A structural, spectroscopic and computational examination of the dative interaction in constrained phosphine–stibines and phosphine–stiboranes

A series of phosphine–stibine and phosphine–stiborane peri–substituted acenaphthenes containing all permutations of pentavalent groups –SbCl_nPh_4-n (5–9), as well as trivalent groups –SbCl₂, –Sb(R)Cl, and –SbPh₂ (2–4, R = Ph, Mes), were synthesised and fully characterised including single crystal diffraction and multinuclear NMR. In addition, the bonding in these species was studied by DFT computational methods. The P–Sb dative interactions in both series range from strongly bonding to non-bonding as the Lewis acidity of the Sb acceptor is decreased. In the pentavalent antimony series, a significant change in the P–Sb distance is observed between –SbClPh₃ and –SbCl₂Ph₂ derivatives 6 and 7, consistent with a change from a bonding to a non-bonding interaction in a response to relatively small change in Lewis acidity of the acceptor. In the Sb^III series, two geometric forms are observed. The P–Sb bond length in the SbCl₂ derivative 2 is as expected for a normal (rather than a dative) bond. Rather unexpectedly, the phosphine–stiborane complexes 5–9 represent the first examples of σ⁴P→σ⁶Sb structural motif.