Donor pair electron counting formalism

There are two filled molecular orbitals on the allyl ligand that act as good fT-donors. Thus, the allyl ligand is a four-electron donor in the donor pair electron counting formalism. 

FI in donor pair electron counting formalism. Both Fischer-type (low oxidation number) and Schrock-type (high oxidation number) compounds are now known to exist. 

Pentahapto-C5H5 is considered by this method as CsHs , a donor of 3 electron pairs it is a 6-electron donor. As in the first example, CO is counted as a 2-electron donor. Chloride is considered CF, a donor of 2 electrons. Therefore, (if) -C5H5)Fe(CO)2Cl is formally an iron(II) complex. Iron(II) has 6 electrons beyond its noble gas core. Therefore, the electron count is Fe(n) 6 electrons... 

The three structurally characterized examples of this type of bonding are shown in Table 16 and all would involve a twenty electron valence count for the metal if the hydrazido(2 —) ligand functioned as a four-electron donor. The structure of [Re( 7s-CsH5)(CO)2(NNMeC6H4OMe)]460 is typical and is shown in (34). The formal localization of a lone pair on Na causes a significant increase in the metal nitrogen distance consistent with decreased multiple bonding. 

---