Jr-Back donation

Figure Schematic representation of the two components of the ij -Hi-metal bond (a) donation from the filled (hatched) CT-H2 bonding orbital into a vacant hybrid orbital on M (b) jr-back donation from a filled d orbital (or hybrid) on M into the vacant a antibonding orbital of Hj.

A number of MO calculations have been performed on carbonyl complexes, with methods ranging from ab initio to DVM-HFS. In any case it was found that both a donation and jr back-donation interactions are important in determining the geometrical structure and physical properties of these complexes. The ab initio calculations of Sakaki et al.110 have shown that the strengthening of jt back-donation is the driving force which stabilizes the pseudotetrahedral geometry vs. the square planar one in [Ni(PR3)2(CO)2] complexes. 

In the tj mode (E and Z) the coordinate bond between the heterocarbonyl ligand and the metal may essentially be described in terms of a cr donation from the ligand (lone electron pair at the heteroatom) to an empty d orbital of the metal (d <— nE) (Fig. 2). jr-Back-donation from an occupied metal d orbital to the 77 CE orbital of the ligand (d - zr CK) in the rf mode is rather unimportant. 

C M interaction (a donation) M - C interaction (d-jr back-donation) Net change... 

The occurrence of stable neutral. binary carbonyls is restricted to the central area of the d block (Table 19.3), where there are low-lying vacant metal orbitals to accept o-donated lone-pairs and also filled d orbitals for jr back donation. Outside this area carbonyls are either very unstable (e.g, Cu, Ag, p. 1199), or anionic, or require additional ligands besides CO for stabilization. As with boranes and carboranes (p, 181), CO can be replaced by isoelectronic equivalents such as 2e , H , 2H or L. Mean bond dissociation energies Z>(M-CO)/kJ mol increase in the sequence Cr(CO)fi 109, MoCCO>6 151. W(CO)6 176, and in the sequence Mn2(CO)so lOO, Fe(CO)s 121, Co2(CO)g 138. NiCCO)4 147. 

In the synthetic iron porphyrin, O2 affinity mainly depends on the strengths of the a-donation from the lone pair of O2 to the heme-iron dz orbital and jr-back donation from the d r orbital on the iron to the tt orbital of 02. To evaluate the O2 affinity and/or O2 binding dynamics in myoglobin and hemoglobin, O2 - protein interaction is a further important factor. For example, the O2 dissociation rate constant for oxymyoglobin is relatively smaller than those of O2 complexes... 

A number of (CO)2(L)2Fe( -carbonyl) complexes have been isolated (113) (rf--C=0). These have been prepared by replacement of N2 from /u.-dinitrogen complexes (112) by the aldehyde in question. Two phosphine or phosphite ligands appear to be necessary for satisfactory stability of these complexes. This is in apparent agreement with the results of both ab initio see Ab Initio Calculations) and EHMO calculations and emphasizes the importance of d-jr back-donation to the overall bonding in these complexes. The EHMO calculations also emphasize the role of the repulsive four-electron ligand-ligand interactions in determining the likelihood of rj coordination. 

Hypso-type spectra follow the regular absorption pattern but are blue shifted (a < 570 nm) due to filled metal d (jt) to Por e (jr ) back donation. Metal ions of this type are limited to those of Groups VIII and IB in low-spin 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 cyanide ion is isoelectronic with CO, and binds to platinum and other metal ions by a donation of a pair of electrons in an sp hybrid orbital on the carbon atom, along with a complementary jr back-donation from filled metal orbitals to empty jc orbitals on GN. The anion is a poorer x acceptor than CO, but stable complexes are formed with platinum(II) and platinum(IV), and the ligand is high in the spectrochemical series. 

Ru(OOl) under UHV conditions [67] show a decrease of CO desorption energy from = 140 kj mol for the pure metal to EfX = 107 kJ mol in the presence of coadsorbed oxygen (Oq = 0.5)) [62, 67]. Such a behavior is expected in the presence of electronegative coadsorbates, which cause a withdrawal of metal electrons and diminish the metal-jr back donation. The consequences of this effect are... 

Transition metals from Groups 8 to 12 bind CO2 and form M (C02)n adducts with n varying from 1 to 4. Metal cations are electrostatically bound to the negative quadrupole moment of CO2 in an end-on way. Obviously, the major stabilizing strength is the donation O-to-metal cation which does not contribute to the stabilization of the adduct as the jr-back-donation is close to zero [70]. The M -C02 bond distances depend on both the size of the cation and its electronic configuration. 

In the case of d-block cus metal cations, the strength of the a-coordinative bond in the carbonyl-like species is reinforced by the jr-back donation of d electrons. The actual spectral position of the vco band turns out to be a compromise between the upward shift due to the polarization/a-coordination and the downwards shift due to the r-back donation [1, 22], As a consequence, the stretching frequency of adsorbed CO can arrive to suffer a downwards shift with respect to the free molecule... 

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