Other Four-coordinated Species

Compared with the unbridged analogues, the centroid cpLncp angles were contracted 3-9°, some 3° less than the effect of a Me2Si tie. See also a similar tied zirconocene42 as a selective polymerization catalyst. 

The Ge—N—N—W chain found in the complexes lrans- WI(NNGcR3 )(PR3 L]52 (4) results from the novel coupling reaction of a coordinated dinitrogen molecule with R3GeCl 

Formation of 1,3,2-diazagermine was identified57 by a crystal study of the novel eight-membered 1,5,2-diazagermocine ring which resulted from acetylene insertion. 

In (ArCMe2CH2)3PbBr59 where Ar is the bulky 3,5-di-t-butylphenyl, the three —CH2CMe2Ar units are in a Manx legs or pinwheel arrangement which minimizes interactions. The crystal was poor, but within larger error limits the configuration at Pb is tetrahedral with Pb-C = 216(5) pm and Pb-Br = 265.6 pm. 

An attempt62 to prepare a jr-bonded product using pentaphenylcyclopentadienyl yielded instead the sigma-bonded, bulky ligand product Ph5C.5GcC.l3 with normal values for GeC = 197.5 pm and GeCl = 213.1 pm. 

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With tren showing a marked propensity to stabilize five-coordinate M" systems, it was of interest to investigate the Pd tren system, as a stereochemistry other than square planar is rare for this central ion. Senoff and his coworkers" have isolated [Pd(tren)Cl]Cl with a suspected five-coordinate structure, but there is some evidence for Pd—N bond rupture in solution to give a four-coordinate species. Pd complexes with the fully iV-methylated ligand Mcg-tren are definitely four-coordinate and structural data are available for both [Pd(Me( -tren)(NCS)]NCS and Pd(Meg-trenH)Cl ", the latter having one arm of the ligand protonated. ... 

Mass transport of species to the electrode surface constantly replenishes their concentrations. The outcome of this effect is the diffusional tail mentioned in Section 3.1 such that the concentrations are artificially inflated for longer periods than can be expected from a first-order process alone. Nicholson s and Shain s description of this feature allows it to be accounted for and enables one to extract first-order rate constants. The power of modem simulation software" can now allow this effect of diffusion to be captured directly. Perhaps more importantly, simulations allow more complex mechanisms to be analyzed in a global and comprehensive manner. Returning to the analysis of the CV, the result remains the same CVs recorded at slower scan rates show how the population of one species (five-coordinate) shifts to the other (four-coordinate). Note also how the peak in the CV at —0.65 V becomes prominent at the slower scan rate, reflecting the increased population of Cu(I)N4. 

The actual hydrogenation [7-3] to [7-4] and [7-4] to [7-1] illustrates a cis ligand orientation effect. Two ligands oriented cis to each other in either a six or four coordinated species are considered favorably situated for subsequent reaction. In some cases a new ligand is formed as in [7-3] to [7-4], and in other cases a new compound is produced [7-4] to [7-1]. 

At this point we shall examine more closely the substitution process represented by structures [7-15] and [7-16]. The principles to be presented are equally valid for similar six and four coordination species substitution reactions involved in hydrogenation, dimerization, polymerization, and oxidation—among others. 

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