ChemInform Abstract: Face-Localized Bonding Models for Borane Cage Ligands in Transition-Metal Coordination Chemistry

Abstract

Abstract Substitution of a borane vertex by an isolobal transition metal vertex in a deltahedron can change the shape and apparent electron count. More specifically, in many metallaborane deltahedra with 9, 10, and 11 vertices some transition metal units are found to occur at degree 6 vertices. In the case of 9- and 10-vertex deltahedra this requires a change in the shape of the deltahedron from the metal-free closo borane deltahedra having only degree 4 and 5 vertices to alternative so-called isocloso deltahedra having a single degree 6 vertex. The change in electron count from 2 n +2 skeletal electrons for metal-free borane closo deltahedra having the maximum number of degree 4 and 5 vertices to 2 n skeletal electrons for metallaborane isocloso deltahedra with the metal at a single degree 6 vertex can be rationalized by a major change in the skeletal bonding topology from globally delocalized in the closo deltahedra to face-localized in the isocloso deltahedra. In this model the skeletal bonding in closo deltahedra with n vertices can be constructed from n two-center two-electron surface bonds and a single n -center core bond whereas the entire skeletal bonding in isocloso deltahedra can arise from n three-center two-electron bonds in n of the 2 n −4 faces. The isocloso metallaboranes with n vertices can be considered to be metal complexes of an open borane ligand having all triangular faces except for a single hexagonal face, which forms a hexahapto bond to the transition metal vertex similar to the hexahapto arenemetal bonds in arene metal complexes. The 10-vertex deltahedra (C 5 H 5 M) 2 C 2 B 6 H 8 (M=Co, Fe) discovered by Hawthorne and co-workers in the 1970s provide an early example of a structurally characterized closo / isocloso deltahedral pair of the same stoichiometry in which the cobalt derivative has the 2 n +2 skeletal electrons and geometry of the closo 10-vertex deltahedron (the bicapped square antiprism) whereas the iron derivative has only 2 n skeletal electrons and the different geometry of the isocloso 10-vertex deltahedron with one of the iron atoms at the unique degree 6 vertex.