An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of approximately 0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson.

We report the preparation, crystallization, and solid-state characterization of a spiro-biphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state and without intermolecular contacts that fall within the van der Waals atomic separation. Magnetic susceptibility measurements show Curie behavior (1 spin per molecule) throughout the temperature range 10−400 K, confirming the presence of noninteracting spins in the solid. The compound shows a room-temperature conductivity of σ = 0.05 S/cm, the highest yet for a neutral radical conductor. We suggest that the ground state of the radical corresponds to a degenerate Mott−Hubbard insulator. We postulate the presence of delocalized energy bands that are responsible for the transport properties but that are separated from the insulating ground state by an energy gap corresponding to the on-site Coulombic correlation energy. Given the presence of isolated molecules in the crystal lattice, the magnitude of the conductivity, the finding of band transport, and the width of the conduction band (0.5 eV) are unprecedented.

A systematic investigation has been made into the degree of silylium ion character in species of the type i-Pr 3 SiY where Yis a weakly coordinating hexahalo-carborane anion, X 6 -CB 11 H 6 -, with X ) Cl, Br, I. By X-ray crystal structural criteria such as the average C-Si-C angle (117.3(4)°, 117.0(13)°, and 115.6(9)°for X ) Cl, Br, I, respectively) and the downfield 29 Si chemical shift (115, 110, and 97 ppm for X ) Cl, Br, I, respectively) the hexachloro-carborane anion is the least coordinating. Thus, the "ion-like" species i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δbecomes the closest structural approach to date to a condensed phase silylium ion. With the aid of ab initio molecular orbital theory and silicon chemical shift calculations we address the question of how much silylium ion character this species represents. From structural, energetic, and NMR criteria (where expectation values for the condensed phase 29 Si chemical shifts are revised downward) we conclude that i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δmay have >50% silylium ion character.