Five-electron donors

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

During the reaction, the diene ring loses an H atom or an R radical and is now to be considered as a neutral five-electron donor or an anionic six-electron system. 

The reaction of H2Os3(CO)10 with Rh(C2H4)2(MeCOCHCOMe) yields the mixed metal cluster RhOsjH2(MeCOCHCOMe)(CO)10 whose structure is shown in Figure 15. One of the acetyl-acetonate oxygen atoms, 0(3), unsymmetrically bridges Os(3) and Rh, while the other, 0(2), is coordinated only to rhodium so that this ligand can be considered as a five-electron donor.509... 

The complex chemistry of the hydrosulfido ligand has emerged during the last decade. The bonding versatility of this ligand is immediately obvious from Figure 1, and for each of the types A-C examples have been reported in the literature. This flexibility is mainly due to the fact that the SH group can act as a one-, three- or five-electron donor. However, polynuclear systems with and... 

Fig. 13. Triply bridging ligands as five-electron donors in Os3H(ji3-L)(CO)9.

The addition of Rh(acac)(CO)2 (acac = MeCOCHCOMe) to Os3H2-(CO)10 gives another rather more complex example of a doubly bridging metal system Os3H2 Rh(acac)(CO) (CO)9 (97), in which the wing tip atoms (Rh and Os) are bridged by the five-electron donor acac ligand. The distance... 

Fig. 36. Ring geometries in two five-electron donor C4R4H and one three-electron donor C4R4H complexes.

The rhodium complex [Rh(acac) (C2Hi )2] reacts with the unsaturated triosmium compound to give the 60-electron cluster species [Rh0s3(y-H)2(y-acac)(CO)10J (A7) in which the acac ligand acts as a five-electron donor, one oxygen atom bridging an Os — Rh separation of 3.292(2) so that the four metal atoms adopt a butterfly configuration. 

The rf-pentadienyl ligand, as a five-electron donor in its semicircular, delocalized form represented schematically in 16-XXXV is capable of forming a variety of metal complexes, including a so-called open ferrocene, (C5H7)2Fe. 

In the ensuing chapters we will consider phospholes as two electron donors when bonding occurs to a metal by donation of the phosphorus lone pair only one electron donors when the > P moiety bonds to a single metal center three electron donors, when this same moiety bridges two metal centers four electron donors, when only the diene system is coordinated five electron donors, for the phospholyl group in phosphametal-locenes four plus two electron donors when the diene system is bound to one metal and the phosphorus behaves as a two electron donor to a second metal four plus three electron donors when the diene system is coordinated to one metal and the > P moiety bridges two others and five plus two electron donors when phosphametallocenes behave as two electron donors. 

As the five-electron donor converts to a one-electron donor. Re accepts an additional four electrons from the two trimethylphosphine ligands. One additional mole of PMcj causes loss of Cp from the metal. It is highly likely that the 77 -Cp complex is an intermediate in the reaction although it was not detected. When Cp ligands are found coordinated in arrangements other than the symmetric we sometimes refer to them as slipped ring systems. The rhenium reaction is an extreme example in which the ring has been induced to slip completely off the metal atom rf n Thus... 

Ligands with lone pairs such as R, R2N, X, and O groups can use their lone pairs to make 77 bonds to the metal, as in M OR <—> M=OR. (Again, formal charges are usually not drawn.) Thus, these groups can be three- (R, R2N, X), four-(O), or even five-electron donors (RO, X), if the metal needs the extra electron density and it has orbitals of the right symmetry to overlap with the donor orbitals. 

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