Electron-deficient multicenter bonding

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

Metallaborane and borylene complexes are related to some extent because both types possess direct metal-boron bonds however, the nature of these interactions is varied. In metallaborane clusters, the framework is made up by nonclassical, electron-deficient, multicenter two-electron bonds, while the borylene ligand (BR) is related to one or more transition metal centers by classicrJ, electron-precise, two-center two-electron (2c-2e) bonds [280 -283]. Since the discovery of various bridged and terminal borylene complexes by Braunschweig and Aldrige, the chemistry of this subarea of transition metal complexes of boron has received significant attention from both structure/bonding and... 

Boron s electron deficiency does not permit conventional two-electron bonds. Boron can form multicenter bonds. Thus the boron hydrides have stmctures quite unlike hydrocarbons. The B nucleus, which has a spin of 3/2, which has been employed in boron nuclear magnetic resonance spectroscopy. 

For low values of the valence electron concentration (VEC< 4 for main group elements), covalent 2c2e bonds are not sufficient to overcome the electron deficiency. We have the case of electron-deficient compounds . For these, relief comes from multicenter bonds. In a three-center two-electron bond (3c2e) three atoms share an electron pair. An even larger number of atoms can share one electron pair. With increasing numbers of... 

A polymeric structure is exhibited by "beryllium dimethyl," which is actually [Be(CH3)2] (see the structure of (BeCl2) shown earlier), and LiCH3 exists as a tetramer, (LiCH3)4. The structure of the tet-ramer involves a tetrahedron of Li atoms with a methyl group residing above each face of the tetrahedron. An orbital on the CH3 group forms multicentered bonds to four Li atoms. There are numerous compounds for which the electron-deficient nature of the molecules leads to aggregation. 

In order to define a borderline between classical and non-classical structures we introduce the following criteria compounds are classified as non-dassical if their framework atoms employ multicenter cr, or multicenter cr and n, or n,cr-distorted multicenter bonding to cope with electron deficiency. 

It is obvious that studying interligand Si-H interactions has reached a great extent of sophistication. At least three classes of nonclassical Si-H bonding can be identified. These are the electron-deficient residual Si H interactions in silane a-complexes and agostic complexes, electron-rich IHI MH SiX, and the more recent multicenter H Si interactions, which are the subject of current debate and have features common to both IHI and a-complexes. This surprising diversity stems from the special role the substituent at silicon can play in tuning the extent of Si H interaction, and from the propensity of silicon to be hypervalent. 

In this review, emphasis will be placed first on definitive proof of structure for compounds of main group organometallic compounds that contain the electron-deficient or multicentered bonds described. Evidence for the occurrence of these systems in both liquid an gaseous states also will be given. Finally, this structural information will be used to elucidate the bonding in the molecules. 

Main-group organometallic compounds are versatile tools in organic synthesis, but their structures are complicated by the involvement of the multicenter, two-electron bonds and ion-dipole interactions that are involved in aggregate formation (5). Electron deficiency or Lewis acidity of the metallic center and nucleophilicity or basicity of the substituents are important considerations in synthesis. The complexity of the structures and interactions is, however, the origin of much of the unique behavior of these organometallic compounds. 

The structures obtained by x-ray and neutron diffraction analysis of compound [U N(SiMe3)2 N(SiMe3)(SiMe2CF[2B(C2B(C6F5)3) ] reveal the electron deficiency of the uranium atom to be effectively compensated by the formation of multicenter bonds between the U atom and Si — CH2 units of the amido ligands the x-ray structure of [U C(Ph)(NSiMe3)2 2 r3-BH4 2] proves unequivocally the -coordination of the BH4 units [444]. The structure of a rare uranium (III) complex with a tripodal aromatic amine tm[(2,2 -bipyridin-6-yl)mcthyl]amine was reported [445]. 

In order to explain the hydrogen bridges in diborane and their apparent electron deficiencies, the concept of three-center bonding, in which one pair of electrons links three atoms, was introduced by Hugh C. Longuet-Higgins and Ronald P. Bell. Lipscomb and co-workers extended the concept to multicenter bonds in general. 

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