Coordination compounds containing bis-di-thiol-ene-chelated molybdenum(IV) and oxalate: comparison of terminal with bridging oxalate.

Two coordination compounds containing tetra--butyl-ammonium cations and bis-tfd-chelated molybdenum(IV) [tfd = SC(CF)] and oxalate (ox, CO) in complex anions are reported, namely bis-(tetra--butyl-ammonium) bis-(1,1,1,4,4,4-hexa-fluoro-but-2-ene-2,3-di-thiol-ato)oxalatomolybdate(IV)-chloro-form-oxalic acid (1/1/1), (CHN)[Mo(CFS)(CO)]·CHCl·CHO or (N Bu)[Mo(tfd)(ox)]·CHCl·CHO, and bis-(tetra--butyl-ammonium) μ-oxalato-bis-[bis-(1,1,1,4,4,4-hexa-fluoro-but-2-ene-2,3-di-thiol-ato)molybdate(IV)], (CHN)[Mo(CFS)(CO)] or (N Bu)[(tfd)Mo(μ-ox)Mo(tfd)]. They contain a terminal oxalate ligand in the first compound and a bridging oxalate ligand in the second compound. Anion is [Mo(tfd)(ox)] and anion , formally generated by adding a Mo(tfd) fragment onto , is [(tfd)Mo(μ-ox)Mo(tfd)]. The crystalline material containing is (N Bu)-·CHCl·oxH, while the material containing is (N Bu)-. Anion 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)Mo. C-O bond-length alternation is observed for the terminal oxalate ligand in : 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 Bu)-·CHCl·oxH, where a difference (for C=O C-OH) of 0.117 (14) Å was observed. In the bridging oxalate ligand in , 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 carb-oxy-lic acid groups, that the bridging oxalate ligand in contains a delocalized π-system and that the terminal oxalate ligand in contains an only partially localized π-system. In (N Bu)-·CHCl·oxH, the F atoms of two of the -CF groups in are disordered over two sets of sites, as are the N and eight of the C atoms of one of the N Bu cations. In (N Bu)-, the whole of the unique N Bu cation is disordered over two sets of sites. Also, in (N Bu)-, a region of disordered electron density was treated with the SQUEEZE routine in [Spek (2015 ▸). C, 9-18].