Cyclopentadienyl complex isoelectronic

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

The complex (C4Hg)Cp2Ta- MdirtV,I 5)3 (38), whose cationic part is isoelectronic with the neutral Zr and Hf complexes (Section IV.A.l), has been prepared by the reaction of complex 37 (R = R1 = R2 = H) with two equivalents of NaCp, followed by abstraction of the cr-bound cyclopentadienyl ligand (Scheme 9)76. Bonding of the butadiene ligand in 38 in the s-trans conformation was determined by X-ray diffraction analysis. 

Rhenium-acyl complexes, such as 1, are isoelectronic with the iron-acyl complexes discussed above and many reactivity patterns are common to the two groups of compounds. Treatment of complex 1 with strong bases, such as butyllithium or lithium diisopropylamide, results in abstraction of a cyclopentadienyl proton which is followed by rapid migration of the acyl ligand to the cyclopentadienyl ring to produce the metal-centered anion 384. Alkylation of 3generates a metal-alkyl species, such as 4. 

The traditional names ferrocene, manganocene, ruthenocene, nickelocene, etc. are given to the respective bis(ri5-cyclopentadienyl)metal complexes. Ocene names should not be coined for isoelectronic species such as chromocene for bis(ri6-benzene)chromium, for structurally analogous... 

A third member of the isoelectronic series to which PlMn(CO)3 and [PlFe(CO)3]+ belong would be the anion [PlCr(CO)3]. All three analogous cyclopentadienyls are well known. Apparently the 3-phenylpentadienyltricar-bonylchromium anion has been prepared (204). Moreover treatment of (diglyme)Mo(CO)3 in thf with 1 equiv Pl K affords a solution which probably contains the anion [Pl Mo(CO)3] " (205). Addition of iodomethane at — 78°C gives a dark solution from which a complex with the surprising structure 52 was isolated in 48% yield (Fig. 6). 

Formal reductive cleavage of the two rings of nickelocene (4H), followed by removal of four protons from the nickel atom would yield a bis(i7S-pentadienyl)chromium open chromocene complex. This transformation could be regarded as a translocation of four protons from the nickel core to the cyclopentadienyl ligands. Bis(i75-2,4-dimethylpenta-dienyl)chromium (42) is an example of just such a product. Therefore, when only the total number of valence electrons is considered (excluding the methyl substituents), this compound is isoelectronic with nickelocene. 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

As in the isoelectronic (h -arene)Cr(CO)j and (h -cyclopentadienyl)Mn(CO)j species, the predominant result of irradiation of (h -cyclobutadiene)Fe(CO)j complexes is carbonyl loss, allowing simple photosubstitution " . In some cases, however, reaction of the cyclobutadiene moiety does take place ... 

Three types of arene r-complexes have seen significant development in synthesis methodology neutral )] -arene-Cr(CO)3, the isoelectronic cationic )] -arene-Mn(CO)3, and the cationic rj -arene) rj -cyclopentadienyl)iron(II) complexes (Figure The... 

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