Hypercoordinated Systems

No minimum on the PES was located for MH5 radicals. The trigonal-bipyramidal Ihh structures168, 8a, are transition states for the hydrogen exchange reaction H + MH4 —  

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The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

For the analogous 1-methyl-substituted cation [C gCFb]"1", recent experimental investigations and quantum chemical ab initio calculations agree that the dynamics of this cation system can be adequately described by considering only one degenerate set of cations, which have the hypercoordinated puckered methylbicyclobutonium ion structure 420 without contributions from a degenerate set of (l -methylcyclopropyl)methyl cation structures 4217,166,167. 

Many more cyclic and polycyclic equilibrating carbocations have been reported. Some representative examples, namely, the bisadamantyl (499),859 2-norbornyl (500),40 7-perhydropentalenyl (501),188 9-decalyl (502),188 and pentacylopropylethyl (503)860 cations, are given in Scheme 3.19. All these systems again involve hypercoordinate high-lying intermediates or transition states. 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

The basic methodology that we have used to study the chemical bonding to hypercoordinate main-group elements is much the same as that described in an earlier Chapter The spin-coupled description of aromatic, antiaromatic and nonaromatic systems . 

Calculations of this type have also been performed for PXn (n=3,5) and SXn (n=2,4) fluorides and chlorides [9]. Much the same basic picture emerges for all of these systems, whether normal octet or hypercoordinate, with the variations in the amount of central-atom character in the two-centre spin-coupled orbital reflecting the polarity of the particular bond. Analogous descriptions were found to apply for XeFn (n=2,4) and for SiX ions (X=H,F). 

We initially believed [23] that a similar mode of description carries over to 03, but we now realise that this system is somewhat more complicated than we had first supposed. Calculations for the four out-of-plane n electron, whether using spin-coupled theory or at the CASSCF level, reveal that there exist two solutions that are remarkably close in energy. The one that lies lowest, provided we optimize properly the description of the inactive electrons, corresponds to a singlet diradical whereas the other corresponds to a hypercoordinate central atom [24]. It is clear that neither description carries much conviction on its own, and that we must consider expanding the active space. 

For most of the systems discussed so far, hypercoordinated carbon atoms have featured in the most stable forms of the compounds in question. For example, the bridged metal alkyl structures found by X-ray studies on crystalline samples of such substances as (AlMe3)2 ° or (l. iMc)/ persist in solutions of... 

Although the coordination numbers are unexceptional, and strictly do not justify treatment of these systems as examples of hypercoordinate carbon, we shall see that the bonding of their carbon atoms is very similar to that of the hypercoordinate atoms in associated dialkyls, in that three carbon valences are essentially occupied in bonds within the bridging ligand, while the remaining valency is used to form a three-center metal-carbon-metal bond. 

Although their hydrogen atoms were not located, their relatively short metal-metal distances and acute M-C-M angles at the hypercoordinated carbon atoms show the metal-carbon bonding to resemble that in A Mce discussed previously. This resemblance to the aluminum system is underlined by the structure of the mixed metal methyl Mg(AlMe4)2 (28), also established by an X-ray study. ... 

In all of these systems, the metal-carbon distances involving hypercoordinated carbon atoms are significantly longer than those involving the four-coordinate carbon atoms of the terminal alkyl groups (monomeric BMe2 has a Be-C distance of 1.70 A as shown by an electron diffraction study of the vapor, while two-center Mg-C bonds are typically about 2.16-2.17 A in... 

In compound 76 and in dimesitylmanganese, which crystallizes as the trimer [Mn(mesityl)2]3 (77) " the degree of association is limited by the bulk of the substituents. All of these systems show the characteristic features of 3c-2c Mn-C-Mn bridge bonding greater Mn-C interatomic distances to the bridging (hypercoordinated) carbon atoms than to their terminal counterparts sensitivity of the metal-carbon distance to the metal coordination number and acute Mn-C-Mn bond angles at the hypercoordinated carbon atoms. 

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

Clear evidence for a C-C protonated C4H1C ion (55) (which would resemble 52) has been obtained by Siskin/ while studying the I II TaFs catalyzed ethylation of excess ethane with ethylene in a flow system [Eq. (5.12)]. n-Butane was obtained as the only product no isobutane was detected. This remarkable result can be explained by C-H bond ethylation of ethane by the ethyl cation, thus producing the hypercoordinate 55 carbocation intermediate, which, subsequently, by proton elimination, yields n-butane (56). The use of a flow system that limits the contact of the product n-butane (56) with the acid catalyst is essential. Prolonged contact causes isomerization of n-butane to isobutane to occur (see Chapter 6). 

Subsequently, as more data accumulated on systems containing hypercoordinated boron and carbon atoms, it became evident that a roughly hnear relationship of slope of approximately 0.33 existed between the boron-11 and carbon-13 chemical shifts [a relationship to which both Eqs. (5.35) and (5.36) approximate]. This held for a wide range of isoelectronic pairs of boron and carbon compounds, as iUustrated in Figure 5.12 and Table 5.6, and in greater detail in Chapter 6 of the hrst edition of this book, written when the structures of carbocationic systems were stih to be resolved. Now that more structures have been determined, others can be calculated reliably, and disputes about them are few, we need to discuss only a few recent examples below, to illustrate what B- C NMR chemical shift relationships hold for... 

Using the GIAO-MP2 method (tzp/dz basis set), Rasul et aU " calculated the B NMR chemical shifts of hypercoordinate boron atoms in selected boron compounds and NMR chemical shifts of the corresponding isoelectronic and isostructural carbocations for systems not yet observed under long-lived superacidic conditions B NMR and NMR chemical shift values for the methonium ion CH5+ (5) and the corresponding neutral boron analog BHs (15) were shown to reproduce well the experimental values. Calculations were also performed for the six-coordinate carbocation (diprotonated methane, 18) seven-coordinate carbocation (triprotonated methane, 27) the... 

In the complexes of transition metals, where the metal is coordinatively unsaturated (i.e., it has access to less than 18 electrons in its coordination shell), the metal becomes electron deficient. In the absence of better n-donor and K-donor systems, such coordinatively unsaturated metals can draw electrons from neighboring a bonds to satisfy the electron deficiency of the metal. In fact, many such stable C-H a-bond-inserted complexes containing hypercoordinate carbons are known (also see Chapter There is even a... 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

Computational studies of the Rh-P,N system have shown that the C-C activation product is the most stable (A < -12kcalmol relative to the C-H activation product) and its formation is fast and irreversible. C-H activation is fast and reversible. When choosing structure 132 as the entry channel, structure 133 is the common intermediate for both C-H and C-C activations and structure 134 is a possible transition state, all having hypercoordinate carbon atoms. 

There were numerous mechanisms proposed for the Ziegler-Natta polymerization. Various valencies have been suggested for the involved titanium (the original catalyst system) ranging form four to two. A popular early concept first suggested by Natta " involved the active catalytic species 161 with hypercoordinate carbon, which is formed between surface T/ ions and the cocatalyst (alkylaluminum). 

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