Electronically equivalent species

Much chemistry of main group and metal carbonyl species can be rationalized from the way in which these species can achieve closed shell (octet or 18-electron) configurations. These methods of achieving more stable configurations will be illustrated. j for the following electronically equivalent species ... 

Electrons Short of Filled Shell Examples of Electronically Equivalent Species ... 

It was stated above that the Schrodinger equation cannot be solved exactly for any molecular systems. However, it is possible to solve the equation exactly for the simplest molecular species, Hj (and isotopically equivalent species such as ITD" ), when the motion of the electrons is decoupled from the motion of the nuclei in accordance with the Bom-Oppenheimer approximation. The masses of the nuclei are much greater than the masses of the electrons (the resting mass of the lightest nucleus, the proton, is 1836 times heavier than the resting mass of the electron). This means that the electrons can adjust almost instantaneously to any changes in the positions of the nuclei. The electronic wavefunction thus depends only on the positions of the nuclei and not on their momenta. Under the Bom-Oppenheimer approximation the total wavefunction for the molecule can be written in the following form ... 

The observation of a bent Cr-H-Cr bond in the tetraethylammonium salt without an accompanying substantial deformation of the linear architecture of the nonhydrogen atoms in the [Cr2(CO)io(M2-H)] monoanion reflects the inherent flexibility of the bond. The deformability of the[M2(CO)io(M2-H)] monoanion species to adopt an appreciably bent, staggered carbonyl structure was first reported by Bau and co-workers (23) from neutron diffraction studies of two crystalline modifications of the electronically equivalent, neutral W2(CO)9(NO)(m2-H) molecule. Subsequent x-ray diffraction studies (15) of the analogous [W2(CO)io(m2-H)] monoanion found that the nonhydrogen backbone can have either an appreciably bent structure for the bis(triphenylphosphine)-iminium salt or a linear structure for the tetraethylammonium salt, with the W-W separation 0.11 A less in the bent form. Crystal packing forces probably were responsible (15) for the different molecular configurations of the monoanion in the two lattices. In solution, however, all known salts of the [W2(CO)io(m2-H)] monoanion exhibit the same three-band carbonyl ir absorption spectrum char-... 

The intermolecular cycloaddition of an electron-deficient species such as a nitrene, a nitrenium ion or a carbene (or their formal equivalents) to the ir-bond of an alkene, alkyne, imine, or nitrile is a significant approach to aziridines and azirines (Scheme 2). 

In light of the great affinity of silylenium ions for electron-rich species, one could be also skeptical about the possibility of the existence of the tricoordinate Si+ species in solvents like acetonitrile (AN) and sulfolane. These solvents are known to have nucleophilic coordination ability. Gut-mann s donicity number (84), 14.1 and 14.8 for AN and sulfolane, respectively, is comparable to that for acetone, 17.0. Coordination of acetonitrile to silicon has been considered in some systems (85,86). The small equivalent conductance of triphenylsilyl perchlorate in CH2C12 may be explained by the domination of the covalent form (18). Consequently, the absence of the coordination of acetonitrile and sulfolane with the tricoordinate Si+ observed in CH2C12 (with 1 or 6 equiv of acetonitrile) may simply indicate that C104 coordinates to Si+ under these conditions more readily than acetonitrile and sulfolane [Eq. (15)]. The reverse situation in acetonitrile... 

Finally, clearly, if instead of the above we were to consider the unpaired-electron chemical species 1Ft°A(1FI0x)3, the same equivalence properties would show up in the simulated fixed-field or fixed-frequency EPR spectra. 

It should be noted that when the 0-0 bond is broken, the O2 molecule bound at Fe + receives four electron equivalents and yields a water molecule and an oxide on Fe +, at least, formally. The stable oxygenated form and the unstable peroxide intermediate provide the four-electron reduction of O2 at once. As is well known, if an O2 molecule receives four electrons one at a time, three active oxygen species, superoxide, peroxide, and hydroxyl radical, will be produced during the O2 reduction. Cytochrome c oxidase must reduce O2 totally without releasing these species, which are extremely toxic to the cell. The four-electron reduction of this enzyme may be the strategy of this enzyme for safe O2 reduction (without damaging cells). 

The stability of EH2 is very species dependent. All of the above results refer to the pig heart enzyme and, where tested, to other mammalian species. It was initially reported that no long wavelength absorption was observed upon reduction of E. coli enzyme with NADH 109), but reduction by 1 equivalent of NADH or dihydrolipoamide leads to the formation of 25% of the maximal 2-electron-reduced species 108) and similar results are obtained with the Azotobacter enzyme 114)- That this species is the catalytically important one in the E. coli enzyme as well as in the mammalian enzyme has also been demonstrated 50). Reduction with dihydrolipoamide in the rapid reaction spectrophotometer at 2° results in the full formation of EH2 followed by the slow k = 13 min, 1 mAf dihydrolipoamide) four-electron reduction. The spectrum of EHa generated in this way is shown in Fig. 7 and is identical with that of the pig heart enzyme. The 2-electron-reduced form, EHj of lipoamide dehydrogenase of spinach 99) may be somewhat unstable however, spectrally it is difficult to distinguish between instability and formation of the EHa-NADH complex (see above) on the basis of available spectral data. Either phenomenon could lead to inhibition by excess NADH. In glutathione reductase it is possible that the complex can be rapidly reoxidized by glutathione 53). 

The useful reaction of oxidized p-phenylenediamines is to generate dye. They can react with an electron-rich species, a coupler anion, to form a leuco dye which either oxidizes or loses a fragment to form the dye. As an example, oxidized p-phenylenediamine can react with a phenol to form a cyan dye as shown in Scheme 2. The leuco dye oxidation usually takes place with the consumption of another molecule of CDox, which is reduced back to the color developer. Thus production of a molecule of dye by this route uses two molecules of CDox or four silver ions and is known as a four-equivalent reaction. Dye formation ean proceed by a two-equivalent route if there is a good leaving group such as a chlorine atom at the reaction site on the coupler anion. 

Similarly, 6-electron main group species show chemical similarities with 16-electron organometallic species. As for the halogens and 17-electron organometallic complexes, many of these similarities can be accounted for on the basis of ways in which the species can acquire or share electrons to achieve filled shell configurations. Some similarities between sulfur and the electronically equivalent Fe(CO)4 are listed in Table 15-2. 

The concept of electronically equivalent groups can also be extended to 5-electron main group elements [Group 15 (VA)] and 15-electron organometallic species. For example, phosphorus and Ir(CO)3 both form tetrahedral tetramers, as shown in Figure 15-1. The 15-electron Co(CO)3, which is isoelearonic with lr(CO)3, can replace one or more phosphorus atoms in the P4 tetrahedron, as also shown in this figure. 

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