Heptacoordinate Structures of Halogenostannanes with Three Phosphine Donors: Significant Influence of Halogen Substituents on Their Geometries

Structural studies were performed on heptacoordinate compounds of halostannanes {(o-Ph2P)C6H4}3SnX (X = F (1), Cl (3), Br (4), I (5)) with three phosphine donors. The fluorostannane (1) has a novel heptacoordinate geometry, in which three phosphine donors interact with the Sn center at the position trans to the Sn−F bond (a form). In contrast, the chloro (3) and bromo (4) analogues have a highly distorted pentagonal-bipyramidal geometry (b form), and the iodo analogue (5) has a tricapped tetrahedral geometry (c form). Although both b and c forms include two phosphine donors coordinating to the Sn center trans to the Cipso atoms and one phosphine donor coordinating to the Sn atom at the position trans to the halogen atom, the geometry around the Sn center is much more distorted from tetrahedral geometry in b form than in c form. DFT calculations showed that the presence of the halogen atom at the Sn center facilitated the coordination at the position trans to the halogen due to strong electrostatic and charge transfer (CT) interactions and the structural modification by changing the halogen substituent arose from the difference in electronegativity of halogen atom because strongly electronegative halogen enabled the formation of highly distorted structure from tetrahedral geometry (Bent's rule).