Segregation of tetracarbon units in low-energy tetracarbindane structures: Major differences from their aluminum and gallium analogs

The structures and energetics of the tetracarbindanes C₄In_n−4Me_n (n = 6-14) have been determined by density functional theory. In contrast to their aluminum and gallium analogs, the lowest energy tetracarbindanes typically have all four carbon atoms segregated into a single C₄ unit. Thus, linear C₄ units resembling butadiene are found in the lowest energy C₄In_n−4Me_n structures. In addition, some higher energy tetracarbindane structures have a structural feature not found in any of the corresponding tetracarbalanes and tetracarbagallanes, namely closed trapezoidal C₄ units resembling cyclobutene. Such trapezoidal C₄ units bind to the In_n−4 subcluster with the CC edge bonding to a single indium atoms as an olefin-metal or 3-center 2-electron bond. These differences may be attributed to the larger size of indium atoms (1.42 Å covalent radius) relative to gallium atoms (1.22 Å covalent radius).