LAUSR

Abstract The lowest energy structures for the Group 9 metallachalcaboranes CpMEB n -2 H n -2 (M = Co, Rh, Ir; E = S, Te) are found by density functional theory to have a central closo MEB n –2 deltahedron consistent with their 2 n  + 2 skeletal electrons. Structures having metal atoms at degree 5 vertices and sulfur atoms at degree 4 vertices are clearly energetically preferred. However, for the 9- and 10-vertex metallatelluraboranes CpMTeB n –2 H n –2 ( n  = 9, 10; M = Co, Rh, Ir) otherwise related structures having tellurium atoms at degree 4 and degree 5 vertices are energetically comparable. The low-energy 11-vertex CpMEB 9 H 9 structures are all based on the 11-vertex closo deltahedron with a unique degree 6 vertex. However, in many of the low-energy 11-vertex structures one of the edges connecting the degree 6 vertex with an adjacent degree 5 vertex is stretched to a non-bonding distance leading to an isonido structure having a quadrilateral face. The three lowest energy structures for the 12-vertex CpMEB 10 H 10 (M = Co, Rh, Ir; E = S, Te) systems are the three possible icosahedral structures with the energy ordering ortho meta para .