Bridging 2-bond 4-electron

This is known as a hydrogen-bridge structure. There are not enough electrons to make all the dotted-line bonds electron-pairs and hence it is an example of an electron-deficient compound. The structure of diborane may be alternatively shown as drawn in... 

The boranes are electron-deficient compounds (Section 3.8) we cannot write valid Lewis structures for them, because too few electrons are available. For instance, there are 8 atoms in diborane, so we need at least 7 bonds however, there are only 12 valence electrons, and so we can form at most 6 electron-pair bonds. In molecular orbital theory, these electron pairs are regarded as delocalized over the entire molecule, and their bonding power is shared by several atoms. In diborane, for instance, a single electron pair is delocalized over a B—H—B unit. It binds all three atoms together with bond order of 4 for each of the B—H bridging bonds. The molecule has two such bridging three-center bonds (9). 

Diarylmethylenecyclopropa[6]naphthalenes 14, unlike their benzene parent counterparts which give cycloaddition reactions at the cyclopropene bridge bond [10a], react on the exo double bond in Diels-Alder cycloadditions (see Sect. 2.1.1) [10b]. The reactions of 14 with the highly electron-deficient acetylenic(phenyl)iodonium triflate 584 give products 586a and 587, which are believed to derive from unstable primary [2 + 2] cycloadducts 585 (Scheme 82) [10b],... 

The addition of Na also impacted the Pt-CO linear u(CO) to bridged u(CO) band ratio, referred to as L/B.52-53 The presence of the Na dopant favored an increase in the amount of bridge-bonded CO, where the ratio of 8.3 for no alkali doping decreased to 1.5 for the 2.5% Na-doped sample. Competing viewpoints (e.g., electronic, geometric) regarding the nature of the change in the L/B ratio are summarized in previous works.1-49... 

In our analysis in the first part, we noted that the four B atoms had but one electron pair to bond them together. To bond these four atoms into a chain requires three electron pairs. Since each electron pair in a bridging bond replaces two normal bonds, there must be at least two bridging bonds in the B4H10 molecules. By analogy... 

Even if one can create the SCO ligand-field around one end of a covalently-bridged dinuclear complex, the SCO might influence the ligand-field at the other end. There are many inter-dependent effects to bear in mind of a bonding, electronic and structural kind, and attempts to delineate these are given below. Inter-cluster or inter-chain effects will play difficult-to-control roles in crystalline SCO polynuclear materials, and these have already been alluded to for mononuclear complexes. 

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

Amongst the hydro-closo-polyborates B H 2- members with n = 6 to 12 are known. All hydro-closo- but also hydro-nido-polyborates act as bases they are, however, not typical Lewis-bases as they miss free electron pairs. However, the negative charge at the hydrogen atoms allows for an interaction with Lewis acids A by formation of hydride bridge bonds (3c2e bonds) ... 

The outcome is a repulsion of the C—H a-bond electrons. The methylene carbon atoms in [3.3]paracyclophane assume a chairlike conformation in solution the chair and boat forms were found to be in equilibrium 12>. The slight distortion of the molecule is reflected in the expansion of the bond angles and in dihedral angles at the bridges. The lengths of the aliphatic C—C bonds are normal (1.507 and 1.517 A). 

In principle, electron-withdrawing isocyanides on metal surfaces are more likely to favor the bridge-bonding mode. Since the migration of a terminal isocyanide to a bridging position formally results in the oxidation of the metal (M) atoms, easily-oxidized metals should favor the bridge-bonding adsorption mode. 

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