Inverted Tricoordinate Carbon

Eleven four-membered ring structures (C₄, C₄H⁺, C₄H₂²⁺, C₃BH, C₃BH₂⁺, C₂B₂H₂, C₃Be, C₃BeH⁺, C₂BBeH, C₂Be₂, and B₄H₂) are discussed in terms of stability properties associated with inverted tricoordinate carbons. These minimum energy structures have short distances between the bridgehead carbons, averaging 1.484 A (6-31G*), and short ring bonds, with average values for C-C, C-B, and C-Be bonds of 1.396, 1.487, and 1.576 A, respectively. The stabilities of these molecules result from four-center, two-electron (aromatic) π bonding and a nonbonding a HOMO between the bridgehead carbons. The Mulliken overlap population between these carbons ranges from -0.236 for C₄H₂²⁺ to +0.495 for C₂Be₂. While there is no bridgehead-bridgehead bonding in C₄H₂²⁺, there is in C₂Be₂. The beryllium isomers possess ionic character.