Trigonal Pyramids: Alternative Ground‐State Structures for Sixteen‐Electron Complexes

Abstract

[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.

Electromeric Rhodium Radical Complexes

“…cal approaches the extreme case of a trigonal pyramid, in which the PRhP angle would be 90°. This structural feature was recently observed for the bistropsilyl complex [Rh(trop 2 SiMe)(PPh 3 )] for which we assumed—because of the strong σ‐electron donation of the silyl group—a strong contribution of the resonance form A (Scheme ) and enhanced electron density at the metal center 13. A more detailed interpretation of the different electronic structures of 6 L .…”

Tris(dibenzo[a,d]cycloheptenyl)phosphane: A Bulky Monodentate or Tetrapodal Ligand

“…The structure of trop 3 P is very rigid and the phosphorus atom is deeply embedded in a concave pocket formed by the three trop substituents. This makes the phosphane infinitely stable against oxidation on air and long reaction times are needed for converting trop 3 P into the corresponding thiophosphorane, trop 3 [49] with M = Rh I , Ir I , and Pd II to which a fifth ligand X = Cl -, H 2 O binds in the remaining axial position. Structural and NMR spectroscopic data suggest that π-backbonding from filled d-orbitals at the transition metal center to the π* orbitals of the coordinated C=C double bonds increases as expected in the order Pd II Ͻ Rh I Ͻ Ir I .…”

Pseudotetrahedral Rhodium(I) Complexes

“…The coordination chemistry of d 8 RhL 4 complexes is dominated by their strong propensity to adopt square‐planar geometries. Nonetheless, some rare sawhorse (SH)1 environments have also been reported, while the amazing compound, [Rh(trop 2 SiMe)(C 2 H 4 )] (trop 2 SiMe=bis(5 H ‐dibenzo[ a , d ]cyclohepten‐5‐yl)methylsilane), remains the sole example for the related trigonal pyramid (TP) geometry 2. In fact, SH and TP structures can be envisaged as derived formally from a trigonal bipyramid (TBPY) geometry lacking either one equatorial or one axial ligand, respectively.…”

“…(trop 2 SiMe = bis(5H-dibenzo [a,d]cyclohepten-5-yl)methylsilane), remains the sole example for the related trigonal pyramid (TP) geometry. [2] In fact, SH and TP structures can be envisaged as derived formally from a trigonal bipyramid (TBPY) geometry lacking either one equatorial or one axial ligand, respectively. Noticeably, some of these complexes exhibit very rich non-conventional chemical reactivity, [3] including dinuclear CÀH bond activation reactions, [4] and unusual electromeric rhodium radical complexes.…”

“…[5] The above mentioned examples provide invaluable information on the parameters that control the electronic structure from a particular geometry and vice versa. [2] Some of these parameters are the ligand topology and bonding effects, such as strong pdonation, as verified by the unique square-planar (SP) iridium(II) and iridium(III) complexes based on pincer ligands, [6] or by the pseudotetrahedral metal environments created by the high-field scorpionate ligands, [PhB(CH 2 PR 2 ) 3 ] À . [7] These scorpionate ligands strongly bind rhodium(I), [8] avoiding dissociation of one arm, behavior that is generally observed for ligands such as the tris(pyrazolyl)borates, [9] and their fac coordination make them ideal candidates to allow rhodium to deviate from its natural tendency to engender a square-planar environment.…”