Phosphine ligands structural parameters

The last series of Ru3C1 family is concerned with the monocations [Ru3C16(PR3)6] + (R = Et, Bu), in which all the terminal chloride ligands of the [Ru3C112]4 precursor have been replaced by phosphines.29 These complexes can be formally represented as Runi-Run-Run arrays, but, as deducible from their typical structural parameters reported in Table 4, with respect to the Runi-Run-Runi [Ru3C18(PR3)4] complexes, the one more electron looks like delocalized on both the two outer Ru atoms. 

We have mentioned that the structural parameters of C2H4 bridged compounds can vary over a wide range. Whereas most examples reported do not have metal-metal bonds, there is one conspicuous exception. Theopold and Bergman succeeded in synthesizing the propane-1,3-d iyl cobalt derivative 125 from the radical anion [(t) ,-C5H5)Co(/z-CO)12 and 1,3-dibromopropane (98, 295) in 40 5 yield. This compound is best described as a dimetallacyclopentane, and its chemistry (thermolysis and reaction with CO and phosphines Scheme 34) supports this view. Formation of cyclopropane (100°C or I2/25°C) is probably the most remarkable feature of this cyclic system. Simple C—C bond formation has never been observed before in ligand-induced or thermal reactions of either mono- or binuclear cyclopentadienylcobalt complexes. The architectural details of... 

Examples of metal complexes containing linear carbon ligands have been characterized for metals from across the transition series (Tables IX, X, and XI) and the structural forms A-D can generally be differentiated on the basis of an examination of the structural parameters. However, the variable precision of the structure determinations, the different size of the various metals and variations in the electrostatic contribution to the M-C bond with metal oxidation state and the nature of the other supporting ligands (e.g., phosphine vs. carbonyl) make detailed comparisons of the molecular parameters within a structural subset somewhat arbitrary. 

Basic trialkyl phosphines are best suited as ligands for the palladium-catalyzed cyclotrimerization of 1 and CO2, and a strong influence of the ligand structure on the performance of the catalyst is observed. As noted earlier [4 c], the Tolman concept [10 a] of electronic (T ) and steric (0) parameters is obviously not sufficient to explain the observed ligand effects. The steric parameter Er, recently developed for phosphine ligands on the basis of molecular mechanics [11], also failed to show any correlation with the experimental results. The understanding of these... 

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