Cr-Donor ability

Interestingly, the inductive effect of the N(CH2CH20)3Si(CH2) groups decreases upon increasing the number of methylene groups (n = 0-3) fivefold faster than that of the (RO)3Si(CH2) group277. From our point of view, this means that the cr-donor ability... 

The activation of a ligand upon coordination is determined not only by the electronic properties of the binding metal site, but also by those of the ligand itself, namely its electron-donor/acceptor character. Both have a prominent effect on the redox potential of the complexes which has been the subject of parametrization methods (see also Chapter 2.19) by Pickett75 and Lever76-79 who have defined an electrochemical ligand parameter (PL or El, respectively) which is a measure of the net 7r-electron acceptor minus cr-donor ability of a ligand. 

In octahedral vanadyl complexes, the weakest ligand is usually found trans- to the 0X0 group. The v(V=O) stretching frequency is indicative of the ligand trans- to the V=O bond. As cr-donor ability increases, v(V=O) decreases [80]. This trend was examined in a series of compounds [VO(DCP)L2] (DCP = 2,6-dicarboxylatopyridine, L2 = (H2O)2, o-phenanthroline, bipyridine) which displayed reversible reduction chemistry... 

This equation will have to take both contributing factors (cr-donor ability and TT-acceptor strength) into account, and will likely ignore steric and substituent effects, as these are smaller than the two dominating electronic contributions. We would expect that the correlation factor R will be rather small, indicative of the unaccounted additional contributions. 

TT-acceptor and lower cr-donor ability of the phosphites compared with the phosphines. More CO groups can be replaced without breakdown of the cluster when the ligand is a chelate. Such substituted clusters include Fe3(CO)io[Me2AsC(C(AsMe2)C2F4)] (111), M3(CO)e(diars)3 (M = Eu, Os) (46, 222), and Ru3(CO)6(diphos)3 (222). 

The absence of strong Ji-interactions and the only normal cr-donor abilities of the thioethers make this class of compounds relatively poor ligands. 

In ligands such as CO that can interact with metal atoms in several ways, the number of electrons counted is usually based on o- donation. For example, although CO is a tt acceptor and (weak) tt donor, its electron-donating count of 2 is based on its cr donor ability alone. However, the rr-acceptor and rr-donor abilities of ligands have significant effects on the degree to which the 18-electron rule is likely to be obeyed. Linear and cyclic organic pi systems interact with metals in more complicated ways, discussed later in this chapter. 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

Comparisons between the electronic structures (using a ZINDO analysis) of [Ru(bpy)3] " and [Ru(bpy)(NH3)4], and between related pairs of compounds where bpy is replaced by 2,2 -bipyrazine or 1,2-benzoquinonediimine, show that bpy is unable to accept extra electron density from the metal center whereas the opposite is true for 1,2-benzoquinonediimine. The acceptor properties of the 2,2 -bipyrazine ligand fall between those of bpy and 1,2-benzoquinonediimine. Using the Fenske-Hall method, the electronic structures of [Ru(bpy)3 (ppy) ] "A (Hppy = 2-phenylpyridine) have been investigated. The coordinated ppy is a C,A-donor. The electronic structures of the heteroleptic complexes exhibit a separation of the Ru—C and Ru—N f7-bonding character. It is proposed that the observed preference for cis- over trans- and for fac- over nrer-isomers may arise from the enhanced cr-donating ability of the C atom when it is trans to an N rather than C-donor. ... 

The donor ability of a cr bond may be improved in two ways, by raising the energy of the orbital and by polarizing the orbital toward one end. The first improves the interaction between it and a potential electron acceptor orbital by reducing the energy difference a — the second by increasing the possibility of overlap and therefore increasing the... 

It appears that the metal-ligand bonds in carbonyl and dinitrogen complexes are similar but somewhat weaker in the dinitrogen complexes. Carbon monoxide is no only a better cr donor but also a better ir acceptor. This is 10 be expected on the basis of whatever polarity exists in the CO molecule (see Fig. 5.18) and the fact that the ir antibonding orbital is concentrated on the carbon atom (see Fig. 5.20), which favors overlap with the metal orbital. The superior r accepting ability of CO also accounts for the instability of carbonyl dinitrogen complexes. Both Cr(CO)sN2 and cis-... 

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