Stabilisation of [W^VF₄]⁺ by N‐ and P‐Donor Ligands: Second‐Order Jahn‐Teller Effects in Octacoordinate d¹ Complexes

Abstract

The first isolated examples of cationic fluoridotungsten(V) complexes are reported as octacoordinate [WF4(L)4]+ (L=C5H5N, P(CH3)3). The [WF4(NC5H5)4]+ cation is synthesised as its [O3SCF3]− salt upon reaction of WF5(NC5H5)2 with [(CH3)3Si(NC5H5)][O3SCF3] in excess C5H5N, whereas [WF4{P(CH3)3}4]+ is accessed directly from WF6 upon reaction with (CH3)3SiO3SCF3 and excess P(CH3)3. These salts were characterised by X‐ray crystallography and Raman spectroscopy in the solid state. New geometry indices for octacoordinate complexes (τ8 and τ8′) are introduced, allowing for the facile differentiation of trigonal‐dodecahedral (TD) and square‐antiprismatic (SA) geometries. This has disambiguated the SA geometries of [WF4(L)4]+ and the geometries of a series of previously reported d0 and d1 MA4B4 complexes. Computational (DFT−B3LYP) studies of [WF4(PH3)4]2+/+ and related model systems demonstrate the occurrence of a second‐order Jahn‐Teller (SOJT) distortion from TD in d0 complexes to SA in d1 complexes, with the degree of SOJT stabilisation being most significant in 5d complexes containing fluorido ligands and monodentate neutral donors.