Guilty on Two Counts: Stepwise Coordination of Two Fluoride Anions to the Antimony Atom of a Noninnocent Stibine Ligand

In our efforts to investigate the
coordination noninnocent behavior
of transition-metal stibine complexes, we have prepared a series of
platinum complexes bearing the tetradentate ligand (o-(Ph2P)­C6H4)3Sb (L). Treatment of (Et2S)2PtCl2 with L affords the lantern complex (o-(Ph2P)­C6H4)3SbCl)­Pt­(Cl)
(1-Cl), which undergoes facile exchange with fluoride
to form the fluorostiboranyl complex ((o-(Ph2P)­C6H4)3SbF)­Pt­(Cl) (1-F). Starting from 1-Cl, anion exchange and
abstraction reactions afford [((o-(Ph2P)­C6H4)3Sb)­Pt­(CyNC)]­[SbF6]2, ([2]­[SbF6]2), [((o-(Ph2P)­C6H4)3SbF)­Pt­(CyNC)]­[SbF6] ([3]­[SbF6]),
and ((o-(Ph2P)­C6H4)3SbF2)­Pt­(CyNC) (4), which are
related by the formal stepwise coordination of two fluoride ligands
to the antimony center. Structural studies of this series show that
the Sb–Pt bond lengthens upon sequential fluoride coordination
at the antimony center, consistent with the weakening of the Sb–Pt
interaction. Natural bond orbital (NBO) calculations performed at
the density functional theory (DFT) optimized geometries suggest that
the Sb–Pt interactions become more polarized across the series,
as part of a larger “spillover” of electron density
from the antimony center to platinum. QTAIM analyses of the DFT-derived
wave functions for this series corroborate the weakening of the Sb–Pt
interaction and suggest that the Sb–Pt bonding pair becomes
increasingly polarized toward platinum upon successive fluoride coordination
at the antimony center.