Electron-deficient hydrogen bridges

HMnRe(CO)i4 forms an electron-deficient hydrogen-bridged bond with Mn and Re [69,70]. 

Hydrogen bonds contain linear 4e,3-center E H M bridge bonds with hypervalent hydrogen, while sigma complexes contain bent 2e,3-center E-H-M bridge bonds with electron-deficient hydrogen. In the former, the metal acts as a weak base, while in the latter it acts as a weak acid. In principle, intermediate situations are possible where both interactions compete but information is still sparse in this area. 

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... 

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is partiy responsible for the great interest surrounding borane chemistry and molecular stmcture. The stmcture of even the simplest boron hydride, diborane(6) [19287-45-7] 2 6 sufficientiy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged stmcture shown. 

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Carbonium Ions as Reaction Intermediates.—The properties of electron-deficient substances may be expected to be of great importance in the theory of chemical reactions. For example, a positively charged (and hence electron-deficient) carbon atom in a complex carbonium ion would be expected to cause adjacent atoms to increase their ligancies, as by the formation of a three-membered ring and by the use of bridging hydrogen atoms. The analysis of the mechanisms of chemical reactions may in the course of time permit much more precise principles to be formulated than are now at hand. 

Boranes, carboranes, and metallocarboranes are electron-deficient compounds in which B-H-B three-center two-electron (3c-2e) bridged bonds are found. The B-H-B bond results from the overlap of two boron sp3 hybrid orbitals and the Is orbital of the hydrogen atom, which will be discussed in Chapter 13. 

The formation of the dioxolanes in the photo-oxygenations of allylic stannanes with electron rich tin centers (i.e., compare 16 and 20) can be attributed to the ability of tin to stabilize and migrate to an electron deficient P carbon (Sch. 9). The reduced yield of dioxolane in the reaction of 22 in comparison to 20 or 21 can be attributed to a steric effect operating in conjunction with an electronic effect of the carbomethoxy group in the bridged (or perhaps open) intermediate 23 which promotes hydrogen abstraction in lieu of sterically more demanding nucleophilic attack (Sch. 9). 

Hydrogen bridges between the beryllium atoms produce a polymeric structure for BeH2, as shown in Fig. 18.6. The localized electron model describes this bonding by assuming that only one electron pair is available to bind each Be—H—Be cluster. This is called a three-center bond, since one electron pair is shared among three atoms. Three-center bonds have also been postulated to explain the bonding in other electron-deficient compounds (compounds where there are fewer electron pairs than bonds), such as the boron hydrides (see Section 18.5). 

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