O-donor orbitals

The basis for a reducible representation is a set of six donor orbitals on the ligands as, for example, o--donor orbitals on six NH3 ligands. Using this set as a basis—or equivalently... 

While direct M-M interactions would stabilize bonding orbitals below antibonding ones, bridging ligands like CO have Jt-acceptor orbitals that stabilize the antibonding M-M orbital and o-donor orbitals that destabilize M-M bonding orbitals. Since the first-row transition metals have more compact if-orbitals, they have weaker... 

Complexes of and The effect of complexation on the splitting of d orbitals is much greater in the case of second- and third-than for first-row transition elements, and the associated effects already noted for Ni are even more marked for Pd and Pi as a result, their complexes are, with rare exceptions, diamagnetic and the vast majority are planar also. Not many complexes are formed with O-donor ligands but, of the few that arc, [M(H20)4] ions, and the polymeric anhydrous acetates [Pd(02CMe)2l3 and [Pt(02CMc)2]4 (Fig. 27.10), are the most... 

Figure 3.77 Leading donor-acceptor interactions in a 3n 7t excited aminodi-alkyl ketone, showing filled (ctqn1) and empty (

Hughbanks has calculated the molecular orbitals of a hypothetical 25-e [Mo6S8L5] cluster and discussed their dimerization (72). The 2eg and 2tIu o--acceptor orbitals lie above the manifold of 12 metal-metal bonding orbitals, which include some s and p hybridization acting to accentuate their projection outward from the cluster. These orbitals are strongly destabilized when the cluster is capped by donor ligands to form the dative exo M—X bonds. There remains one 2e (z2) acceptor... 

Unsaturated organic molecules such as alkenes, alkynes, dienes, polyenes and arenes can also stabilize low oxidation states in metal complexes, being both o donors (filled bonding jt orbitals) and jt acceptors (empty antibonding jt orbitals). In these so-called Jt complexes, only jt orbitals are involved in the metal-to-ligand bonds. This latter type of complex is beyond the scope of this chapter and only a few examples will be given. 

Both coordination modes, if and if, are combined in binuclear fj.-7jl rf complexes (C and C ) in which the heterocarbonyl coordinates to one metal in the rf and to the other in the if fashion. Complexes with (C ) and complexes without a metal-metal bond (C) are known. Complexes of type D (n-rf rf) have not been structurally characterized presumably due to the poorer donor properties of the second lone pair at the heteroatom E (o-CE) as compared to the zrCE orbital. The energy of the o-CE orbital is well below that of the zrcE orbital. In heteroformaldehydes, e.g., the energy separation between the o-CE and the zrCE orbital is 1.25 eV (E = S) and 1.85 eV (E = Se).33 However, complexes of type D might be intermediates in a possible isomerization process of complexes C (site exchange of LnM and LnM2). 

The mode of 02 coordination was for some time a subject of controversy. It now seems settled that the ligand molecule is bonded via one O atom, with an Fe-O-O angle of c. 120°. This is consistent with the prediction of a rather crude VB description of the 02 molecule, which allocates two lone pairs to each atom the valence state of O is (sp2)2(sp2)2(sp2)1(p)1. This description is inconsistent with the observed paramagnetism of 02 (which can readily be explained in terms of MO theory). However, oxygenated haemoglobin Hb.402 is diamagnetic and we may fairly describe the Fe-0 bond in terms of a filled sp2 hybrid donor orbital from the O atom. 

As Si is electropositive with respect to carbon, an Si-C bond is polarized Si+C- Si is therefore a o-donor. The Si-C bond being weaker than the C-H bond by 22 kcal mol-1, this means that JtSiMe lies higher than nMe and 7CSiMe lies below 7t Ie (cf. p. 98). SiMe3 is thus both a better n-donor and a better 7t-acceptor than Me. However, as Si bears a partial positive charge, its orbitals are all lowered (cf. Rule, p. 103). The acceptor effect is therefore dominant. 

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