Orbitals of metallocenes

The frontier orbitals of metallocenes (Figure 3.22) are similar to those of bis(benzene) chromium discussed in Chapter 1. One E," pair is a bonding combination of the metal and orbitals with ligand orbitals, and the other / pair is tfie antibonding combination of these orbitals, which is denoted Ej Three frontier orbitals are largely metal-centered and nonbonding a degenerate E orbital pair (d and d ) and an A, (d ) orbital. These orbitals in ferrocene are filled and are the HOMOs. 

Frontier orbitals of metallocenes containing parallel Cp ligands (the electron occupancy shown is that of ferrocene) the spin states of several metallocenes. 

On a more qualitative level, the bonding in the more stable isomer lb can be explained on the basis of the general molecular orbital scheme for bent (C2v) metallocenes containing 14 valence electrons, as shown in Fig. 5. The localization of three electron pairs in bonding orbitals (lal, 2 i, 2b2) is primarily responsible for the Si-Cp interaction the absence of a silicon orbital of a2 symmetry imposes the presence of a ligand-based non-bonding orbital. Structural adjustment from D5d (ferrocene type) to C2v... 

FIGURE 14.1 (See color insert following page 302.) Frontier molecular orbitals of (a) bent metallocenes (Cp2M M = Ti, Zr) and (b) carbene. 

Figure 2. Contour diagram in the yz plane of the three most important molecular orbitals of the generic d° bent metallocene Cp Mt of Gi symmetry. Solid and dashed lines correspond to positive and negative contour of the wave function.

The e (antibonding), a (nonbonding) and e 2 (bonding) are assumed to be the frontier orbitals for metallocenes. The non-bonding nature of the a orbitals, the HOMO, is in agreement with the observation that for ferrocene (18 valence electrons terminal electronic configuration e2a f) the removal of one electron to form the ferrocenium ion (17 valence electrons) does not substantially destabilize the molecular frame. 

Molecular orbitals in heteronudear diatomic molecules. 167-175 in homonudear diatomic molecules, 160-166 of metallocenes. 670-673 in octahedral complexes, 414-418... 

On the basis of Cp and its heteroanalogues, a wide series of metallocene structures of types 3, 5, 126, 127 [2-5,7,8,13,132,133,192-205] have been obtained. These complexes are typical for Fe, Co, Ni, Mn, V, Cr, and Ru, and are the standard examples of complex compounds with multicenter delocalized coordination bonds formed as a result of combined grouping orbitals of the cyclopentadienyl rings with rf-metal orbitals. 

If the ollide ions are or can rearrange to open icosahedral species (Fig. 2), the open face of these structures would have the characteristics of the cyclopentadienide ion. The five nearly equivalent sp3 atomic orbitals pointing toward the missing apex of the ollide ion should yield five 7r-type molecular orbitals which correspond to the Al, the two j, and the two E2 molecular orbitals of the C5H5 ion. This type of ligand would then be expected to form sandwich-bonded complexes similar to the metallocenes. 

FIGURE 27. General molecular orbital diagrams showing the construction of the orbitals of 14 electron Cp2M metallocenes, in parallel (D d) and bent (C2V) conformations, from the orbitals of the Cp2 and M fragments (only the occupied orbitals are shown). Adapted from Reference 389... 

Continuing advances in the theoretical treatment of metallocenes and substituted derivatives can be anticipated. Despite the historical difficulties with molecular orbital calculations on ferrocene, the problems are now recognized to stem from failure to account for electron correlation effects. New approaches to addressing this issue, coupled with inevitable increases in computing power, should make more metallocenes with substituted groups systems amenable to accurate calculation. More realistic predictions of donor ability, and thus better estimates of their effects in nonlinear optical (NLO) systems, will be possible. 

It is generally assumed that MAO firstly undergoes a fast ligand exchange reaction with the metallocene dichloride ( - Me). Either CH or CH3 is further abstracted from the complex by an Al-center from MAO, thus forming a cationic species (step 1). In the step 2, the monomer (alkene) coordinates to the empty orbital of the complex. Step 3 is related to the insertion see Insertion) of the alkene into the zirconium-alkyl bond which generates a new free... 

Fio. 1. Valence orbitals of the bent metallocene unit (from ref. 24), and schematic representation of the energy profile of the (butadiene)ZrCp2 system. 

Molecular Orbitals The importance of mctiillocenes and the complexity of their bonding make it worth-of Metallocenes while to describe tliem with molecular orbital theory. The properties of the molecular... 

---