X-Back donation

Metal-phosphine bonds can generally be modeled in much the same way as any other metal-heteroatom bond. The fact that phosphines participate in x-backbonding (filled dn (metal) -> empty d or a (phosphorus) interaction) is only of importance for generic force field parameterization schemes, and half-integer bond orders have been used to describe the effect of x-back-donation[ 153). In the usually adopted empirical force field formalism, x-bonding effects, like most of the other structural/elec-tronic effects, are accommodated by the general parameter-fitting procedure (see Parts I and III). 

The results indicate that the bonding between the metallic fragment and the unsaturated molecule lies at the very extreme of the Chatt-Dewar-Diincanson model [2], being dominated by the x-back-donation. Dinitrogen prefers to bind to a low valent metallic fragment in a side-on fashion rather than in an end-on one, since the former mode implies a more pronounced x-back-donation than the latter. 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

Lithium complex bearing a r 1 1 -[ 1,2,4]diazaphospholide ligand, [(ri1 ri1-dp)-(ji-Li) (DME)]2 (DME= 1,2-dimethoxyethane) was first reported by Gudat and coworkers [43], Lithium [l,2,4]diazaphospholide further binds to two M(CO)5 fragments by coordination via the lone pair of the P and one N atom. Formation of mononuclear P-coordinated complexes as intermediates is supported by indication of more efficient M—>L back donation for P- than for IV-bound fragment by X-ray and spectroscopic studies. 

P 162 x 1CT4 cm-1, k0 0.5, c2 0.9, and fry 0.9 (= V23 0.5), indicating only a small degree of metal-ligand interaction for the level. It was suggested by Rettig etal. (65) that the metal orbitals (5 ) of Mn(Cp)2 were more involved in back donation to the ring than in Fe(Cp)2+ (q.v.) by virtue of the smaller apparent k values found for the former (ca. 0.5 vs. ca. 0.8). However, since the calculated parameter is actually fcy 23 this conclusion cannot be substantiated since only the product of k and V23 is currently known for Fe(Cp)2+. 

As for ethers and alcohols, the ease of hydrogen abstraction is determined by polar factors when operating with electrophilic radicals (X ). The polar character is influenced by the same factors as for ethers and alcohols, i.e., a back-donation of charge from nitrogen to the a-C radical accentuates the nucleophilic character. The influence of the abstracting species in the case of dimethylformamide is shown by the results given in Table V, where the attack of carbamoyl and a-amidomethyl radicals... 

X-Ray diffraction shows that the [Pt(CN)6]2 ion is regular octahedral with linear Pt—C—N bonds.3"2 The IR and Raman spectra of K2[Pt(CN)6] have C—N stretching vibrations at higher frequencies and C—N force constants which are larger than the corresponding divalent complexes.303,304 This correlates with less n back-donation from the metal to the empty n orbitals of the cyanide ligand in the PtIV complexes. 

---