N back-donation

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

Furthermore, protonation results in a significant distortion of the coordination polyhedron, i.e., the metal ion is displaced from the plane formed by the four cyano ligand carbon atoms toward the oxo along the M = 0 axis by as much as 0.34 A, which represents about 20% of the total metal-oxo bond length. In spite of this distortion stronger metal-cyano bonds are observed crystallographically, suggesting a better n back-donation by the metal center to the cyano carbons since d-ff overlap is increased. This observation is in line with both the 13C and 15N chemical shift and kinetic data (Section V) for the protonated complexes (8). 

Bacillus ferredoxins, cysteine residues, 38 246 Bacillus thermoproteolyticus ferredoxins, chain topology, 38 244, 246 Back donation and bonding, 16 92, 93 thallium(III) cyano complexes, 43 17-18 (d-d)n Back donation, 33 335 Bacteria... 

For coordination of CO to FeP and CoP, the interaction can be rationalized in terms of a traditional c-donation/n-back-donation... 

The XPES of a number of ethylenediamine and cyano complexes, including [Cr(en)3]Cl3 and K3Cr(CN)6, have been measured in order to obtain the difference between die Nls and Cis binding energies. For the en complexes the AE values are related to o donation, and for the cyano complexes are mainly controlled by the nitrogen charge, which depends on the n back-donation. 27... 

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. 

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. 

The electronic structures of the metal carbonyls have been investigated vigorously over the past few years because the interaction between CO and metals (76, 137) is essential for an understanding of both organometallic chemistry and CO chemisorption phenomena. There is continuing debate concerning the relative importance of o-donation from the carbon lone pair of carbon monoxide and back donation from metal d electrons into vacant rr ligand orbitals. The most frequently cited data in favor of n back donation are... 

The situation changes, however, if the electronegativity of the group X increases drastically as in the series X = NH2+, OH+, F+ 9. n-Back donation is stepwise decreased to zero and n-complexes are formed. This has been demonstrated via the electron density analysis,which reveals that convex bent bonds change into concave (inwardly curved) bent... 

Hypso-type spectra follow the regular absorption pattern but are blue shifted (a < 570 nm) due to filled metal d (it) to Por eg (n ) back donation. Metal ions of this type are limited to those of Groups VIII and IB in low-spin d5-10 states. The relaxation is radiationless (Fe, Co, Ni, Ag, some Ru and Os), phosphorescent (Rh, Pd, Ir, Pt, Au, some Ru and Os) or luminescent (Cu). 

The side-on bonding of H2 to the metal involves the transfer of a bonding electrons of H2 to a vacant metal d orbital (or hybrid orbital), together with the transfer of electrons from a tilled metal d orbital into the empty a orbital of H2, as shown in Fig. 11.5.2. This synergistic (mutually assisting) bonding mode is similar to that of CO and ethylene with metal atoms. The n back donation from... 

The possibility of synergic n back donation from a nonbonding pair of electrons on M into a vacant 3djr orbital on P varies in the sequence ... 

Frequencies of intermediate spin complexes are close to those of low spin complexes because the dz2 orbital is occupied but the dx2 yi orbital is empty. The core size is also influenced by the coordination number and the degree of Fe (t/7t)-porphyrin (n ) back donation in Fe(II) complexes. Thus, v3 must be compared with caution. 

When 7r-acids such as CO and 02 are bound to Fe(II), this frequency is upshifted to the Fe(III) region because the Fe( 7i)-porphyrin (n ) back donation is reduced by the Fe( 7i)-axial ligand (n ) back donation. Since v4 is sensitive to the occupancy of the porphyrin(7i ) orbital, it is more generally called a 71-electron density marker. For more detailed discussion of these structure-sensitive bands, the reader should consult Ref. 9. 

Carbon monoxide and ethylene are common substrates involved in homogeneous catalysis. The bonding of carbon monoxide to a transition metal has been depicted in Fig. 4.4. The bonding of alkenes to transition metals is described by the Chatt-Dewar-Duncanson scheme involving c donation by the filled it orbital of the alkene, and n back donation from the metal into the n orbital of the alkene (see Fig. 4.6). 

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