7r-Donor ligands

Table 7.11 lists the predicted BDEs of TM compounds with 7r-donor ligands [4, 54, 55, 68-71], The complexes of W(CO)5 with acetylene, ethylene, and formaldehyde belong to the donor-acceptor class. The compounds of WC14 with the same ligands are metallacyclic molecules. 

Water has two lone pairs of electrons. Once one of these is used to form the cr-bond to the metal ion, the second may be used to form a ir-bond. This arrangement causes the set of orbitals to move up in energy, thereby making A0 smaller hence water is a weak-field ligand. Ammonia does not have this extra lone pair of electrons and consequently cannot function as a 7r-donor ligand. 

Ligands such as alkynes, and diphenyldiazoalkanes can act as 7r-donor ligands (as well as -acceptors). The 1 1 adducts of these derivatives are relatively stable compared with those of allenes, which, like olefins, are not generally viewed as rc-acceptor and rc-donor ligands. 

Excitation to the LC states may also result in population of the CT-excited states, especially the LLCT states. These phenomena are frequently encountered in complexes containing both rc-acceptor (eg 1,10-phenantroline or 2,2 -bipyridyl) and 7r-donor ligands (eg aromatic thiols). Then the LC excitation can induce charge transfer between these ligands through central atom (LLCT) that leads to a photoredox reaction. Such reactions were reported in the case of heteroleptic organometallic compound [Rh CylLXCsHs)]3 [37], heteroleptic Re1 complex fac-[Re (L)(CO)3(bpy)]n+ [46] and metal-carbon-bonded platinum complexes [47]. 

Asymmetrical nitrido Tc(V) complexes (simply defined as heterocomplexes) are defined as coordination compounds in which two different bidentate ligands are bound to the same Tc=N group, and are represented by the general formula [Tc(N)(L)(L )]"+/0/". The attempt to develop a high-yield synthesis of these types of complexes may first appear to be prevented by basic chemical considerations. Actually, it is reasonable to expect that the reaction of two different bidentate ligands, A and B, with the same Tc=N group would always yield a statistical mixture of symmetrical and asymmetrical complexes, namely [Tc(N)(A)2], [Tc(N)(B)2] and [Tc(N)(A)(B)]. However, the peculiar properties of mixed 7r-acceptor-7r-donor ligands offered the route to the solution of this synthetic problem. The key approach can be outlined as follows. 

A 7r-donor ligand possesses a high-lying filled orbital that can serve to interact with and destabilize a member of the nonbonding metal d set. As an iUnstrative example we present the trans and cis dioxo problem. Fignre 11(a) constmcts the n orbitals for a trans dioxo ML4 complex. Two of the three members of the t2g set can interact with two of the fonr SALCs of oxygen p AOs. Thus, one metal d and two 0x0 orbitals are left nonbonding. The orientation... 

The historical move to early transition metals in rather high oxidation states and the use of a- and 7r-donor ligands, contrary to what was observed in the late and low-oxidation-state transition metals, has been the preferred approach to achieve a real electron-transfer from the metal to the N2 ligand. In this context, two major strategies have been particularly successful, namely the use of the ancillary ligands mentioned above and the macrocyclic effect. 

Fig. 20.13 TT-Bond formation in a linear L—M—L unit in which the metal and ligand donor atoms lie on the x axis (a) between metal and ligand orbitals as for L = I, an example of a 7r-donor ligand and (b) between metal and ligand tt -orbitals as for L = CO, an example of a 7r-acceptor ligand.

Fig. 20.14 Approximate partial MO diagrams for metal-ligand 7r-bonding in an octahedral complex (a) with 7r-donor ligands and (b) with 7r-acceptor ligands. In addition to the MOs shown, cr-bonding in the complex involves the a g and ti MOs (see Figure 20.12). Electrons are omitted from the diagram, because we are dealing with a general ion. Compared with Figure...

Let us complete this section by considering the occupancies of the MOs in Figures 20.14a and 20.14b. Six 7r-donor ligands provide 18 electrons (12 a- and six 7r-electrons) and these can... 

Octahedral clusters of the electropositive metals, groups 3 to 7, are stabilized by 7r-donor ligands such as halides, chalcogenides, and alk-oxides, but the majority accessible to solution chemistry are the halide complexes. These highly s5Tnmetric and aesthetically pleasing MeYs " and clusters contain a robust core of six metal... 

Saito, T. In Early Transition Metal Clusters with 7r-Donor Ligands" Chisholm, M. H., Ed. VCH New York, 1995. 

It is found that the reaction is best described as an olefin insertion, rather than a hydrogen migration, and that it is promoted by good 7r-donor ligands in the coordination sphere. Note that from the rate law (66) one cannot say whether the slowest step in the inner loop of Figure 2.5 is due to olefin insertion ke or reductive elimination kj. 

---