Bonding multicenter

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. 

Formation of group-IIIB-group-IB and -IIB element bonds is limited almost exclusively to the element B and is observed predominantly in metalloboranes and metallocarboranes. The boron-IB and boron-group-IIB bonding interactions either are of the a type or can be described as multicenter bonds ... 

In these examples the metal atom is connected to the borane frame via multicentered bonds. 

The stoichiometry product does not agree with that used in the experiment, as Ni and B compounds are mixed in diethylether in a 1 1 mole ratio. The (Ph3P)2NiBPh2 0.5 OEt2 product is a dimer with a Ni — B—Ni multicentered bond. ... 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

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... 

For example, the bonding in B4C14 can be interpreted in the following way every boron atom takes part in four bonds, one 2c2e B-Cl bond and three 3c2< Cl bonds on the faces of the B4 tetrahedron. In this way every boron atom attains an electron octet. Eight of the valence electrons take part in the multicenter bonds the other eight are needed for the B-Cl bonds. 

One common feature of all M + hydrocarbon systems mentioned in Sec. 1.2.2 is that none of the products resulted from cleavage of a C-C bond. This is a result of several factors. First, C-H bonds are less directional than C-C bonds (Sec. 1.1), allowing for multicentered bonding at the transition state, which tends to lower the barrier for C-H insertion relative to C-C insertion.2,18,22 Second, since M-H bonds are usually stronger than M-C bonds, intermediates resulting from insertion into a C-H bond are usually thermodynamically favored.141 Third, there are typically more C-H bonds in hydrocarbons than C-C bonds, so C-H insertion is also statistically favored. Finally, C-H bonds are more accessible to an incoming metal atom and are therefore more susceptible to insertion. 

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. 

Multicenter bonding is the key to understanding carboranes. Classical multicenter n bonding gives rise to electron-precise structures characteristic of Hiickel aromatics, which are planar and have 4n + 2 n electrons. Clusters are defined here as ensembles of atoms connected by non-classical multicenter bonding , i.e., all... 

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. 

Classical aromatics like the electron-rich, cyclobutadiene dianion A or cydo-pentadienyl anion B and electron-precise hydrocarbons (e.g., benzene C, Figure 3.2-1) have pure n multicenter bonds and therefore are generally not regarded as clusters. 

The multicenter bonding interaction in 2A is classic in a sense that it results in a well known Hiickel aromatic, which is planar with cyclic delocalized An + 2 n electrons (no hyper-coordinate atoms). The most favorable site for a proton to bind to 2A is not the carbon atom, but the B-B edge. The non-dassical 1C is 47.6 kcal mol-1 more stable than the classical structure 1A [5],... 

When some boron atoms in non-classical boranes are exchanged by isolobal C+ units, the multicenter bonding MOs look qualitatively the same, but the contribution of carbon hybrid orbitals is larger than those from boron atoms [compare Figures 3.2-3(b) and (c)]. This polarization is due to the higher electronegativity of carbon versus boron atoms. 

Multicenter bonding is the key to understanding carboranes. The series [CB H i and C2B H 2 (Schemes 3.2-44) contain mainly polyhedral clusters (trigonal bipyramids 15 and 39, octahedral [CBsHg]- and C2B4H6, icosahedra [CB11 H,2 and C2B10H12) which are three-dimensional a aromatics, however, the... 

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