Homonuclear bonding

The remainder of this chapter covers reactions by which homonuclear bonds between the group-IIIB elements are formed and includes a few examples of heteronuclear bonds, e.g., B—Al and Ga—In in compounds of these elements. 

H. G. v. Schnering, Homonuclear bonds with main group elements. Angew. Chem. Int. Ed. 20 (1981) 33. 

The new developments in homonuclear bonding described in this section for A1 -> T1 have precedent in the synthesis of molecular species with boron-boron multiple bonds. In essence, it has been shown that tetraor-ganodiboron compounds38-40 can undergo two successive one-electron reductions9,41 to give multiple B—B bonds as shown by Eq. (1). 

Table 2.7 Mean Bond Enthalpies (kJ mol ) Homonuclear Bonds ...

A heteronuclear bond length (one between two different atoms) is equal to the average of two homonuclear bond lengths (one between two like atoms) of the same order (both single, both double, or both triple). 

Table 8.1 lists covalent radii obtained by dividing homonuclear bond distances by two. In many cases the appropriate homonuclear single bond has not been measured and the assigned covalent radius is obtained indirectly by subtracting the covalenl radius of element B in a heteronuclear bond AB to obtain the radius of atom A. 

Cr2(DMP)4 not only falls below comparable ratios for other multiply bound metal dimers but also is slightly less than the formal shortness ratio calculated for the strongest homonuclear bonds known, for example, N2 (0.786), C2 (0.783), and P2 (0.860). The utility of the formal shortness concept as a guide to the assessment of homonuclear attractive forces makes it a valuable addition to comparative chemistry of elements forming such bonds. 

Hypothetical Nv cages based on ipp polyhedra have 6 BB, 3BB + 3NN, or 6 NN homonuclear bonds, and overall stoichiometries Bn/2+2 Nn/2 2, Bb/2Nb/2, Bre/2 2 Nn/2+2, with (n —4), n or (n + 4) n electrons respectively. This is proved as follows. Associate one third of each atom with each of its three bonds. In the total atom count, homonuclear BB bonds then contribute B + 0N each, heteronuclear bonds contribute B+ N, etc. Therefore the number of homonuclear bonds of BB type is... 

Figure 7. A pentagon has one forced homonuclear contact, which forces a second in a neighbouring hexagon. A pentagon pair has only one forced homonuclear bond which is without effect on neighbouring rings.

Although the XeJ cation is known to exist from Raman spectroscopy, with i (Xe-Xe) = 123 cm-1, its bond length was only determined in 1997 from the crystal structure of Xe2Sb4F2i. The Xe-Xe bond length is 309 pm, the longest recorded homonuclear bond between main-group elements. 

The energy expression for homonuclear bonds (ai = 0 2) in atomic units, reduces to... 

Figure 5.9 Schematic representation of the relationship between bond order and dissociation energy of homonuclear bonds, according to eqn. (8). The stippled lines show degree of flexibility.

To gain information about the active sites in a catalyst, a common approach is to use probe molecules. In the case of IINS spectroscopy, the molecule of choice is dihydrogen, H2. Because this is also often a reactant, it may provide some assurance of the relevance of what is measured. The low mass, 2amu, and short homonuclear bond, 0.746 A, of dihydrogen provide this molecule with a uniquely high rotational constant, B — 59.3 cm Figure 2a shows the IINS spectrum of solid H2 at 13 K. The intense, resolution-limited line at 118.6cm is the IR- and Raman-forbidden... 

Covalent radii estimated from homonuclear bond lengths where available and from selected heteronuclear bonds otherwise. Bond lengths from Tables of Interactomic Distances and Configuration in Molecules and Jons) Sutton. L., Ed. Spec. Publ. Nos. 11 and 18 The Chemical Society London, 1958, 1%5, except where noted. Values in parentheses are for noble gases not known to form compounds and are extrapolated from the values of neighboring nonmetals Allen, L. C. Huheey, J. E. J. Jnorg. NucJ. Chem. 1980, 42, 1523. 

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