Three-center bridges

The NBO donor-acceptor theory of hypovalency three-center bridge bonds... 

Figure 3.119 Leading donor-acceptor interactions in AI2H6 (left) and Ga2H6 (right), showing overlap contours in the plane of three-center bridge-bonding (the pi plane, top row) and two-center skeletal bonding (the sigma plane, bottom two rows), with associated second-order stabilization energies in parentheses.

Figure 4.58 The three-center bridge bond of Y2H6, showing (a) overlapping NHOs, (b) the three-center bond tyiiy- and the associated antibonds (c) tYhy(A) and (d) TYHY(7t). ...

Figure 13.22 Carbonium ion as a two-electron three-center bridge.

Before discussing the structural features of tetraorganomagnesiates in the solid state it should be noted that structures in which the presence of separated anionic and cationic moieties can be distinguished are rare. In most cases such units are linked via electron-deficient bonds, i.e. two-electron three-center bridge-bonded carbon atoms between magnesium and the counter cation. 

One of the main differences between our proposed data-structure and the original concept of ie-matrices is that there are no restrictions regarding the number of allowed types of bonds. Not only single, double or triple bonds are permitted. So far the following types have been included nonbonded, single, double, triple, quadruple, aromatic, three-center, double-three-center, bridge, hapto and connected. If desired, other types can easily be defined and appended to this list, e.g. intermetallic or cluster. 

Each B-H-B three-center bridge bond corresponds to a filled three-center localized bonding orbital requiring the hydrogen orbital and one hybrid orbital from each boron atom. 

Soon after the synthesis of mononuclear transition metal carbonyls, polynuclear metal carbonyls featuring bridging carbonyls, such as Fc2(CO)9 and Co2(CO)g, were obtained and cyclopentadienyl anirni containing complexes like [(Cp)Fe (CO)2l2 followed. The first example, Fe2(CO)9, features three identical bridging carbonyls and six terminal carbonyls in a high-symmetiy (D3 ) strucmre that facilitated theoretical treatment and was extensively studied [16—22], though its X-ray stmcture was obtained quite late [4, 23]. These studies examined two fundamental questions (1) what is the electronic nature of Fe-C(0)-Fe three-center bridge and (2) what role does the metal—metal interaction play in the complex, as the short distance between two irons and the 18-e mle imply a metal-metal bond ... 

Figure 20.7 (a) The structure of B2H6 with its two three-center B—H—B bridging bonds and four normal B—H bonds, (b) The structure of B5H9. There are five normal B—H bonds to terminal hydrogens and four three-center bridging bonds around the base. 

Boron hydrides thus contain four different kinds of bonds ordinary boron-hydrogen bonds, boron-hydrogen-boron three-center bridge bonds, ordinary boron-boron bonds, and boron-boron-boron three-center bonds. W. N. Lipscomb developed a method for determining what combinations of these structural features are possible for a specific boron hydride formula and what are the possible structures. This system is equivalent to the simple bonding rules in organic chemistry that make it possible to translate an empirical formula into possible organic structures. 

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