Boron coordination

Table 2. Structure Types, Boron Coordination and Representatives op Metal Borides with Isolated B Atoms (Filled Metal Host Lattice Compounds)...

Space group Boron coordination representatives Ternary representatives Refs. 

Structure type Boron coordination Binary representative... 

Nickel atoms in BajNi B form distorted, puckered 3.6.3.6-kagome nets stacked in six layers perpendicular to the c axis. The densely packed framework of trigonal-Ni prisms again result in boron-pair formation, although Ba atoms are too large to be sandwiched between two Ni layers, and only four Ba can be accommodated within six Ni layers. Superconductivity is found for ( a, Sr, Ba)2pt9Bg borides with a structure related to Ba2Ni9Bfi and e o,B2 however, with respect to crystal chemistry and boron coordination, only the subcell is derived so far. 

Trigonal-prismatic boron coordination predominates in borides, (Mj, Mj)2B with such structures as FcjP, anti-PbCl2 and NbCoB, which all are closely related NbCoB can be derived as the topochemical sum of the former two, and at higher T Nb(Ta)CoB transforms into the FcjP type. ... 

Table 2. Crystal Structures and Boron Coordination of Platinum Metal Borides WITH Isolated B Atoms (Owing to Defect Boron Sublattice)...

Structure type Binary representatives Ternary representatives. Boron coordination Type of boron defects... 

Boron atoms in infinite boron zigzag chains form covalent B—B bonds at 165-190 pm with bond angles of 115°. The boron coordination is trigonal prismatic, giving rise to coordinated rows of face-connected trigonal metal prisms (Table 1). Most typical are the binary transition-metal monoborides such as the FeB, CrB and MoB structure types. Transformations between pairs by simple geometric shifts ... 

Structure type Boron aggregation boron coordination Binary representatives Ternary representatives Refe. 

Infinite zigzag B chains with C strongly bonded to B eonstitute the basic building element of transition-metal carboborides (M -BC). The boron coordination is trigonal prismatic BM, whereas carbon coordination is a M -sBOh- However, with an increasing ratio of radii the B-chain elements are replaced by B pairs or... 

The type of interaction and its chemical influence are determined by phosphorus and boron coordination, by the type of the fragment separating them, and by molecular structure. Thus, it is reasonable to classify P,B-containing heterocycles according to the type of fragment separating phosphorus and boron. 

Diborane is also a useful reagent for reducing amides. Tertiary and secondary amides are easily reduced, but primary amides react only slowly.59 The electrophilicity of diborane is involved in the reduction of amides. The boron coordinates at the carbonyl oxygen,... 

Recently, Silver and Bray (52) were able to differentiate and to estimate the relative proportions of three- and four-coordinated borons in binary borate glasses. This technique was thus adopted by the present author in order to ascertain the presence or absence of four-coordinated boron in liquid B203 (35). Glassy samples were quenched in liquid mercury from temperatures up to 1400°C but no four-coordinated boron was detected. (The lower limit of detectability was estimated to be about 2%.) Experiments were also carried out on liquid B203 up to 500°C but again no four-coordinated boron was found. It thus appeared that at least up to 500 or 600°C, the structural variation of liquid B203 with temperature is not primarily the result of a boron coordination change of from three to four. 

In the examination of crystalline borates, caution has been exercised over the structural interpretations of spectroscopic studies. Certain features pertaining to the boron coordination and water/hydroxyl entities can be deduced, but absolute configurations of molecules require more exacting techniques. 

Infrared spectroscopy is the favored technique for characterizing borates, and several compilations of data have been made (171, 333,417, 423,431,432). The intense absorption at about 970 cm-1 is characteristic of tetrahedrally coordinated boron, as measured for the magnesium borates MgO B203 nH20 (153), and bands at 700 to 900 and 1200 to 1500 cm-1 can correspond to triangular boron coordination. Absorptions in the region 400 to 700 cm-1 have been attributed to chain structures (423). 

The proper metal boride structures with isolated boron atoms include approximately 20 structure types with many ternary representatives. The boron coordination is trigonal prismatic (in the Fe3C, TisP, ResB ThvFeB structure types), octahedral (in RhsB4), or Archimedian prismatic (in the CuAl2-type structure with many M2B representatives). 

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