Extended Hükel molecular-orbital calculations on the model complexes [B4H4(CoL3)4]4+ and B4H4(CoL3)4, where L = a two-electron σ-donor ligand, have established the electronic factors responsible for the breakdown of the polyhedral skeletal electron-pair theory when applied to B4H4[Co(η-C5H5)]4 and B4H4[Ni(η-C5H5)]4. In addition the observation that B4H4[Co(η-C5H5)]4 and B4H4[Ni(η-C5H5)]4 adopt alternative D2d structures based on the dodecahedron has been rationalised in terms of the different electronic requirements for stabilising the flattened metal tetrahedral structure in B4H4[Co(η-C5H5)]4 and the elongated metal tetrahedral structure in B4H4[Ni(η-C5H5)]4.