Crystal field arguments to explain the trans labilisation within transition metal–borane complexes

There has been great interest in metal–borane complexes following the discovery of metallaboratrane complexes in 1999. These complexes contain unusual Z-type (σ-acceptor) coordination of borane functional groups and have been shown to provide unexpected coordination geometries and properties for a transition metal in a particular oxidation state. Additionally, such ligands exhibit unusually high trans influences only in certain geometries. Herein, a simple qualitative description, based on crystal field theory arguments, is presented for the explanation of the geometries and atypical trans labilisations observed.