Construction principles of "hyparenes": families of molecules with planar pentacoordinate carbons

Sigma aromaticity of the bimetallic Au5Zn+ cluster

Experimental and theoretical evidence for a "sigma aromatic" bimetallic cluster is presented. A mass spectrometric analysis of AuNZn+ (N = 2-44) photofragments shows Au5Zn+ to be very abundant, proving its high stability. Calculations predict that Au5Zn+ has a planar geometry and six valence s electrons occupying delocalized sigma-bonded molecular orbitals in a manner similar to that of aromatic organic compounds, except for their nodal properties in the molecular plane. The delocalized sigma electrons provide diamagnetic ring currents, suggesting Au5Zn+ is aromatic.

The theoretical design of neutral planar tetracoordinate carbon molecules with C(C)(4) substructures

Using a new charge-compensation strategy, we designed neutral molecules with perfectly planar C(C)(4)-type tetracoordinate carbon arrangements (ptC) employing DFT computations. These designs, based on the planar preference of methane dications, replace two remote carbons in spiroalkaplanes by borons or two remote hydrogens by BH(3) groups; the two formally anionic boron units which result compensate the formal double positive charge on the central ptC's. The LUMOs correspond to the "wasted" lone pair HOMOs of the alkaplanes. As compared to the latter, pi occupancies on the central carbon are much smaller (less than 0.7e), and the IPs are much larger. The newly predicted compounds utilize all of the electrons more effectively. There are no lone pairs, and the ptC-C bond lengths are ca. 1.50 A. The Wiberg bond index sums of the ptC's are near 3.2, and the boron sums are close to 4.