Oxalate bridges

After forming C-C, C-0, and 0-M a bonds, planar oxalate has just six n molecular orbitals and only two of these have the appropriate energy and symmetry match to interact with the MM8 orbital. These orbital interactions are schematically represented by X and XI. 

Of these the in-phase combination of the two M28 orbitals with the oxalate n orbital, XI, is by far the most important. (The filled CO2 k bonding orbitals lie several eV below the energy of the M28 orbitals). Effectively, the fQled M28 orbitals back-bond to the oxalate tt system and stabilize the planar O2CCO2 unit by placing electron density in an orbital which has C-C it bonding character. The free oxalate dianion has the twisted D2d geometry which minimizes oxygen lone pair repulsions. 

Calculations on the model compounds [(HC02)3M2]2( x-02CC02), where formate substitutes for pivalate, employing DFT support this view and the frontier MOs are shown in Fig. 5. 

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Within the area of molecular magnets, oxalate-bridged 3D networks of the type X" "[M M C204)3] are important, and of significance is the introduction of chiral cations X" ". Examples have included enantiomers of [Ru(bpy)2L]+ where HL = 2-phenylpyridine or 8-hydroxyquinoline, which are able to act as chiral templates in the construction of 3D polymeric networks. ... 

Diamagnetic Reduction of the 11,13-diene complex SqPl Y = I, PF(, with Raney Ni and H2 cis Oh oxalate bridge folded macrocycle... 

Oh dinuclear with oxalate bridges folded macrocycle... 

Bianchini et al 1 have obtained triphos-capped tetrathio- and tetraseleno-oxalate bridged Rh dimer dications of the structure (triphos)Rh(C2E4)Rh(triphos)3, where triphos = 1,1,1-tris-(diphenylphosphinomethyl)ethane and E = S, Se. 

Figure 45 (Left) Structure of (H2TMPD)o,5[Ga3(C204)o.5(P04)3] viewed along the b-axis showing the tetrahedral layers, oxalate-bridged octahedral dimers and double 6-ring units. The Ga06 octahedra, Ga04 tetrahedra and P04 tetrahedra are presented, respectively.

There are also carboxylates15 (Section 17-E-10), phosphates, catecholates, and oxalates. Most of the divalent iron oxalato compounds have a chain structure but the product of photoreduction of mononuclear [bipyH] [Feni(ojc)2(H20)2] gives a three-dimensional anionic polymeric network of [Fe2I(ox)3]2 1 and [Fen(bipy)3]2+ ions.16 Magnetic exchange typically occurs through the oxalate bridges.17 Six-coordinate iron(II) a-amino acid complexes are readily obtained from the reaction of the 4-coordinate Fe(mes)2(phen) and the protic amino add, AH, in THF 18... 

Thus, it is easy to see how the dimer is related to the chain (ID) and the chain to the honeycomb structure (2D). Connecting the 2D layers by oxalate bridges gives the 3D structure. 

Figure 22 Oxalate bridged three-dimensional structure of K4[Th(C204)4] 4H20 oxalates bridge in each plane, either directly or via hydrogen-bonding (Akhtar and Smith Acta Crystallographica, Section B 1975,...

A number of ternary complexes with simple dicarboxylic acid complexes supported by other ligands have been reported. These include the mixed valence Viv,v complex with 2,2 -bipyridine (bpy) as the supporting ligand134 and the dinuclear vanadium(V) oxalate-bridged complex.135 The structure-reactivity relationship in the redox series Vv,iv ni chelated to 2-ethyl-2-hydroxybutanate... 

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