Back donation effect

The 5cr level is primarily located on the carbon atom and can be identified with the carbonyl lone pair orbital, but in addition it is somewhat antibonding between carbon and oxygen. The population of this orbital has a significant influence on the carbonyl stretching force constant and, therefore, changes in the carbonyl force constant cannot be assigned solely to back donation effects. Force constants k in the M(CO)5L molecules can be described by the following equation ... 

The v(CN) stretching frequencies of the diazo complexes are shown in Table 12. They are as high as that observed in the Bu NC group bonded to Ni11 and reflect the importance of n back-donation effects. 

A further consequence of the back-donation effect consists of a weakening of the C—O bond strength that manifests itself in a lowering of the C—O stretching frequency as compared with the value of free CO (= 2143 cm ). Vibration spectra from CO adsorbed on single crystal surfaces have thus far been obtained with Pd (60), Pt (59, 61-64), and Ru (65). In all cases bands below 2100 cm"1 were observed. Some data for Pd are included in Table I and a more detailed discussion will be presented in the following two sections. 

Because of the orthogonality of donor and acceptor 7t-orbitals in the ideal TICT conformation, the electron transfer from donor to acceptor has to be virtually complete [2], possibly somewhat reduced by o--back donation effects [134]. 

A similar analysis of the (HPdCO)+ - (PdCHO)+ reaction (3 showed that in these systems, and in agreement with known experimental data (41), the d - tzqq back donation effects arc rather weak and that they should not interfere significantly in the theoretical description of insertion reactions involving Pd(II) transition metal... 

Altogether, the bond-distance data give evidence that, in the case of axial anomeric substituents, a back-donation effect occurs, with the fluoro compound behaving similarly to the chloro and bromo derivatives. The same proves true with oxygen-bound substituents, although here the enhanced bond-distances point to a substantially smaller effect. 

The oxidation of 1,2,4-triazines may result in the formation of three isomeric V-oxides. 1,2,4-Triazine V-oxides are still electron-deficient aromatic compounds and, due to the electron-withdrawing character of the V-oxide function, the ring carbon atoms are activated for nucleophilic attack. On the other hand, the electron deficiency of the 1,2,4-triazine ring is partially compensated by the back-donation effect of the V-oxide moiety (Scheme 2), which is clearly reflected in H, and NMR spectra (for a detailed discussion, see Section 9.02.3.3) <1989AHC(46)73, 1998RCR633, 2002AHC(82)261>. 

Due to the strong electron-deficient character of the 1,2,4-triazine system, the ring carbon atoms in a vast majority of derivatives are resistant toward electrophilic attack. However, electron-donating substituents and, especially, the A -oxide function with its back-donation effect (see Sections 9.02.5.4 and 9.02.3.3.1), may activate the 1,2,4-triazine ring to such an extent, that electrophilic substitution becomes possible. Indeed, 3-methoxy- and 3-amino-substituted... 

These interactions are efficient when the metal center has empty (or partially empty) o-type s and d, orbitals and occupied d orbitals. Since Fe+, Co+, Rh+, and Ir+ (and aU late transition metal atoms) have double occupied d orbitals but Sc+ (and all early transition metal atoms) has none, the back donation effect is expected to be larger for late transition metals compared to the early ones. 

In contrast, transition metal cations such as Co+ and Rh+, have the s°d" ground state with empty s-orbital as opposed to Sc+, Ir+, and Fe+, and one may expect the strong o(C-H) (s,dJ (M) donation effects for the former cations. Thus, availability of the s°d" state (with doubly occupied 6 and empty s-orbitals) for Co+ and Rh+, as well as Fe+, facilitates both donation and back donation effects and makes the o-bond activation significantly easier for these cations compared to Sc+ and Ir+. This statement is consistent with the calculated trends the reverse reaction HMCHJ—>M+(CH4) occurs without barrier and is controlled by thermodynamic factors for M=Fe+, Co" and Rh". Meanwhile reaction M+(CH4)— HMCHf occurs with energetic barrier of 38.5 and 2.1 kcal/mol, for M=Sc and Ir, and is controlled by both thermodynamic and kinetic factors. The reverse reaction HMCHJ—>M+(CH4) occurs with 18.2 and 10.8 kcal/mol barriers for M=Sc and Ir, respectively. 

The groups ML formed by transition metals have a MO of 2-symmetry whose energy level is very close to that of the ethylene 7i -MO (Fig. 4.4), which gives rise to the so-called back donation effect stabilizing the structure XII ... 

Bistoni G, Belpassi L, Tarantelli F (2013) Disentanglement of donation and back-donation effects on experimental observables a case smdy of gold-ethyne complexes. Angew Chem IntEd 52(44) 11599 11602. doi 10.1002/anie.201305505... 

Since lanthanides cannot back-donate effectively, at least in the M(III) state, any carbene hgands have to be stabilized by their own substituents instead of the metal, and such species as 15.14 have thus been prepared. ... 

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