[1] However, apart from tetrahedral structure T, which is observed for high-spin transition metals from the 4th period, [2] sawhorse (SH, C 2v ) and trigonalpyramidal structures (TP, C 3v ) are alternatives (Scheme 1).[3]The highly reactive transient carbonyls [M(CO) 4 ] (M = Fe, Ru, Os) have SH structures, [4a-c] as do a few recently isolated Ru 0 and Rh I complexes, which show remarkable reactivity.Even computationally, the TP structure has been rarely considered. Pidun and Frenking investigated by DFT methods [RhX(CO) 3 ] complexes with X = H, HC = O, CH 2 OH, and OMe. For [Rh(HC = O)(CO) 3 ], TP is the (slightly) preferred ground-state structure, whereas all other complexes have either a P or an SH form in the global minimum.[5] Within the complementary spherical electron density (CSED) model, Mingos identified SH and TP forms as the next best choices to planar structures for 16-electron complexes of late transition metals. [6] In computations using the orbital ranked symmetry analysis method (ORSAM), Bayse and Hall considered d 8 -MH 4 hydrides with SH and TP forms.[7] Proposals for making Scheme 1. Planar (P), tetrahedral (T), sawhorse (SH), and trigonalplanar (TP) structures.