N backbonding

Another example of the correlation between the isomer shift and covalent bonding properties is n-backbonding. The observed isomer shift of ferrous cyanides [Fe(ll)(CN)5X"] " [24] becomes more negative with increasing... 

The structure of rhodium(II) carboxylate-derived carbene complexes has been assessed both by quantum mechanical calculations [19,20] and by the study of rhodium(II) carboxylate isonitrile complexes [20,21]. Recent investigations [20] suggest, that also in these highly electrophilic carbene complexes there is a significant n backbonding from rhodium to carbon. 

Scheme 4 Significance of n-backbonding at the stage of the aikyne-metai complex...

Me3Si)2C5H3]U CO [(Me3Si)2C5H3]U(CO) n Uranium-to-carbonyl n-backbonding takes place. The product dissociates reversibly 268... 

Note NUC can engage in metal- carbene n-backbonding to some degree, but it is not necessary for the exceptional stability of the M-NHC bond. 

Of course, this simple bonding model, which implies a certain similarity of silenes tvith compounds of aluminium, explains inadequately the intermediate formation of a complex with carbon monoxide. As is indeed known, n backbonding is essential for the stability of CO complexes. In contrast to aluminium compotmds, silenes possess an occupied n molecular orbital which may interfere with a n state of CO (Scheme 8). Should then silenes also be qualified for intermediate formation of a- and n-complexes, respectively I will now present two reaction examples on this theme ... 

The formation of C FsPdBr proceeds similarly, but the product is stable in the solid state to > 100°C. Thus, there is a remarkable thermal stability difference between the CgHj and C Fj complexes owing to n-backbonding by the better acceptor, CgF, CgFjPdBr is polymeric in the solid state but dissolves in ether to form monomers and in acetone to form trimers, etc.. It has limited stability in benzene. 

The second protonation step is accompanied by loss of the carbonyl ligand, since the alkylidyne complex 22 is a system (considering the alkylidyne ligand as a trianion) and there are no available electrons for n backbonding to carbon monoxide. 

Figure 3 Simplified schematic of metal-ligand a and n bonding, as well as n backbonding...

The absence of 5d electrons and the inertness of the lanthanides 4f shell makes n backbonding energetically unfavourable and simple carbonyls, for instance, have only been obtained in argon matrices at 8-12K. On the other hand, essentially ionic cyclopentadienides are well known and an increasing number of u-bonded Ln-C compounds have been produced (see section 30.3.5). 

Fig. 20] [61]. Noteworthy are the very low field nB NMR chemical shifts for these compounds (5b = 141.2 and 139.0 for 8.60 and 8.61, respectively), indicating a very deshielded boron centre in each case. Further reaction of 8.60 with TlF yields the structurally authenticated fluoroboryl complex CpFe(CO)2B Si(SiMe3)3 F (8.62), the nB resonance showing a diagnostic upfield shift on going from 8.60 to 8.62 (5b 141.2 to 5b 113.2), reflecting the substitution of chloride with the better 7r-donating fluoride substituent. The Fe-B distances for 8.60-8.62 are relatively short [1.964(8), 1.985(11) and 1.983(9) A, respectively], with torsion angles implying that any n backbond-ing must originate in the HOMO-2 of the [CpFe(CO)2]+ fragment.

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