Boron atoms, coordination

Except for Sr2B509Cl Eu, researchers have conducted some detailed study on M2B509X-based phosphors. In the crystal structure of M2B5O9X (M = Ca, Sr, Ba X = Cl, Br), boron atoms coordinate with three or four oxygen atoms to form [BO3] and [BO4] such that these infrastructures comprise the (B5O9) network. In contrast. 

The boron atom in boron trifluoride is hybridized to the sp planar configuration and consequently is coordinatively unsaturated, ie, a Lewis acid. Its chemistry centers around satisfying this unsaturation by the formation with Lewis bases of adducts that are nearly tetrahedral sp [ The electrophilic properties (acid strengths) of the trihaloboranes have been found to increase in the order BF < BCl < BBr < BI (3,4). 

The a-rhombohedral form of boron has the simplest crystal stmcture with slightly deformed cubic close packing. At 1200°C a-rhombohedral boron degrades, and at 1500°C converts to P-rhombohedral boron, which is the most thermodynamically stable form. The unit cell has 104 boron atoms, a central B 2 icosahedron, and 12 pentagonal pyramids of boron atom directed outward. Twenty additional boron atoms complete a complex coordination (2). 

The a-tetragonal form of boron has a unit cell B qC2 or B qN2 it always has a carbon or nitrogen in the crystal. The cell is centered a single-boron atom is coordinated to four icosahedrons (4Bj2 + 2B). The -tetragonal form has a unit cell of 192 boron atoms but is not, as of this writing, totally defined. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

The radius of the 24-coordinate metal site in MBs is too large (215-225 pm) to be comfortably occupied by the later (smaller) lanthanide elements Ho, Er, Tm and Lu, and these form MB4 instead, where the metal site has a radius of 185-200 pm. The structure of MB4 (also formed by Ca, Y, Mo and W) consists of a tetragonal lattice formed by chains of Bs octahedra linked along the c-axis and joined laterally by pairs of B2 atoms in the xy plane so as to form a 3D skeleton with tunnels along the c-axis that are filled by metal atoms (Fig. 6.11). The pairs of boron atoms are thus surrounded by trigonal prisms of... 

Figure 6.10 Cubic MBs showing (a) boron octahedra (B-B in range 170-174 pm), and (b) 24-atom coordination polyhedron around each metal atom.

Similar possibilities arise for 10-atom clusters. Thus, dimerization of the c/oso-CtBj claster l,5-Me2C2B3Et3 (56) by means of K metal then I2 in thf yields the classical adaniantane derivative Me4C4B6Et6 (f) when this is heated to 160° the mdd-tetracaibadecaborane cluster (g) is obtained rapidly and quantitatively. It will be noted that in (f) all four C atoms are 4-coordinate and all six B atoms are 3-coordinate, whereas in (g) the three C atoms in the C3 triangular face are 5-coordinate while the boron atoms are variously 4, 5 or 6 coordinate. 

The di- and triborolyl ligands tend to i -coordination in sandwichforming reactions. There is a clear-cut tendency for stacking processes followed by the formation of multidecker species and often stabilization of the unusual oxidation states of the transition metals. The route to the linked sandwich and multidecker complexes is attractive for materials chemistry. Thia- and azaborolyl organome-tallic chemistry follows the same trends, although in the azaborolyl complexes the i -rather than i -coordination is sometimes realized. Moreover, coordination via the boron atom is known. In the B, N, Si-heterocycles, the heteroring is j " -coordinated. 

MP2, MAs2 and MSb2 all have a compressed form of the marcasite structure, while the carbides MC have trigonal prismatic coordination in the WC structure. Several borides are known MB2 has nets of boron atoms. RunBg has branched chains while RU7B3 has isolated borons. 

The boron atom in BF5 can complete its octet if an additional atom or ion with a lone pair of electrons forms a bond by providing both electrons. A bond in which both electrons come from one of the atoms is called a coordinate covalent bond. For example, the tetrafluoroborate anion, BF4 (31), forms when boron trifluoride is passed over a meral fluoride. In this anion, the formation of a coordinate covalent bond with a fluoride ion gives the B atom an octet. Another example of a coordinate covalent bond is that formed when boron trifluoride reacts with ammonia ... 

The Tetrahedral Carbon Atom.—We have thus derived the result that an atom in which only s and p eigenfunctions contribute to bond formation and in which the quantization in polar coordinates is broken can form one, two, three, or four equivalent bonds, which are directed toward the corners of a regular tetrahedron (Fig. 4). This calculation provides the quantum mechanical justification of the chemist s tetrahedral carbon atom, present in diamond and all aliphatic carbon compounds, and for the tetrahedral quadrivalent nitrogen atom, the tetrahedral phosphorus atom, as in phosphonium compounds, the tetrahedral boron atom in B2H6 (involving single-electron bonds), and many other such atoms. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

Borasiloxanes are derivatives of the well-studied class of siloxanes (R2SiO) , in which part of the four-coordinate silicon atoms have been substituted by three-coordinate boron atoms. They are therefore characterized by the presence of Si-O-B units and can have one-dimensional oligomeric [120] or polymeric [121], two-dimensional cyclic [122-126], or three-dimensional cagelike [127-131] structures 83-92 as outlined in Figs. 23 and 24. 

Similar to the four- and five-coordinate complexes 120-126, for RCo (dioxime-BR2)2L 127 and Fe(dioxime-BR2)LL 128 different conformations are possible in solution and in the solid state, in which the substituents of the boron atoms may adopt cis- or trans-configurations and in which the alkyl group R may have a parallel or an antiparallel orientation with respect to the BR2 substituents [173-180]. 

In 159 and 163-166 the tertiary amine function is coordinated to the boron atom and transmits the electronic change due to the ester formation to the chromophore. In 160-162 the boron atom is directly connected to the chromophore. After the complexation of the saccharide, the change of the charge transfer, e.g., for 159 [249-251], or the fluorescence bands, e.g., for 160-166 [252-255], can be measured and interpreted. The most selective binding of n-glucose has been achieved with host 164 that forms a 1 1 complex with a macrocyclic structure (Scheme 1). 

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