Polyhedral Trimetallaboranes of the Group 9 Metals: Isocloso versus Capped and Uncapped Closo Deltahedra

The structures and energetics of the trimetallaboranes Cp3M3Bn–3Hn–3 (Cp = η5-C5H5; M = Co, Rh, Ir; n = 5–12) have been investigated by density functional theory. The capped- octahedral structures of the experimentally known 7-vertex trimetallaboranes (η5-R5C5)3M3B4H4 (R = H, M = Co; R = Me, M = Co, Rh, Ir) with the three degree 5 vertices occupied by metal atoms correspond to the lowest-energy structures by substantial margins. The lowest-energy structures for the 8- and 9-vertex Cp3M3Bn–3Hn–3 (M = Co, Rh, Ir) systems have central closo deltahedra despite their 2n rather than the 2n + 2 skeletal electrons suggested by the Wade–Mingos rules for closo deltahedra. The lowest-energy structures for the 10-vertex Cp3M3B7H7 as well as the experimentally known Cp*3Rh3B7H7 have a central M3B7 isocloso deltahedron with one of the metal atoms located at the unique degree 6 vertex. All of the low-energy 11-vertex Cp3M3B8H8 (M = Co, Rh, Ir) structures, including the experimentally known Cp*3Co3B8H8, have a central M3B8 closo/isocloso deltahedron with one of the metal atoms located at the unique degree 6 vertex. The central M3B9 polyhedra in the lowest energy structures for the 12-vertex systems Cp3M3B9H9 (M = Co, Rh, Ir) are not regular icosahedra but instead 12-vertex deltahedra with two degree 6 vertices for two of the three metal atoms balanced by two degree 4 vertices. This deltahedron is derived from the regular icosahedron by a diamond–square–diamond transformation.