Can One σ*-Antibonding Orbital Interact with Six Electrons of Lewis Bases? Analysis of a Multiply Interacting σ* Orbital

Abstract

One empty orbital normally interacts with one lone pair of a Lewis base to form one dative bond, and we do not know any example where one antibonding orbital interacts with more than two lone pairs of Lewis bases. We wish to report here the first example which is beyond our aforementioned common knowledge of dative bonds. We synthesized heptacoordinate tris{(o-diphenylphosphino)-phenyl}tin fluoride and found that three lone pairs of phosphine donors equivalently interact with the antibonding σ*(Sn−F) orbital of the Sn center. The nature of this "multiply interacting σ* orbital" was theoretically elucidated by DFT calculations.

Heptacoordinate Structures of Organotin Halides with Three Phosphine Donors: Halogen‐Substituent Effect on Geometry

“…Previously, the similar site exchange was reported to take place more smoothly in { o ‐(Ph 2 P)C 6 H 4 } 3 GeF than in the Si analogue because the longer Ge–C bonds than the Si–C bonds facilitate the rotation of ortho ‐diphenylphosphinophenyl groups. [6b] Similarly, the longer Sn–C bonds may be responsible for the faster site exchange in 3 to 5 over NMR time scale. Further, the presence of an equilibrium including other isomers may contribute to the broadening of the signals.…”

“…The chemistry of hypercoordinate compounds of group 14 elements has been developed due to their interesting geometries (similar to transition states or intermediates of S N 2‐type reactions) and their essential roles in molecular transformation and catalytic functions An ER 4 molecule of a heavier group 14 element (E) can interact with Lewis bases using the σ *(E–R) orbital to form a hypercoordinate geometry. The characteristics of the heavier group 14 elements results in several and curious geometries involving not only penta‐ and hexacoordination but also higher coordination . These geometries and bonding interactions would be useful for discussing the reaction mechanisms and/or reasons for stereoselectivity.…”

“…We have been interested in the use of a phosphine‐based LX‐type ligand for the synthesis of novel hypercoordinate compounds because of its strong multiple LP(P)→E donor–acceptor interactions induced by the effective overlap between the larger lone pair orbital of phosphine relative to a nitrogen donor and the σ *(E–X) orbital . Previously, we reported that fluorosilane and fluorogermane { o ‐(Ph 2 P)C 6 H 4 } 3 E(F) (E = Si, Ge) adopted a different type of heptacoordinate geometry, whereby one of the three phosphine donors was coordinated to E at the coordination site trans to the E–F bond owing to the significant LP(P)→ σ *(E–X) interaction (type IV in Figure ).…”

“…Previously, we reported that fluorosilane and fluorogermane { o ‐(Ph 2 P)C 6 H 4 } 3 E(F) (E = Si, Ge) adopted a different type of heptacoordinate geometry, whereby one of the three phosphine donors was coordinated to E at the coordination site trans to the E–F bond owing to the significant LP(P)→ σ *(E–X) interaction (type IV in Figure ). [6b] Furthermore, we found that tin fluorostannane { o ‐(Ph 2 P)C 6 H 4 } 3 SnF ( 1 ) adopted an unusual heptacoordinate structure, in which the three phosphine donors equivalently interacted with one σ *(Sn–F) orbital. [6a] FSnC 3 core in 1 strongly distorted from a tetrahedral geometry and its structure adopted a trigonal pyramidal geometry with the axial fluoride atom [Σ(CSnC) = 358.23(7), Σ(CSnF) = 282.96(15)] (type V in Figure ).…”

“…[6b] Furthermore, we found that tin fluorostannane { o ‐(Ph 2 P)C 6 H 4 } 3 SnF ( 1 ) adopted an unusual heptacoordinate structure, in which the three phosphine donors equivalently interacted with one σ *(Sn–F) orbital. [6a] FSnC 3 core in 1 strongly distorted from a tetrahedral geometry and its structure adopted a trigonal pyramidal geometry with the axial fluoride atom [Σ(CSnC) = 358.23(7), Σ(CSnF) = 282.96(15)] (type V in Figure ). In contrast, the hydride analogue { o ‐(Ph 2 P)C 6 H 4 } 3 SnH ( 2 ) of 1 has a different heptacoordinate geometry in which each phosphine donor coordinates via the LP(P)→ σ *(Sn–C) interactions in a way similar to type I in Figure .…”