Two coordination compounds containing tetra-n-butylammonium cations and bis-tfd-chelated molybdenum(IV) [tfd2− = S2C2(CF3)2
2−] and oxalate (ox2−, C2O4
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), (C16H36N)2[Mo(C4F6S2)2(C2O4)]·CHCl3·C2H2O4 or (N
n
Bu4)2[Mo(tfd)2(ox)]·CHCl3·C2H2O4, and bis(tetra-n-butylammonium) μ-oxalato-bis[bis(1,1,1,4,4,4-hexafluorobut-2-ene-2,3-dithiolato)molybdate(IV)], (C16H36N)2[Mo2(C4F6S2)4(C2O4)] or (N
n
Bu4)2[(tfd)2Mo(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)2Mo(μ-ox)Mo(tfd)2]2−. The crystalline material containing 1
2− is (N
n
Bu4)2-1·CHCl3·oxH2, while the material containing 2
2− is (N
n
Bu4)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)2MoIV. 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
Bu4)2-1·CHCl3·oxH2, 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
Bu4)2-1·CHCl3·oxH2, the F atoms of two of the –CF3 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
Bu4 cations. In (N
n
Bu4)2-2, the whole of the unique N
n
Bu4
+ cation is disordered over two sets of sites. Also, in (N
n
Bu4)2-2, a region of disordered electron density was treated with the SQUEEZE routine in PLATON [Spek (2015). Acta Cryst. C71, 9–18].
An investigation of the interrelationships between linear and nonlinear polarizabilities and bond-length alternation in conjugated organic molecules
