Pentahapto ligands

Cyclobutadiene complexes afford a classic example of the stabilization of a ligand by coordination to a metal and. indeed, were predicted theoretically on this basis by H. C. Longuet-Higgins and L. E. Orgel (1956) some 3 y before the first examples were synthesized. In the (hypothetical) free cyclobutadiene molecule 2 of the 4 rr-electrons would occupy and there would be an unpaired electron in each of the 2 degenerate orbitals 2. P l- Coordination to a metal provides further interactions and avoids this unstable configuration. See also the discussion on ferra-boranes (p. 174). 

The importance of bis(cydopentadienyl)iron [Fe(jj - 5115)2] in the development of organo-meiallic chemistry has already been alluded to (p. 924). The compound, which forms orange crystals, mpI74°, has extraordinary thermal stability ( 500°) and a remarkable structure which was unique when first established. It also has an extensive aromatic-type reaction chemistry which is reflected in its common name ferrocene . The molecular structure of ferrocene in the crystalline state features two parallel cyclopentadienyl rings at one time these 

A very convenient though somewhat less general method is to use a strong nitrogen base to deprotonate the C5H  

An enormous number of rj -CsHs complexes is now known. Thus the isoelectronic yellow 

Innumerable derivatives have been synthesized in which one or more group is present 

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In general, the solubility of solvent-free complexes is very low. However, coordination of Zr with donor solvents gives more soluble compounds. Depending on the extent of the steric effect of the pentahapto ligand, solvent mono or bis adducts are obtained. However stronger electron-donating molecules such as phosphanes do not form adducts, according to the Tewis acidity of the metal. 

In cyclopentadienyl compounds of copper such as [Cucp(CO)] and [Cucp(PR3)] (R = Et, Bu, Ph) the C5H5 group is a pentahapto ligand. Formally these compounds have 18 valence electrons. However, because of a considerable lowering of the energy of M copper orbitals and a substantial increase in the energy difference between the Ap and As orbitals, the orbitals of copper probably interact weakly with the orbi-... 

Two pentahapto ligands that are closely analogous to Cp are cyclohex-adienyl 5.32 and pentadienyl 5.33. In the hrst, the uncomplexed methylene unit of the ring is bent 30-40 out of the plane of the rest of the ligand, but the ligand is otherwise much like Cp itself. The pentadienyl group on the other hand, is easily able to shuttle back and forth between the n, and Ti structures.52 The form, being a substituted allyl, can have syn and anti... 

Oxidative coupling of two alkynes to give the metallole, followed by CO insertion and reductive elimination. The dienone should be a good pentahapto ligand. 

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