Electronically Unsaturated Molecules

The compounds described to this point have generally been electron precise, obeying the octet rule at E and the 18-electron rule at M. Exceptions 

An interesting tautomerism is possible for compounds of type 19 (or 18). As shown in Eq. (1), the lone pair of electrons could be moved onto the metal fragments, resulting in breakage of the M-M bond, creating a planar EM3 unit (40). This creates an electron-deficient E center, but such molecules have precedent in [E Fe(CO)4 3]2 (E = Sn, Pb). Such intermediates could well be stabilized by partial multiple bonding as for the other ini-dene compounds. 

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In unsaturated molecules electronic effects can be transmitted by mesomerism as well as by inductive effects. As with the latter, the mesomeric properties of a group are described by reference to hydrogen. Groups which release electrons to the unsaturated residue of the molecule are said to exert a +Af effect, whereas groups which attract electrons are said to exert a —Af effect. In aromatic structures the important feature of an M-substituent is that it influences the 0- and p-positions selectively. 

A Semi-Empirieal Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules R. Pariser and R. G. Parr... 

Cycloaddition reactions are those in which two unsaturated molecules add together to yield a cyclic product. For example, Diels-AJder reaction between a diene (four tt electrons) and a dienophile (two tt electrons) yields a cyclohexene. Cycloadditions can take place either by suprafacial or antarafacial pathways. Suprafacial cycloaddition involves interaction between lobes on the same face of one component and on the same face of the second component. Antarafacial cycloaddition involves interaction between lobes on the same face of one component ancl on opposite faces of the other component. The reaction course in a specific case can be found by looking at the symmetry of the HOMO of one component and the lowest unoccupied molecular orbital (LUMO) of the other component. 

Pariser, R., and Parr, R. G., 1. J. Chem. Phys. 21, 466 II. J. Chew. Phys. 21, 767, "A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules."... 

A very useful class of intermediates in synthetic organic chemistry are carbenes CX2 (X = H, R, F, Cl, Br etc.)13). These molecules are typical representatives of molecular, monomeric, highly reactive and electronically unsaturated compounds. Their main characteristics are the following ... 

Table 2. Ionization potentials (I.P.), electron affinities (EA), and computed orbital energies of unsaturated molecules...

In addition to the physiological reaction of N2 reduction, nitrogenase catalyzes a wide variety of reactions involving small unsaturated molecules(56). Table III lists key reactants and products for FeMo nltrogenases. All substrate reductions involve minimally the transfer of two electrons. Multielectron substrate reductions may involve the accretion of such two-electron... 

Another possibility is that we can get radical addition to an unsaturated molecule, e.g. an alkene. Writing in all the electron movement arrows, we have one of the double bond n electrons being used to make the new a bond with the original radical species, whilst the second n electron becomes located... 

Our focus in this chapter shall be primarily upon the surface electrostatic potential, specifically its most positive and most negative values, denoted by Vs, max and Vs,min, respectively. There may be several local and absolute maxima and minima on a given surface. They indicate the most positive and negative sites. The former are often associated with hydrogens, especially acidic ones, and the latter with lone pairs, tt electrons of unsaturated molecules and strained bonds. We have demonstrated that Vs niax and Vs nim correlate well with measures of hydrogen bond donating and accepting tendencies . 

There is considerable evidence that at least many of these reactions require coordinately unsaturated compounds to proceed. In those cases, a dissociation step may be the first part of the reaction. Then the.se coordinately unsaturated compounds react with an unsaturated molecule—it can be most anything as long as it has an available pair of electrons—and this inserting molecule goes in between the metal atom and one of the groups initially bonded to the metal. If the starting material is coordinately unsaturated, so is the product. A final step must be the formation of a coordinately saturated product by some final reaction, either with another ligand or by decomposition of this insertion product. 

Several studies have examined the reactions of Mg+" with unsaturated molecules. Under the lower pressure conditions of FT-ICR mass spectrometry, Mg+" reacts with the polycyclic aromatic hydrocarbon, coronene, via a combination of radiative associative adduct formation (equation 16) and electron transfer (equation 17). The latter reaction is 8 times faster, consistent with it being exothermic. Adduct formation (equation 16) also readily occurs in reactions with Theoretical calculations suggest that related radiative... 

As was the case for alkenes, alkyl substituents lower the enthalpy of the unsaturated molecule. Hence, ketones, with two R s, have lower enthalpies than aldehydes, with one R. The electron-releasing R s diminish the electrophilicity of the carbonyl C, lessening the chemical reactivity of ketones. Furthermore, the R s, especially large bulky ones, make approach of reactants to the C more difficult. 

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