Atoms bridging

The species at the centre of tire rings is usually Si or Ge and tire bridging atom is oxygen. In one study tire peripheral hydrogens on tire phtlialocyanine molecules were replaced by alkyl groups and tire resulting polymers could be rendered soluble in ordinary organic solvents [108, 109 and 110]. Successful deposition of several of tliese materials has been achieved and different techniques were employed to study tlieir stmctural properties [109, ill, ill, ill and 1141. 

A bridging group should be indicated by adding the Greek letter p immediately before its names and separating this from the rest of the complex by a hyphen. The atom or atoms of the characteristic element to which the bridging atom is bonded, is indicated by numbers. 

Carbon-6, which bears two hydrogens, is pentacoordinate and serves as the bridging atom in the cation. 

The B3Hg ion (p. 166) is a triangular cluster of Cj (rather than C2 ) symmetry (see Fig. 6.15a) the bridging atoms are essentially in the B3 plane with Ht above and below. While it has been conventional to represent the cluster bonding in terms of two BHB and one B-B bond (Fig. 6.15b), recent high-level computations suggest the presence of a 3-centre BBB bond, as depicted approximately in Fig. 6.15c. 

VI. Tellurium-Containing Heterocycles with N-Bridged Atoms... 

An interesting additional example of the interchangeability of the bridging atoms in the neuroleptic series comes from the finding that the biologic activity of the phenothiazines is maintained in a compound that contains an extra atom on the nitrogen... 

The occurrence of oxyfluoride chains in MF - MNbOF4 molten mixtures was confirmed by IR emission spectra [379]. Fig. 94 presents a typical example, the spectrum of molten LiF - LiNbOF4. The strong band at about 780-800 cm 1 is characteristic of Nb-O-Nb vibrations of the octahedrons that are linked into chains via oxygen bridge atoms. 

In /r-bridged dimers, two phthalocyanine moieties are linked by one or more bridging atoms such as oxygen or halogens. Niobium(V) oxide tribromide (NbOBr,) when treated with phthalonitrile in 1-chloronaphthalene at 230CC for 3 hours produces /r-bromo-/t-dioxobis[nio-bium(V) phthalocyanine] tribromide in 52%.232... 

In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4e-donor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as... 

If, even in this extremely unfavourable case involving chlorine, a poorer bridging atom than bromine, bridging is enforced, bromine bridging must occur a fortiori for generally less crowded alkenes. 

Picolylphenylketone S-benzyldithiocarbazate, 48, yielded paramagnetic [ Ni(48-H)A 2] (A = Cl, Br) and diamagnetic [Ni(48-H)I] [207]. All three compounds are non-electrolytes and the iodo complex is planar while the other two complexes involve sulfur bridging atoms and five-coordinate nickel(II) centers. All three complexes can be converted to monomeric, octahedral complexes by addition of pyridine, 2-picoline or quinoline. 

The different behaviour of difluorophosphoric acid which is of a higher degree of association, is probably induced by the ability of the fluorine ligands to act, besides the 0-atoms, as bridging atoms in the hydrogen bridges. 

These results are consistent with the expectation that fluorine would not be an effective bridging atom. 

Larger structural units can be described by connected polyhedra. Two polyhedra can be joined by a common vertex, a common edge, or a common face (Fig. 2.3). The common atoms of two connected polyhedra are called bridging atoms. In face-sharing polyhedra the central atoms are closest to one another and in vertex-sharing polyhedra they are furthest apart. Further details concerning the connection of polyhedra are discussed in chapter 16. 

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