Five-center, four-electron bonding

Five-Center Four-Electron Bonding Structures. The potential for the existence of 5-center 4-electron (5c-Ae) bonding structures 558 have recently been surveyed by Tantillo and Hoffmann980 [calculations at the B3LYP/6-31G(d) level], A cation with three anthracenyl units joined around the C -H-C -H--C core with two approximately trigonal pyramidal carbon atoms and one five-coordinate trigonal bipyramidal carbon was found to have 5c-Ae bonding. The anticipated existence of... 

Organotellurium(II) compounds can also contain a three-center, four-electron bond as shown for 39 to 42 in Fig. 19. Typically, these molecules contain an odd number ligands around the central atom and an electronegative atom helps to stabilize a tellurenyl halide or selenenyl halide bond through chelation to form a four, five, or six membered ring. " Such molecules are described as lO-Te-3 and lO-Se-3... 

To analyze the bonding first remove the four BHB bridging H as protons to give [BgHg]4-. This species has a total of 32 AOs and 30 valence electrons. If we remove 16 orbitals and 16 electrons to form the eight two-center-two-electron B-H bonds, we are left with 16 orbitals and 14 electrons. Hence, two three-center-two-electron bonds (six orbitals and four electrons) plus five two-center-two-electron bonds (ten orbitals and ten electrons) nicely utilizes them all and is consistent with the fact that there are two four-connect vertices in the framework. 

While compoimds with three coordination are known, these species are strong Lewis acids and usually achieve four coordination by formation of three-center two-electron bonds, for example, AI2CI6 (1) (see Bridging Ligand). Aluminum is not restricted to an octet of electrons in its valence shell and compounds with coordination numbers of five and six may be prepared, for example, Cl3Al-2NMe3 (2), [AlFg] " (3). 

In the ensuing chapters we will consider phospholes as two electron donors when bonding occurs to a metal by donation of the phosphorus lone pair only one electron donors when the > P moiety bonds to a single metal center three electron donors, when this same moiety bridges two metal centers four electron donors, when only the diene system is coordinated five electron donors, for the phospholyl group in phosphametal-locenes four plus two electron donors when the diene system is bound to one metal and the phosphorus behaves as a two electron donor to a second metal four plus three electron donors when the diene system is coordinated to one metal and the > P moiety bridges two others and five plus two electron donors when phosphametallocenes behave as two electron donors. 

The molecular structures of 4(C1) (74) and 5(BF4) (70) have been confirmed by X-ray means. Formally, these structures can be considered to contain a P(III) atom which is stabilized through proton-induced chelation by the tertiaiy nitrogen, rendering the P(ni) atom five-coordinate. Although a more sophisticated view of die bonding in these cations involves three-center four-electron MOs along the three-fold axis, the oxidation state of the phosphorus can be viewed as trivalent since electron withdrawal... 

The probability of four-center or five-center bonds completely accounting for the skeletal electron deficiencies in the B9H13 structure (IV-N9) is a probability that must be rejected. Structure IV-N9 for B9H13 is extremely unlikely. By contrast, others have described the same B9H13 structure (IV-N9) as eminently satisfactory (83, 86). This is but one of several examples that illustrate the diametrically opposing predictions resulting from other theories as compared to CNPR theory. 

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