Coordination compounds containing bis-di­thiol­ene-chelated molybdenum(IV) and oxalate: comparison of terminal with bridging oxalate

Two coordination compounds containing tetra-n-butyl­ammonium cations and bis-tfd-chelated molybdenum(IV) [tfd(2−) = S(2)C(2)(CF(3))(2) (2−)] and oxalate (ox(2−), C(2)O(4) (2−)) in complex anions are reported, namely bis­(tetra-n-butyl­ammonium) bis­(1,1,1,4,4,4-hexa­fluoro­but-2-ene-2,3-di­thiol­ato)oxalatomolybdate(IV)–chloro­form–oxalic acid (1/1/1), (C(16)H(36)N)(2)[Mo(C(4)F(6)S(2))(2)(C(2)O(4))]·CHCl(3)·C(2)H(2)O(4) or (N(n)Bu(4))(2)[Mo(tfd)(2)(ox)]·CHCl(3)·C(2)H(2)O(4), and bis­(tetra-n-butyl­ammonium) μ-oxalato-bis­[bis­(1,1,1,4,4,4-hexa­fluoro­but-2-ene-2,3-di­thiol­ato)molybdate(IV)], (C(16)H(36)N)(2)[Mo(2)(C(4)F(6)S(2))(4)(C(2)O(4))] or (N(n)Bu(4))(2)[(tfd)(2)Mo(μ-ox)Mo(tfd)(2)]. They contain a terminal oxalate ligand in the first compound and a bridging oxalate ligand in the second compound. Anion 1 (2−) is [Mo(tfd)(2)(ox)](2−) and anion 2 (2−), formally generated by adding a Mo(tfd)(2) fragment onto 1 (2−), is [(tfd)(2)Mo(μ-ox)Mo(tfd)(2)](2−). The crystalline material containing 1 (2−) is (N(n)Bu(4))(2)-1·CHCl(3)·oxH(2), while the material containing 2 (2−) is (N(n)Bu(4))(2)-2. Anion 2 (2−) lies across an inversion centre. The complex anions afford a rare opportunity to compare terminal oxalate with bridging oxalate, coordinated to the same metal fragment, here (tfd)(2)Mo(IV). C—O bond-length alternation is observed for the terminal oxalate ligand in 1 (2−): the difference between the C—O bond length involving the metal-coordinating O atom and the C—O bond length involving the uncoordinating O atom is 0.044 (12) Å. This bond-length alternation is significant but is smaller than the bond-length alternation observed for oxalic acid in the co-crystallized oxalic acid in (N(n)Bu(4))(2)-1·CHCl(3)·oxH(2), where a difference (for C=O versus C—OH) of 0.117 (14) Å was observed. In the bridging oxalate ligand in 2 (2−), the C—O bond lengths are equalized, within the error margin of one bond-length determination (0.006 Å). It is concluded that oxalic acid contains a localized π-system in its carb­oxy­lic acid groups, that the bridging oxalate ligand in 2 (2−) contains a delocalized π-system and that the terminal oxalate ligand in 1 (2−) contains an only partially localized π-system. In (N(n)Bu(4))(2)-1·CHCl(3)·oxH(2), the F atoms of two of the –CF(3) groups in 1 (2−) are disordered over two sets of sites, as are the N and eight of the C atoms of one of the N(n)Bu(4) cations. In (N(n)Bu(4))(2)-2, the whole of the unique N(n)Bu(4) (+) cation is disordered over two sets of sites. Also, in (N(n)Bu(4))(2)-2, a region of disordered electron density was treated with the SQUEEZE routine in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9–18].