Empty n orbitals

Class-b acceptors on the other hand are less electropositive, have relatively full d orbitals, and form their most stable complexes with ligands which, in addition to possessing lone-pairs of electrons, have empty n orbitals available to accommodate some charge from the d orbitals of the metal. The order of stability will now be the reverse of that for class-a acceptors, the increasing accessibility of empty d orbitals in the heavier halide ions for instance, favouring an increase in stability of the complexes in the sequence... 

In low oxidation states, transition metals possess filled or partly filled d shells. The Dewar-Chatt-Duncanson model envisages some of that electron density in (local) d (e.g. d., d y) orbitals being donated into the empty n orbitals of the carbon monoxide ... 

The characteristics of the vinylboranes as dienophiles can be rationalized in terms of a strong interaction of the diene with the empty n orbital at boron. Molecular orbital calculations show a strong interaction between B and C-l in the transition state, and the transition state shows little charge separation, accounting for the relative insensitivity to substituent effects. As for regiochemistry, the para-like selectivity would also be expected to be reduced because the LUMO of the dienophile is nearly equally distributed between B and C-2.36... 

An alkene bearing a heteroatom also shows multiplicity selectivity. Moderately electrophilic singlet carbenes interact with the lone-pair electrons on the heteroatom to generate an ylide by accepting an electron pair in the empty n orbital, and this interaction is more efficient than that with n electron of the double bond. Thus,... 

Fig. 15 J Overlap of a and it orbitals of bridging carbonyl ligands with the d orbiials of metal atoms. The cr orbital of CO can donate electron density to the metal orbitals and the empty n orbital of CO can accept electroi density trom the d orbitals. (From Kostk. N. M. Fcnske.

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. 

Conversely, when the ligands have low-lying empty n orbitals for bonding, examples of which include CO, CN-, PR3,..., etc., the originally nonbonding 2g orbitals now become n bonding in nature, while the eg orbitals remain... 

The third case includes those ligands which possess both filled and empty n orbitals. Examples are the Br, Cl" and CN" anions. However, in order to understand the chalcophilic properties of transition elements, only the second case involving n-bonding with metal t2g orbitals needs to be considered. 

When an unsaturated ligand such as an alkene approaches the metal sideways to form a n complex, similar interactions lead to bonding. The filled 7t orbitals of the ligand bond to empty d orbitals of the metal, while filled d orbitals on the metal bond to the empty n orbitals of the ligand. The result is a % complex with the metal-alkene bond perpendicular to the plane of the alkene. The bond has both a and 7t character. 

Osmium(IV) ammines have unusually low pXa values (see Table 5) and so persist only in strongly acidic media the complexes are more acid by an order of five to six than their platinum(IV) counterparts but only slightly more acidic than the corresponding iridium(IV) complexes. This has been correlated with the decreasing t2g electron occupancy in the series PtIV-Irlv-OsIV and attributed to charge transfer from ligands to the empty n orbitals on osmium(IV). The cis isomers are more acidic than the trans, and this can be correlated with differences between the osmium(III) and osmium(IV) complexes for the isomers.55... 

In contrast, lJ02 oxygen, often just referred to as singlet oxygen , reacts as an electrophile, via the empty n orbital. Thus its reactions... 

The explanation for this is believed to be that, as shown in Fig. 16-2, the C— M cr dative bond is reinforced by a donation of electrons in the jr-type d orbitals of M to empty n orbitals of the CO. Indeed, some calculations ascribe more importance to the jr-type back-bonding than to the cr donation. 

The activation of C—C multiple bonds essentially involves electron donation via a coordinate bond to the empty orbital on the metal and a 7i-back donation from the metal to empty n orbitals on carbon atoms resulting in the well-known s)mergetic effect (13). As a result of these interactions, C—C multiple bonds are activated. 

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