Comparison of the structures of two metallaboranes possessing the same borane fragment and ancillary metal ligands but differing transition metal atoms defines the geometric consequences of the addition (or removal) of two valence electrons from a bicapped tetrahedral metallaborane cluster structure. Likewise the effects of the cluster distortion on electronic structure is explored utilizing approximate molecular orbital calculations on hypothetical (CpMn)2B4H8, Cp=η5-C5H5, as it is changed from the shape characteristic of five skeletal electron pair (sep) (Cp*Cr)2B4H8 to that of six sep (Cp*Re)2B4H8, Cp=η5-C5Me5. In doing so it is demonstrated that the observed changes in the metal-metal distance (a counter-intuitive increase with smaller sep) and endohydrogen positions (more like B-H with smaller sep) are required to electronically accommodate the removal of a pair of electrons from a saturated bicapped tetrahedral cluster. © 2000 Elsevier Science B.V.