Squarate coordination modes

B. Coordination Modes of the Squarate Ion II. Squaric Acid Compounds... 

Examples of the squarate ion exhibiting different combinations of mixed coordination modes occur in the tetranuclear complex [(C4Hg)4N]4[V406(C404)5(H20)4] BHgO (65) Synthesized by Khan et al. where the squarate ion adopts both /U.-l,2- and /u,-1,3-bridging roles (Fig. 9), while in [Eu(H20)4]2(C404)3 synthesized by Petit et al. (55),... 

Fig. 11. The adoption of both mono- and bis-chelating coordination modes by the squarate ion to Ce(III) in the polymeric structure [Ce(H20)2J2(0404)3 n (57).

Infrared and UV/vis data have been used by several authors to identify the C=C, C=0, and M—O stretches in the complexes synthesized 15, 18-21, 37, 38, 41, 44, 50, 54, 56, 58, 59, 64-66, 69, 74, 78, 80, 82, 103). Except in the initial research on first-row transition metal complexes of squaric acid, where these data were used in proposing structures, IR and UV/vis analysis have been used as supporting evidence for the particular coordination mode of the ligand 19,21,22, 45, 52, 59, 65). Infrared spectroscopy has also been utilized in the study of mixed oxalate/squarate complexes 118), although not to the same extent as in complexes of the oxalate ion. For example, Scott et al. studied the IR properties of Co(III) oxalate complexes with the hgand in a variety of chelating/bridging situations 119). 

The reduction in the number of potential coordination sites from four to three on monosubstitution of squaric acid restricts the variety of possible coordination modes possible for these hgands. In the majority of the complexes prepared by HaU et al., the monosubstituted squarate ligands adopt a /u.-l,3 role, although in a few instances the ligands coordinate monodentately, adopting a pendant role 111, 123,132-137). Expectedly, as a consequence of bite-parameter considerations, chelation is observed only in the case of the large lead(II) ion (76). In... 

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

In their pioneering work, West and Niu assumed that the squarate complexes of divalent cations Mg ", Ca, Mn, Fe, Co ", Ni ", Cu " and Zn form an isostructural series (Figure 1). They suggested a bis-chelating coordination mode for the squarate dianion in all the squarate complexes, but their proposed structure does not correspond to the actual coordination mode of squarate. 

These studies have helped to elucidate the coordination modes of squarate by establishing the following facts ... 

As far as squarate coordination chemistry with 3d ions is concerned, a wide variety of modes has been found monodentate, -1,2-bis-(monodentate), /.i-l,3-bis(monodentate) and tetrakis(monoden-tate). ... 

Table 5.3 Coordination modes of squarate dianion 2 toward transition metals and lanthanides.

Fig. 5. The bismonodentate coordination and /r-1,3-bridging modes of the squarate ligand to Zn(II) in the polymeric structure [Zn(C404)(H20)4]n (30).

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