The equilibrium geometries and fundamental frequencies of the Li2F+,

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Potassium reduction of RZn(mu-I)2Li(OEt2)2 (R = [{(2,6-Pri2C6H3)N(Me)C}2CH]) affords the second compound with a Zn-Zn bond, RZn-ZnR. The air- and moisture-sensitive title compound was characterized by 1H NMR, elemental analyses, and single-crystal X-ray diffraction. The Zn-Zn bond was determined to be 2.3586(7) A; this value is only about 0.05 A longer than the Zn-Zn bond reported for Cp*Zn-ZnCp* (Cp* = C5Me5), the first reported compound with a Zn-Zn bond. In addition, density functional theory (DFT) computations on related model RZn-ZnR compounds provide insight into the intriguing Zn-Zn bond.

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N(2)S(2) is a four-membered-ring system with 6pi electrons. While earlier proposals considered N(2)S(2) to be aromatic, recent electronic structure calculations claimed that N(2)S(2) is a singlet diradical. Our careful reexamination does not support this assertion. N(2)S(2) is closed shell and aromatic since it satisfies all three generally accepted criteria for aromaticity: energetic (stability), structural (planarity with equal bond lengths), and magnetic (negative nucleus-independent chemical shift due to the pi electrons). These characteristics as well as the electronic structure of N(2)S(2) are compared with those for an isoelectronic pi system, Li(2)C(4)H(4), motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N(2)S(2) and Li(2)C(4)H(4) are both essentially 2pi-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6pi electrons. This is because four of the six pi electrons occupy the nonbonding pi HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antibonding effect can be said to reduce the aromatic character by approximately 7% and 4% for N(2)S(2) and Li(2)C(4)H(4), respectively.

Sets of boron rings enclosing planar hypercoordinate group 14 elements (ABn(n-8); A = group 14 element; n = 6-10) are designed systematically based on geometrical and electronic fit principles: the size of a boron ring must accommodate the central atom comfortably. The electronic structures of the planar minima with hypercoordinate group 14 elements are doubly aromatic with six pi and six in-plane radial MO systems (radial MOs are comprised of boron p orbitals pointing toward the ring center). This is confirmed by induced magnetic field and nucleus-independent chemical shift (NICS) computations. The weakness of the "partial" A-B bonds is compensated by their unusually large number. Although a C7v pyramidal SiB8 structure is more stable than the D8h isomer, Born-Oppenheimer molecular dynamics simulations show the resistance of the D8h local minimum against deformation and isomerization. Such evidence of the viability of the boron ring minima with group 14 elements encourages experimental realization.

Neutral and especially dianionic 6-and 12-vertex closo ortho-carboranes (o-carboranes) 1,2-R2-1,2-C2BnHn (R ) H, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, as well as e -, CH2 -, NH -, O -, SiH2 -, PH -, and S -) exhibit extremely large variations (over 1 Å!) of the cage CC distances, from 1.626 to 2.638 Å, at the B3LYP/6-31G*//B3LYP/6-31G* DFT level. These CC "bond lengths," among the longest ever reported, generally are greater in the icosahedral than those in the corresponding octahedral systems and depend strongly on the substituents. While 1,2-(NH2)2-1,2-C2B10H10 has the longest Cc‚‚‚Cc separation in neutral species (1.860 Å), Cc‚‚‚Cc distances can be much larger in the corresponding dianions. These range fromRemarkably, there is no abrupt discontinuity over the entire range of CC lengths. Consequently, the relationship between the gradual changes in the distances and the nature of the bonding was analyzed by means of the form of the Kohn-Sham orbitals, the Wiberg Cc‚‚‚Cc bond indices, and Bader AIM method. Cluster carboranes, and possibly other heteroboranes, thus appear to offer unique opportunities for modulating Cc‚‚‚Cc distances.

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