Carboranes bond distances

Carboranes, 18 2, see also specific types analogs and derivatives, 18 97-132 BH group-substituted, 18 78 CNPR theory, 18 125-127 bond distances in, 18 46 bonding in, 18 7-16 bridge hydrogen, 18 90-93... 

A recent X-ray characterization of the cyclopentyl cation 33 generated with methyl carboranes has shown a perfect planar structure around the carbocation center [bond angles CH3-C+-CH2= 124.9 and 125.2°, CH2-C+-CH2= 109.9° bond distances CH3-C+ = 1.46 A, CH2-C+ = 1.45 A).123... 

However, the external (CH)4 link between the carborane hypercarbon atoms shows the alternation in C-C bond distances (1.48,1.34,1.45,... 

The C—C and C—B interatomic distances in carboranes can also be related to the coordination numbers of the skeletal atoms. Two factors tend to make these distances shorter than the B —B distances in comparable boranes the preference of the carbon atoms for sites of low coordination number and the greater electronegativity of carbon than boron, which increases the electron density in the region of the carbon atoms and so strengthens the bonds that they form. Table IX lists some C—C distances for closo- and wido-carboranes 13, 20, 21, 26, 98,121,168) and metal-acetylene 50, 58,112) complexes, relating them... 

Complex 4 (6), (MeC2BioHio)Li(pmdeta) [pmdeta = (CH3)2N(CH2>2-N(CH3)(CH2)2N(CH3)2], a lithiated carborane, also is terminally bonded and represents a simple example of a carbon-lithium o -bond. The C—Li distance is 2.18 A. The structural features at the lithium atom are analogous to those for compounds 1-3. 

Fig, 1 The optimized HF/6-31G structures of the o-, m-, and p-carboranes and their mono-anions and di-anions. The framed numbers are the realtive energies in kj/mol of each structure with respect to the neutral p-carborane. Note that in structure (3) the B9-B10 bond is hidden exactly behind the B8-B9 bond. The distance limit for drawing a bond was set to 1.95 A. 

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