Bis- 5-cyclopentadienyl complexes,

Of the 24 lines expected for coupling between V (I =7/2) and two equivalent C (I = V ) nuclei, 18 were resolved in the epr spectrum of VO(S CNEt2)2, showing that the C(2s) orbital can also participate in transannular interactions (61). [Cp2V(S2CNEt2)BF4] and dithiophos-phate have been used in an extension of studies on the C4V oxovana-dium(IV) chelates to yield C2V bis(cyclopentadienyl) complexes. 

Bis(cyclopentadienyl) complexes are central to the organometallic chemistry of the early transition metals and feature in applications such as alkene polymerization chemistry. Parallels can be drawn between a porphyrin ligand and two cyclopentadienyl ligands, in that they both contribute a 2— formal charge and exert a considerable steric influence on other ligands in the same molecule. Several of the metalloporphyrin complexes discussed below have bis(cyclopentadienyl) counterparts, and authors in some ca.ses have drawn quite detailed comparisons, although these discussions will not be repeated here. 

Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

Electrochemical oxidation of the M(IV) bis-cyclopentadienyl complexes such as [Mo( j -C5H5)2X2], where X is an anionic ligand, have been widely studied. Invariably, such complexes undergo reversible oxidation to the M(V) species... 

The tetramethylethanediyl-bridged, f-butyl-substituted bis-cyclopentadienyl complex Me4C2(3-f-BuC5H3)2Mg(THF) (129) is present in the solid state as a me o-diastere-oisomer. It has a structure in which both cyclopentadienyl groups are /7 -bonded to magnesium while only one THF molecule is coordinated to magnesium (Figure 62) . 

The stereocontrol in the polymerization of MMA mediated by chiral organolanthanide metallocenes has been investigated. Flydride, hydrocarbyl, and amido bis(cyclopentadienyl) complexes (Scheme 268) were used as precatalysts for the processes. The achiral hydride precatalysts yielded syndiotactic PMMA with narrow polydispersities (Mw/Mn = 1.02-1.05). The polydispersities of PMMA from the chiral and achiral hydrocarbyl precatalysts are larger then those from the hydride precatalysts, and amide precatalysts gave the largest polydispersities.982... 

Broad or bimodal molecular mass distributions have been obtained using mixtures of metallocenes,452 487-491 linked homo- and heterobimetallic bis(cyclopentadienyl) complexes for broad molecular mass distribution,492 or amino-functionalized bis-Cp complexes.476... 

In addition to ansa-Cp-amido and bis(cyclopentadienyl) complexes, several monocyclopentadienyl complexes... 

E/O co-polymerization reports with bis(cyclopentadienyl) complexes include ethylene/1-butene, ethylene/l-pentene,254 ethylene/4-methyl-l-pentene,516,522,523 ethylene/1-hexene,229,230,254,446,447,478,481,484,509,512-515, 517-519,522,524-530 ethyiene/1.octene(254,504,505,515,522,531-535 ethyiene/i decene,254,520 ethylene/1-dodecene,254,536 ethylene/1-tetradecene,254,532 ethylene/1-hexadecene,511,512,518,525 ethylene/1-octadecene.532,536... 

Although a large share of the EP co-polymers and EPD terpolymers is still manufactured with vanadium-based catalysts at low temperature, metallocene catalysts have added a whole new dimension to EP co-polymerization, and to the range of material properties that can be achieved, as has been the case for the ethylene-based low-density copolymers discussed in the previous section. The subject of EP co-polymerization with group 4 bis(cyclopentadienyl) complexes has been reviewed in detail up to 1998.59... 

Table 13 Selected results from syndioselective bis-cyclopentadienyl complexes...

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