Cp2M bent

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

One of the most important carbon 7i-donors are the cyclopentadienes and their heteroanalogues, for instance 15 and 16. In general, cyclopentadiene itself forms three general types of mononuclear Cp transition metal complexes Cp2M 3 (symmetric molecules with mutually parallel Cp rings examples M = Fe, Cr, Ni), Cp2ML 124 [bent metallocenes, L = H, R, CO, etc. (n=l-3)], and half-sandwich compounds CpML 125 (n — 1 4) [21b], In the bent sandwich complexes... 

Cyclopentadienyl complexes of Ge, Sn, and Pb exist in a wide variety of composition and structure, such as half-sandwiches CpM and CpMX, bent and parallel sandwiches Cp2M, and polymeric (Cp2M)x. 

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... 

There are many complexes of the type [Cp2ML ] (n = l, 3), where M is a transition metal towards the left of the periodic classification (M = Ti, y Zr, Hf, Mo, for example). In these complexes, the CP2M fragment is bent, rather than linear as in the metallocenes. The other bgands are all located in the plane that is perpendicular to that defined by the M atom and the centres of the two Cp rings (3-47 and 3-48). If n = 3, this arrangement imposes very small L—M—L angles (about... 

Figure 3.12. Energy changes for the three lowest-energy orbitals of the d block for a CP2M complex passing from a linear to a bent arrangement of the Cp ligands.

The last mononuclear transition metal fragment that we shall study in some depth is bent Cp2M. There exists a vast body of chemical information on Cp2ML complexes where n = I -3 and M is an early transition metal atom (e.g., Ti, V, Zr, Hf, Mo, etc.). There have been many theoretical treatments of these molecules [41] the one we shall follow was given by Lauher and Hoffmann [42]. These Cp2ML complexes are unique for several reasons the L groups are forced to lie in a common plane, see 20.30 for one example. There are obvious steric requirements in 20.30 the... 

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