Bidentate mode

Modeling this diversity computationally is a challenge for FF methods, especially for the bidentate modes, since the method should automatically determine the preferred mode as well as flip —i.e., smoothly switch from short-long asymmetric bidentate through symmetric to long-short asymmetric bidentate. 

In the binuclear complex, [(bpy)2(0H2)Zn(p2-C03)Zn(bpy)2] (N03)2-7H20 (F), the two N4-bound Znx and Zn2 ions are bridged by carbonate in monodentate and bidentate modes, the Znx being coordinated to one H20. Both the zinc ions are octahedrally coordinated with a relatively greater degree of distortion at Zn2 due to the chelating mode of the carbonate bridge. In these binuclear carbonate complexes of Zn(II) and Cu(II) an unexpected behavior, viz. Zn-0 distances < distances for carbonate coordination, was reported (341). 

Fig. 20. The familiar bidentate mode of attachment of the (B3H8) ligand, as found in [Cr(CO>4-(B3H8)] (Ref. 104)...

N (5) is a very weak basic site in FloX (pK < 0), a moderately strong one in FI" (pK 8.5) and again very weak in Flred. In both HFl red (pK 6.5) and FIoX (pK 0.5), N(l) is the preferred protonation site. This explains why enforced blockade of the N(5)-lone pair favors the radical, i.e., le"-transfer, while enforced N( 1)-blockade favors Flox and Flred and, therefore, 2e"-transfer. If, however, the blockade is achieved by metal instead of protons, the attachment site at Flox is switched from N (1) towards N (5) because of the 4 ,5-bidentate mode of metal fixation. Hence, metal complexation as such is sufficient to shift the 2e -transfer system into a le"-transfer system. For redox-active metal,... 

Kitajima et al. (21) also reported a more efficient catalyst through a serendipitous modification of complex 2. They observed that an increase in the catalytic activity (by a factor of 1.5) occurred when hexafluo-roacetyl acetone (hfacac) was used instead of acetic acid. The reaction between 2 and hfacac produced [ Fe(HBpz3)(hfacac) 20], 4 in 50-60% yield that was structurally characterized to reveal a ferric dimer with a single n-oxo bridge (21). Each iron center is in a six-coordinate N303 environment. The two octahedral units are more distorted in this coordination environment than that of 2 the FeOFe unit is bent with an angle of 169.4°. Each ferric center has one hfacac ligand bound in a bidentate mode. 

Prior to the crystallographic demonstration that carbonate binds directly to the iron atom, in bidentate mode, in diferric lactoferrin (78), much debate surrounded the nature of the metal-anion-protein interactions and the functional role of the synergistic anion. Experiments directed at elucidating these questions, together with the crystallographic results, are now beginning to give a much clearer picture. 

Confirmation has come through crystallographic studies of oxalate binding to lactoferrin. The crystal structure of a hybrid complex of copper-lactoferrin, at 2.2-A resolution, has carbonate in the N-termi-nal site and oxalate in the C-terminal site (192). The oxalate ion is bound to the metal ion (Cu2+ in this case) in 1,2-bidentate mode, as anticipated (190,191), i.e., through both carboxylates (Fig. 26a). One... 

Based on the IR, Sn NMR and Sn Mossbauer spectroscopy, the tin atom in tetraorganotin derivative 363a is suggested to be tetracoordinate, but it is pentacoordinate both in the iodide 363b due to the O Sn interaction and in the dithiocarbamate 363c due to the bidentate mode of the thiocarbamate ligand . ... 

The most widely studied of these oxoanions is the nitrate anion, NOb". Some of the known coordination modes of this anion are shown in Figure 4(a). The most common is the symmetrical bidentate mode (27). Examples of nnidentate (28) and asymmetrical bidentate (29) coordination modes are also qnite plentiful. The most common bridging coordination mode for these and all oxoanions is the symmetrical syn-syn mode involving two oxygen atoms of the oxoanion (30). The other modes (31-33) shown in Figured are relatively rare. The mode with a single oxygen... 

The complexes that are formed are nonionic, and coordination to the Pu metal occurs in a bidentate mode through the carbonyl oxygens of the malonamide ligand. IR spectroscopy indicates C2v geometry of the extracted complex due to nitrate stretching bands at 1530-1540 cm" and 1280 cm The extracted species from complexation with DMDBTDMA are different than those that would be obtained with monamides, where the anionic complex Pu(N03)gH2(amide) t would be observed in the organic phase. Monamides are weaker complexants for Pu than are malonamides, due to nitrate/metal competition at high acid concentrations." ... 

XAFS data for complexes of Cm with the HC301 extractant indicate only sulfur donation to the metal in the inner sphere of coordination. HC301 forms 3 1 complexes with the trivalent actinides and are coordinated in a bidentate mode as seen in Figure 91. Data indicates a hexacoordinate structure that resembles Z>3 symmetry in lanthanide dithiophosphinic acid complexes. 

Coordination of the diamide with the metal ion can occur in both a bidentate mode through both carbonyl oxygens or one carbonyl oxygen and the bridging oxygen/sulfur, or in a tridentate mode via both carbonyl oxygens and the bridging atom. ... 

Complex [Am(MPBIZ)2] is the prevailing component in ethanol solution as shown by means of EXAFS experiment [81]. The MPBIZ molecule coordinates to Am through the nitrogen atoms in bidentate mode. The Am-N distance is equal to 2.63 A, while the Am-0 bonds are shorter. By this means, the MPBIZ ligand appears to be bonded to metal atom weaker than water molecules. The authors made the conclusion that the contribution of the covalent bond to the chemical bonds between Am and N is small. 

---