Success and pitfalls of the Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) isolobal analogy: Depth and breadth of the boron connection.

The extent and depth of the so-called boron connection suggested recently by the present author [J. Chem. Phys. 128, 184305 (2008)] for the Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) isovalent species [following similar connection of (Si(n))(2-)-(B(n)H(n))(2-) dianions] are further investigated by considering larger species up to n=12 and additional isovalent moieties. Here we consider, using density functional and coupled clusters theory, isovalent Si(n-2)C(2)H(2) and Ge(n-2)C(2)H(2) clusters, in comparison to the corresponding C(2)B(n-2)H(n) carboranes and to each other for n=3-8, 12. Special attention is given to the n=8 species, where the corresponding carborane is highly fluxional, and to n=12, where the "parent structures" of the corresponding (Si(12))(2-) and (B(12)H(12))(2-) dianions have drastically different symmetries. The structures generated by substitutions on (Si(12))(2-), as well as (B(12)H(12))(2-), are compared and interrelated for both C(2)B(10)H(12) and Si(10)C(2)H(12). The carborane generated from the C(s) (Si(12))(2-) dianion provides an illustrative example of carborane rearrangements, reverting after geometry optimization to the second lowest C(2)B(10)H(12) meta isomer of C(2v) symmetry, 0.12 eV above the lowest energy para isomer. This demonstrates the amphidirectional character of the boron connection. It is found that n=8 is the upper limit of the n range in which the isolobal Si(n-2)C(2)H(2)-C(2)B(n-2)H(n) analogy is fully operative not only for the lowest but also for the second and third lowest energy states. For n<or=8 all three pairs of structures are isovalent, isostructural, and isolobal. For large values of n (n>8) the boron connection, although not fully isolobal, is still valid in a broader and more general sense, still providing deeper and broader fundamental understanding and insight for both species. It was also found that the Ge(n-2)C(2)H(2) clusters are not fully homologous (isolobal) neither to the isovalent Si(n-2)C(2)H(2) clusters nor to the corresponding C(2)B(n-2)H(n) carboranes, preferring structures in which the two carbon atoms are always in adjacent positions. This is attributed to the relative weakness of the Ge-Ge and Ge-C bonds relative to Si-Si and Si-C bonds and the "inert pair effect."