Retrodative bonding

Relatively high stability in ligand substitution reactions is associated with the rather high ability of platinum to form retrodative bonds. Thus the acetylene is a 77-acceptor and should therefore facilitate the trans-ligand A-type substitution 1). This has been confirmed by the data of Allen and Theophanides 18) on the hydrolysis of K[(acetylene)PtCl3],... 

The nigher value of the 2-AmPta complex (363 mV) is consistent with stronger retrodative bonding between Ru(II) and 2-AmAii than with Ado (342 mV) ... 

In fact, the sulfur 3d orbitals, in contrast to those of nitrogen, are energetically available, thus allowing the formation of a p — d retrodative bonding between oxygen and sulfur atoms,... 

The covalent part of the bonding character of both Cp ligands with Ee(ll) can be summarized by c-bond interactions (Cp slpjd22), strong 7i-interactions (Cp d jdyjpjpy), and weak retrodative 8-interactions (d y/d 2-y2 Cp )... 

Of particular interest are o-organyls having empty orbitals of it symmetry (with respect to the M-C vector) available for overlap with occupied metal orbitals of n symmetry (t2t,). Such ligands, e.g. alkenyls (vinyls), acyls and alkynyls (acetylides), can in principle supplement their o-bonding with a Jt-retrodative component. This may be reflected in an increased M-C bond strength (Table 1.5), a decrease in the M-C bond length (Figure 4.2) or in restricted rotation about the M-C bond, as sometimes observed in VT NMR experiments. 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

The ferraboratrane [Fe K4-(B(mtfBu)3) (CO)2] is the first example of complex having a retrodative Fe —> B bond. It has been prepared by... 

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