Effect of electron count on the structures of (benzene)metallaboranes with 9–12 vertices: Comparison of the iron and ruthenium systems

Abstract

The metallaboranes (C6H6)MBn−1Hn−1z (n=9, 10, 11, 12; z=−2, 0, +2; M=Fe, Ru) have been studied by density functional theory. The most spherical closo deltahedral structures were found as global minima in the dianions (η6-C6H6)MBn−1Hn−12− in accord with the Wade–Mingos rules for these 2n+2 skeletal electron systems. However, novel alternative low energy “slipped” structures were found for (η4-C6H6)MB9H92− with a tetrahapto rather than hexahapto benzene ring and the 10-vertex isocloso deltahedron with the metal atom at the unique degree 6 vertex. The expected isocloso structures were found for the neutral 10 and 11 vertex systems (η6-C6H6)MBn−1H n−1 (M=Fe, Ru; n=10, 11) with the metal at the unique degree 6 vertex in accord with their 2n skeletal electrons. Similar non-icosahedral 12-vertex (η6-C6H6)MB11H11 (M=Fe, Ru) structures with the iron or ruthenium at a degree 6 vertex were found to be of lower energy than the corresponding icosahedral structures. For the 9-vertex (η6-C6H6)MB8H8 (M=Fe, Ru) the closo tricapped trigonal prism structure with the metal at a degree 5 vertex is predicted to be of lower energy than the isocloso structure with the metal at a degree 6 vertex. Capped isocloso structures with the metal at a unique degree 7 vertex are predicted for the lowest energy structures of the dications (η6-C6H6)MBn−1Hn−12+ (n=10, 11; M=Fe, Ru). A related bicapped pentagonal bipyramid structure with the iron at a unique degree 7 vertex is found for the 9-vertex ferraborane dication (η6-C6H6)FeB8H82+. The isocloso and capped isocloso structures with the metal atoms at degree 6 and 7 vertices, respectively, for the 2n skeletal electron (η6-C6H6)MBn−1Hn−1 and 2n−2 skeletal electron (η6-C6H6)MBn−1Hn−12+, respectively, appear to be more favorable for the ruthenium species (M=Ru) than for the corresponding iron species (M=Fe).