Five-Center, Four-Electron Bonding Structures

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... 

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. 

Ionic and covalent materials can combine in any group of valence compounds forming a class centered on simple or complex phases with four electrons per atom. Valence compounds with three and five electrons per atom are the nearest nel bors. In each of these subgroups we find that an increase in the atomic weight tends to increase the metallic interaction between the atoms and to alter the structure, However, most of the valence compounds are substances with mixed (ionic-covalent) bonds and with a gap in the electron energy spectrum, i.e., they are semiconductors. 

In these cases, I-, II-, and III-Al 9, conservation of the 3-center bonds would require five (or six) 3-center bonds to be expressed within the skeleton matched with four (or three) 3-center bridge hydrogens, respectively. There are several examples demonstrating that the nine-vertex arachno system is flexible enough to adapt to both electronic alternatives. The structures for i-BgHjgrLB (X-A19) and i-B9Hi4 (A-VI9) are both known in the encumbered crystalline phases and both must accommodate six 3-center bonds within the skeletal framework. By contrast, i-C2B7Hx3 (II-A19) absorbs only five 3-center bonds within the skeletal framework. 

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