Three-centre two-electron bonding

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... 

Carbenium ions generally have a planar, or close to planar, sp -hybridized electron-deficient centre. The carbocation centres in carbonium ions are essentially less electron-deficient carbon atoms bound with other atoms by three single bonds and by a fourth two-electron three-centre bond. The latter may be directed either to the two other cation centres (left, pentacoordinate ions) or to the carbon atom to which they are also bound by a single bond (right, tetracoordinate ions). 

Beryllium hydride, chloride, and di-methylberylhum form polymeric bridged species but, whereas the bridging in the chloride is via an electron pair on chlorine atoms and can be regarded as an electron-pair donor bond, the bonding in the hydride and in the methyl compound involves two-electron three-centre bonds. Coordi-... 

According to Olah an extremely active proton can be attached to single C-C or C-H bonds in an alkane molecule.A transition state complex is formed in which a proton is bound to the alkane by a two electron-three centre bond. The possibility of the existence of such a state had been already suggested by Lewis and Symons in 1958. "... 

M—cr-bond, via the formation of a two-electron, three-centre bond, not through direct linear hydride-ion transfer. ... 

The distorted octahedral cluster [Cug(M3-oxl)4 /t-Br)2l (oxl - 4,4-dimethyl-2-oxazoline-4-methylphenyl) has four two-electron three-centre bonded aryl bridges.Clusters [Au8(PPh3)7(CNR) ]2+. [Au9(PPh3)g(CNR)2] + and... 

Five-electron three-centre bonding units involve the distribution of five electrons amongst three overlapping AOs that are located on three atomic centres. The 7t-electrons of triatomic systems with 19 valence-shell electrons provide examples of these types of bonding units when only valence-shell pjt AOs are used to accommodate the electrons. Another example is provided by the indirect interactions of two nitrogen atoms via a hydrogen atom in medium-ring bicyclic compounds [23]. We consider here symmetrical systems, for which there are three canonical Lewis structures, XXVII-XXIX,... 

Thus five-electron three-centre bonding units have three different types of VB representations, the wavefunctions for which are equivalent [52,53]. As an example, we display them in Figure 8 for the five pi electrons of sym CIO2, when it is assumed that these electrons only occupy pit AOs. Spin-pairing of the 03 odd electrons of two CIO2 molecules generates CI2O4, for which the asym isomer... 

Example of increased-valence structures, and the Lewis structures and three-electron bond structures from which they may be derived, are displayed in Figure 10 for NO2 and N2O4. These two molecules involve four-electron three-centre and five-electron three-centre bonding units. N2O4 also possesses six-electron four-centre bonding units that are components of a ten-electron six-centre bonding unit. 

In a more recent proposal, a description of the bonding as a two-electron three-centre Mn( H)Si has been suggested65,76,78,79 (Figure 5). Such a representation would account for the above-mentioned chemical and physical properties of these complexes. A similar suggestion has been put forward in the case of silyl-diplatinum complexes, where a Si-H distance of 1.72 A was found82. 

FIGURE 5. Representation of hydrido silyl-manganese as a two-electron three-centre Mn(/ H)Si bond... 

On the basis of all the data obtained Olah has ascribed to the 2-norbomyl cation the structure of a comer-protonated nonclassical cation. The bond nature can be best described with a two-electron three-centre molecular orbital (2e3C) ... 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

Ahlrichs and Ehrhardt75 calculated shared electron numbers for 1. While bonding is normally reflected by two-centre contributions and negligible contributions from three and four-centre terms, a CCC shared electron number of 0.3 was calculated for 1, which is indicative of significant three-centre bonding. 

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