Jt-backbonding

The Kterature of Lewis add-base chemistry is loaded with incorrect concepts and wild speculations. This situation arose because imaginative chemists have attempted to explain the results of their experiments in qualitative ways. After the fact rationalizations always work-Uke the plays of Monday morning quarterbacks. Whenever the investigator is surprised, a new effect or one of the multitude available is offered as the explanation". The concept of jt-backbonding has been overworked in... 

That this is so can be seen from the investigation of some beryllium NHC complexes [13,134] where the carbene was seen to replace bridging chloride hgands and coordinate to the Be cation despite the lack of suitable electrons for jt-backbonding on the beryllium atom. 

Metal-phosphine bonds can generally be modeled in much the same way as any other metal-heteroatom bond. The fact that phosphines participate in Jt-backbonding (filled dn-(metal) empty dn or a (phosphorous) interaction) is only of importance for generic force-field parameterization schemes, and half-integer bond orders have... 

Early MO calculations provided the composition of the frontier MOs and the character of the electronic transitions in different metallonitrosyls. Briefly, the HOMO has been described as metal-centered for SNP ( 65-70%) (67) and for many other systems (45). HoAvever, in the series of tw 5-[Ru(NH3)4(NO) (L)] complexes, the HOMO is still metal centered for L = NH3, H2O, Cl, and OH, though it is mainly py- or pz-centered for the N-heterocyclic ligands (68). For the [Ru(Me3[9]aneN3)(bpy)(NO)] ion (Table 1), a combined experimental UV—vis/(TD)DFT approach allowed assigning the detailed ordering of frontier orbitals (15), with the HOMO defined as a t(bpy) orbital. On the other hand, the LUMO and LUMO +1 are mostly located on the nitrosyl moiety with 27% contribution firom metal center orbitals, reflecting a substantial Jt-backbonding, comparable to the one calculated in [Ru(tpm)(bpy)(NO)] (25-30%) (30) and [Ru(DMAP)4(OH) (NO)] (30%) (38). As a comparison, recent calculations with [Fe(por) (Ml)(NO)] afforded a pair of LUMO and LUMO +1 orbitals with composition close to 68% Jt x,Jty and 27% dxz,dyz(69j. 

The broadness of the phosphine resonances assigned to the catalyst in Fig. 4 is due to the exchange between the different dihydride isomers Hr and Hr, discussed earlier, and the phosphine arm-on/arm-off equilibrium. Once one of the external phosphines dissociates, CO can coordinate to form E. The replacement of a o-donating alkylated phosphine with a jt-backbonding CO reduces the electron density on that Rh center. This will promote reductive eliminaticai of the two hydrides to produce the Rh(I) bimetallic complex F. The CO saturation of the one Rh center leads to loss of [Rh(CO)4], which probably goes on to form unreactive cluster complexes and monometallic species G. Under low-concentration autoclave conditions (catalyst concentration equal to 1 mM),... 

Figure 1.11 Model complexes studied by Jacobsen and coworkers to evaluate the contributions of ii-donation and jt-backbonding to metal-carbene bonds [32].

Figure 1.14 Spectroscopic investigation of jt-backbonding in [RhCI(COD)(NHC)l and [RhCI(CO)2(NHC)l complexes.

The lone pair on carbon is antibonding for the C-N bond. Upon coordination, it is given as a a bond to the metal. The antibonding character is then suppressed, which leads to the strengthening of the C-N bond (in CO, this lone pair is not antibonding for the C=0 bond). The jt backbonding exists as in metal carbonyls and reduces the strength of the C-N bond. These two effects, a and Jt, are opposite to each other. 

When the Jt backbonding becomes very (too) important, it is necessary to change the formalism and to adopt the writing M-N=N-M, as in the following complex ... 

These pKa are 2.5 units lower than those of the corresponding ammoniums, i.e. the amines are better donors than phosphines (because of the absence of Jt backbonding in amine complexes). 

The Jt backbonding of the vacant d orbitals of silicon strengthens the bond. 

The complexation of unsaturated hydrocarbons by transition metals is a powerful activation method that plays a fundamental role in their stoichiometric and catalytic transformation. The bonds formed are governed by jt backbonding. Olefins, dienes and arenes are usually 2-electron L, 4-electron L2, and 6-electron L3 ligands respectively, and alkynes are 2-electron L or 4-electron L2 depending on the needs of the metal. The odd-electron ligands are the 3-electron LX allyl radical and 5-electron L2X dienyl radicals. Cyclopentadienyl and arene sandwich complexes will be examined in Chap. 11. 

The Jt bond from the olefin to the metal is the a bond (L ligand), whereas the filled d metal orbitals give back an electron density from the side into the antibonding Jt ligand orbitals, which makes the Jt backbonding (see Chap. 2). 

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