Backbonding

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

With the increase in electronegativity of the element M the degree of covalence of the bonds M —O and M—0 should increase, as a result of which an increase in electron density on the ion M can be expected. As in the formation of the ir-bond with olefin the ir-backbonding mechanism plays a large role, that should result in an increase in the ir-complex stability. 

Group (5) Acids that are a-acceptors but capable of n-donation in backbonding. This group includes cations with mobile d electrons e.g. Cu +, Co +, Fe"-". 

The vacant orbital in 16e -zirconocene(IV) complexes allows a Ji-interaction with an incoming alkene or aUcyne. However no metal— alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex (>10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

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... 

Backdonation also explains why the isomer shifts of K4[Fe(CN)6] 3H20 and K3[Fe(CN)6] are nearly equal [36] the degree of 7t-backbonding changes upon oxidation or reduction of the metal site and compensates for the change in the number of valence 3d electrons. 

The strong backbonding from the chelated diphosphine Pd(0) metal center to the electron-poor exocylic C=C bond of the QM moieties results in remarkable stability of the complex, with the QM ligand remaining unaffected in water or alcohol, even upon heating. 

An EPR study of the monomeric 02 adducts of the Schiff base complexes of Co(bzacen)(py) (71a) and the thiobenzoyl analog Co(Sbzacen)(py) (71b) characterized the five-coordinate mono (pyridine) precursors and the six-coordinate 02 adducts.327 Increased covalency in the Co—S bonds was seen in the EPR parameters, indicative of 7r-backbonding. Substituent effects on the aromatic rings had no effect on the EPR spectra, but these were reflected in the observed redox potentials. Furthermore, the S-donors stabilize the Co ion in lower oxidation states, which was consistent with destabilization of the 02 adducts. 

The orange, air-stable, homoleptic tetrakis( 71-phosphabenzene)nickel (1046) is tetrahedral (point symmetry 54) and can be obtained from phosphabenzene and [Ni(cod)2].2 25 It features a short Ni—P bond length of 2.1274(5) A with considerable N i P 7r-backbonding and a i/(Ni—P) stretch at 168 cm-1. In solution, partial dissociation of one phosphabenzene ligand is observed. 2-Diphenylphosphino-3-methylphosphinine forms with [Ni(cod)2] in the presence of the CO the dinuclear complex (1047) with a W-frame structure.2526... 

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