Donor pair method

In classifying these reactions, it will frequently be necessary to determine formal oxidation states of the metals in organometallic compounds. In general, method A (the donor pair method) described in Chapter 13 can be used in assigning oxidation states. Examples will be given later in this chapter in the discussion of oxidative addition reactions. 

For the donor-pair method, treat C as a chloride anion, CP, and CH>=CH2 as a neutral, two-electron donor as stated in the exercise (and Table 22.2). Considering that the anion of Zeise s salt has -loverall charge, the charge on Pt centre must be +2. Since Pt has 10 valence electrons, Pt(II) must have eight and electronic configuration d. Then, the electron count is 8 (from Pt(ll)) + 3x2e (from 3 Cl") + l 2e (from CHi CHj) = 16 electrons. This is a common electron count for d square planar complexes like this one. 

Many reactions of organometallic compounds involve a change in metal coordination number by a gain or loss of ligands. If the oxidation state of the metal is retained, these reactions are considered addition or dissociation reactions if the metal oxidation state is changed, they are termed oxidative additions or reductive eliminations. In classifying these reactions, it is often necessary to determine oxidation states of the metals the donor pair method (Chapter 13) can be used. 

Ligand Neutral ligand method Donor pair method... 

The number of electrons contributed by some ligands by the neutral ligand method and donor pair method are summarized in Table 4. 

Using the donor pair method, perform the electron count for [Cr(CN)5(NO)]", considering... 

In summary, the H-acceptor pairs appear to be very similar to their silicon counterparts, which we have discussed in depth. The H-donor pairs are similar in that the H occupies a silicon-antibonding site however, this is an antibonding site to the defect and not to the host as is found in silicon. It is also interesting to note that the computed hydrogen frequencies appropriate to the latter pairs are better described by theory than the silicon counterparts discussed earlier. It is not clear whether this is a consequence of the electronic-structure method used here, a natural consequence of the differences between the silicon and compound-semiconductor hosts, or simply an accident. 

A formal analogy is often drawn between the linear bonding modes of both ligands NO+ is isoelectronic with CO therefore, in its bonding to metals linear NO is considered NO a two-electron donor, by the donor pair (ionic) method (method A, Chapter 3). By the neutral ligand method (method B), linear NO is a three-electron donor (it has one more electron than the two-electron donor, CO). 

In the donor pair model (method A), NO in the bent coordination mode is considered NO-. The nitrogen in NO" is considered sp2 hybridized and can donate an electron pair to a metal d orbital, as shown in Figure 4-8. In the linear mode, the ligand is considered NO+, with sp hybridization on nitrogen.20 An electron pair is donated through this hybrid. The metal (which is formally considered to have one more electron than in the covalent, neutral ligand case) can then donate an electron pair to an empty n orbital on the ligand. 

The most easily accessible experimental approach to this kind of modulation is offered by the study of solvent effects. This problem has recently been successfully faced by several authors [153,255,256]. In Fig. 28.33 it is possible to observe the modulation of the NLO responses obtained by changing the polarity of the solvent. It can be shown that a similar modulation is observed when the vibrational method is used to estimate molecular hyperpolarizabilities. Obviously, not every push-pull polyene exhibits solvent-dependent behavior. The analysis of the intensity behavior will reveal which systems are more suitable for modulation of bond alternation. Again it is the acceptor-donor pair that makes the molecule more responsive to the action of the solvent. In the polyene regime the chain structure is less affected by the interaction with the solvent. This can be rationalized if one remembers the two limiting canonical structures whose weighted combination reproduces the molecular structure. The interaction with the solvent is reflected in stabilization of the zwitterionic structure. 

Since S -T - energy gaps in organic molecules vary considerably, it is easy to find acceptor-donor pairs in which only triplet excitation transfer is energetically feasible. Such systems have been extensively exploited for study of the factors which influence the efficiency of Reaction (8). Three methods have been used to measure rates of triplet excitation transfers in solution. The first involves measurement of the phosphorescence lifetimes of the donor molecule as a function of the concentration of the acceptor molecule (Sandros and Backstrom, 1962). The general applicability of this method is severely limited by the fact that few molecules phosphoresce in solution. The second, more generally applicable, method uses the flash... 

This method (sec Table 1, method C, 8.2.2.2) yields Au clusters in high yield e.g. (AuyfPPh,) ] as prepared by the Au evaporation or the borohydride route is an attractive starting material for the synthesis of most Au clusters. Scheme I shows the reaction pathways of [AUgfPPh,) ] with electron-pair donor bases. 

Benzene is the prototype aromatic Lewis base. It offers formally three pairs of equivalent, conjugated tt bonds as the potential electron donor. Symmetric-top-type rotational spectra have been observed for the benzene HX complexes, where X is F [139], Cl [140] or Br [141], by methods (molecular-beam... 

Majumdar, Z., Hickerson, R., Noller, H. and Clegg, R. (2005). Measurements of internal distance changes of the 30S ribosome using FRET with multiple donor-acceptor pairs Quantitative spectroscopic methods. J. Mol. Biol. 351, 1123-45. 

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