Built up from ML5 Fragments

In this section these valence C, ML5 fragment orbitals are used to build up the orbitals of more complex units. First, we will look at the level structure of a simple dimer M2L10 (17.41). The ML5 d orbitals neatly partition into o(z )ti(xz. 

FIGURE 17.6. Interaction diagrams fcir two MoLio sy.stcms. Notice liow the rr levels in Re(C ())5 lie to lower energy than in RcC lj , a direct result of the rr acceptor and donor nature of the ligands respectively. Combined with a shorter metal-metui distance in (he halide the final level diagrams arc quite different. 

How can we increase the bond order between the two ML5 fragments for the case of L= acceptor By shortening the MM distance tlic relevant orbitals change in energy in the obvious way shown in 17.43.1 or the case of JO electrons (a metal) a formal triple bond is predicted Indeed Cp2M2(CO)4 species (M = 

isoelcctronic with the unknown V2(CO)io molecule, with lliis electron configuration have very short metal-metal distances. As we will see later Cp is equivalent to three coordinated ligamls. 

The energy levels of an H(ML5)2 complex (17.44) may be derived in a similar way by adding the hydrogen k orbital to the orbital picture produced by two MLg units set at the metal-metal distance expected in a molecule of this type (f igure 17.7). Note the small interaction between orbitals of 3 type at this distance. We would expect a stable molecule to result for the electron configuration shown at 

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