt3 (which precedes y-H migration) may considerably relieve steric strain. This may be an important factor in related ligand-loss processes. 

The mechanisms of cis-trans isomerization of Pd(II) and Pt(II) complexes (except organometallics) have been considered in Chapter 4. The uncatalyzed cis-trans isomerization of [PtXR(PEt3)2] (X = halide, R = aryl) occurs by an initial release of halide ion. The dependence of rate on the solvent (acetonitrile or alcohols) has been attributed to hydrogen bonding which assists the dissociation of halide to give T-shaped three-coordinate intermediates, rather than to nucleophilic displacement of halide by solvent.However, others have shown that the first step has a value of -12 cm mol (X = Br, R = mesityl. 

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