Unidentate coordination

The molecular structures of several [TpBut]ZnX derivatives have been determined by x-ray diffraction. For example, x-ray diffraction studies confirm that the acetate ligand in [TpBut]Zn(r)1-02CMe) is bound to zinc in a unidentate mode, similar to that proposed for [TpBut]Mg(7j1-02CMe), but in contrast to the bidentate coordination proposed for the copper analogue [TpBut]Cu(T)2-02CMe) (86,87). Such a change in coordination mode for copper and zinc derivatives is to be anticipated on the basis of structural studies on the nitrate derivatives [TpBut]M(N03) (M = Co, Ni, Cu, Zn), as described in Section V,B,2,e. The thioacetate [TpPh]-Zn V-SC(0)Me (81), and cyanoacetate [Tp lZnlr -C CCH N) (88) derivatives also exhibit unidentate coordination. 

Secondly, the molecular structure of [TpPr 2]Zn 2(/u,-7)1,7)2-C03), as determined by x-ray diffraction (151,153), reveals that the carbonate ligand is coordinated in an asymmetric manner, with unidentate coordination to one zinc center and bidentate coordination to the other zinc center, i.e., fi-ri1,ri2-C03 (Scheme 24). Such coordination is noticeably distinct from that of [TpBut Me]Zn 2(/x.-r)1,7 1-C03), in which the carbonate ligand bridges in a symmetric unidentate mode. 

The structural variations observed for [TpRR ]M(N03) (M = Co, Ni, Cu, Zn Cd) reveal that, for a given [Tp1 ] ligand, the preference for bidentate coordination increases across the series Zn < Co Cu, Ni, and Cd. Significantly, these structural preferences of the nitrate ligand correlate with the activity of the metal-substituted enzymes Zinc, the metal with the greatest tendency to exhibit unidentate coordination of the nitrate ligand, is the most active, while nickel, copper, and... 

Tris(pyrazolyl)hydroborato ligands are also known to adopt bidentate and uniden-tate coordination to a lesser degree. For example, a rare example of unidentate coordination is provided by [i7l-TpBu,]Ni(PMe3)2(p-C6H4X) (X = H, Me, OMe). See Gutierrez, E. Hudson, S. A. Monge, A. Nicasio, M. C. Paneque, M. Carmona, E. 

Monomeric carboxylato complexes with Av values greater than 200 cm"1 invariably have unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

IR spectroscopy is often used for distinguishing between unidentate and bidentate coordination of carboxylate (02CR) ligands. For monomeric carboxylate derivatives the separation between the symmetric and asymmetric C02 stretching bands, At = [t asym(C02) - t syJCOa)], provides a useful indication of the coordination mode complexes which exhibit values of greater than 200 cm-1 invariably possess unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

Symmetric bis(unidentate) coordination of bridging carbonate ligands has been observed previously. See ... 

This suggestion is not necessarily intended to imply that the bicarbonate intermediate of the carbonic anhydrase cycle must exhibit unidentate coordination in the ground state. Rather, it is intended to imply that a unidentate species should be readily accessible. Moreover, it is not intended to suggest that this factor alone is responsible for influencing the activity of metal-substituted carbonic anhydrases,... 

The X-ray structure of the unsubstituted tris(pyrazolyl)borato zinc nitrate has been solved showing a unidentate coordination mode for nitrate, in contrast with the t-butyl substituted ligand, which shows anisobidentate nitrate coordination due to the steric effects.232 A partial explanation of the reduced activity of cadmium-substituted carbonic anhydrase is offered by Parkin on the basis of the comparison of nitrate coordination to cadmium and zinc trispyrazo-lylborate moieties. A contributing factor may be the bidentate coordination supported by the cadmium that does not allow the facile access to a unidentate bicarbonate intermediate, which could be highly important to carbonic anhydrase activity.233... 

Asparagine and glutamine residues constitute only 6% and 2% of the Ca -coordinating ligands in the sites of Table II. In theory the amide side chains of these amino acids can provide one oxygen for unidentate coordination to Ca as well as interaction with hydrogen-bond donors ... 

Much coordination chemistry has been carried out with simple pyrimidines and the nucleic acid bases. The crystal structure566 of tetrakis(l-methyl-pyrimidine-2-thione)zinc(II) perchlorate bis(propanone) demonstrates unidentate coordination by the non-methylated (N-3) nitrogen atom, with r(Zn—N) at 2.058 and 2.060 A. The structure of dichlorobis(l-methylcytosine)cadraium(II)567 involves two Cd—Q bonds (2.497 and 2.485 A) and two Cd—N(3) bonds (2.281 and 2.296 A) with approximately tetrahedral stereochemistry. 

Unidentate coordination (B) is well established and indeed extensive particularly in circumstances where only one coordination site is available, e.g. cobalt(III) pentaammine series.12 Within the various forms of coordination available to carboxylates the unidentate has been the poor relation until recently, but over the last few years there has been an upsurge of interest triggered by its possible role as an intermediate in the formation of more complicated structures.13... 

Complexes containing unidentate coordination have depended for their characterization upon physicochemical methods rather than structural determinations. The monomeric octahedral co-balt(III) complex [Co(en)2X(ox)], where X = halogen or OH, provides one of the few examples of unidentate oxalate coordination the conformation was determined from IR data.65 By contrast, there are numerous examples of structurally characterized chelating dicarboxylate systems. There are several structural determinations of different forms of oxalic acid and its ions but the most useful for comparative purposes are those of a-oxalic acid and anhydrous sodium oxalate. Using the planar ( >2 ) oxalate ion and acid as benchmarks it can be seen (Table 4) that the rj4 oxalate ligand (bite = 265 pm)66 has two long and two short C—O bonds. The coordinated C—O(M) bond resembles the C—O bond of the acid while the free C=0 bonds are similar to those in the oxalate... 

Cu(en)2(C104)2 complex by Pajunen (107, 108) has shown unidentate coordination of both perchlorates in the long tetragonal positions above and below the planar [Cu(en)2]2+ unit. 

Sandstrom (146) has established by X-ray analysis unidentate coordination of two of the perchlorates to one of the metal ions (Hg—O length = 2.78, 3.04 A Hg—O—Cl angle = 161,123°) in the binuclear complex Hg2(dmso)8(C104)4 The metal ions bridged through two of the eight dmso molecules assume distorted octahedral geometries. X-ray evidence has been presented by Epstein et al. (147) for the presence of a weakly bonded perchlorate in a seven-coordinate mercury(II) complex... 

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. 

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