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

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)].CHC13.C2H2O4 or (NnBu4)2[Mo(tfd)2(ox)].CHC13.C2H2O4, and bis(tetra-n-butylammonium) --oxalato-bis[bis(1,1,1,4,4,4-hexafluorobut-2-ene-2,3-dithiolato)molybdate(IV)], (C16H36N)2[Mo2(C4F6S2)4(C2O4)] or (NnBu4)2[(tfd)2Mo(--ox)Mo(tfd)2]. They contain a terminal oxalate ligand in the first compound and a bridging oxalate ligand in the second compound. Anion 12- is [Mo(tfd)2(ox)]2- and anion 22-, formally generated by adding a Mo(tfd)2 fragment onto 12-, is [(tfd)2Mo(-- ox)Mo(tfd)2]2-. The crystalline material containing 12- is (NnBu4)2- 1.CHC13.oxH2, while the material containing 22- is (NnBu4)2-2. Anion 22- 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 12-: 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 (NnBu4)2-1.CHCl3.oxH2, where a difference (for C O versus C-OH) of 0.117 (14) Åwas observed. In the bridging oxalate ligand in 22-, 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 22- contains a delocalized .-system and that the terminal oxalate ligand in 12- contains an only partially localized .-system. In (NnBu4)2-1.CHC13.oxH2, the F atoms of two of the -CF3 groups in 12- are disordered over two sets of sites, as are the N and eight of the C atoms of one of the NnBu4cations. In (NnBu4)2-2, the whole of the unique NnBu4 + cation is disordered over two sets of sites. Also, in (NnBu4)2-2, a region of disordered electron density was treated with the SQUEEZE routine in PLATON [Spek (2015). Acta Cryst. C71, 9-18]. [ABSTRACT FROM AUTHOR]