Cyclopentadienyl ligands bonds

The symbol ri1 is not used. For a cyclopentadienyl ligand bonded by only one a-bond one uses cyclopenta-2,4-dien-l-yl or cyclopenta-2,4-dien-1 -yl-icC1. 

Thorium.—The production of nearly comparable quantities of propene and propane on photolysis of [Th(PrO( -C 5H 5)3] contrasts with the results of thermal decomposition studies and suggests that -hydride elimination pathways become available on photochemical excitation. The weakening of the metal-cyclopentadienyl ligand bonding e.g. promotion to 7 or ri excited states) and consequent reduction of metal co-ordinative saturation is a possible explanation for this change in behaviour. 

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

Borabenzene metal complexes, like their cyclopentadienyl counterparts, are not readily amenable to breaking of the metal-ligand bond. Such bond breaking will occur more easily in complexes with antibonding electrons. By fortuitous chance the cobaltocene route to borabenzene chemistry (Section V,A) provided 19-e complexes with the useful property of inherently weakened metal-ligand bonds. Thus most of this section centers on the reactivity of Co(C5H5BMe)2 (7) and Co(CsH5BPh)2 (13). 

The zinc atom is pseudo-tetrahedrally surrounded by the cyclopentadienyl and methyl groups and the chelating TMEDA ligand, while the dimethylpyrrole pendant is not coordinated to the metal. The zinc-methyl bond (1.985(2) A) and the (vO-bound cyclopentadienyl ligand (2.185(2) A) are both slightly elongated, possibly due to the steric crowding about zinc. 

Quite surprisingly, 28a and 29a are formed from 28 and 29 with about the same reaction energy (A E -4.0 kcal mol" ), even though secondary radicals are more stable than primary radicals by approximately 3 kcal mol-1 based on their bond dissociation energies. This must be due to steric interactions with the cyclopentadienyl ligand in 29a, which fully counterbalances the radical s increased stability. A similar trend of product stability is observed in the formation of the less favored primary radicals 29b and 30b. The formation of 30a is more favorable by 4.5 kcal mol 1 compared to 29a. This is even higher than the stability difference between a tertiary and a secondary... 

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. 

---