Nitrate coordination modes

Fig 46. Variation of nitrate coordination mode for the complexes [TpBut]M(N03) (M = Co, Ni, Cu, Zn). Reprinted with permission from Ref. (184). Copyright 1991 American Chemical Society. 

For some additional studies on acetate, formate, nitrite, and nitrate coordination modes in zinc and copper [M(phen)2] and [M(bipy)2] derivatives, see ... 

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. 

The coordination modes of the nitrate ligand in the complexes [TpBut]M(N03) (M = Cu, Ni, Co, Zn) are summarized in Fig. 46. (171, 184). Evidently, the coordination mode varies from unidentate for Zn to symmetric bidentate for Ni and Cu, with the cobalt derivative exhibiting an anisobidentate coordination mode. Moreover, the related cadmium derivative [TpBut,Me]Cd(N03) also exhibits bidentate coordination of the nitrate ligand, with Cd-0 bond lengths of2.272(6) A and 2.295(7) A (91). Such symmetric bidentate coordination contrasts with the significantly different Zn-0 interactions [1.978(3) A and 2.581(3) A] in unidentate [TpBut]Zn(N03). The coordination modes for a variety of [TpRR ]M(N03) complexes are summarized in Table VIII. 

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... 

There has been particular recent interest in zinc nitrate complexes as coordination models for bicarbonate binding in carbonic anhydrase. The mono- or bidentate coordination modes have been studied with tris-pyrazolyl borate complexes and can be rationalized in the context of the enzyme activity.433 Caution in this comparison is introduced by ab initio calculations on these model systems demonstrating both monodentate and bidentate coordination energy minima for nitrate binding to zinc 434... 

In all other complexes of lanthanide nitrates, the mode of coordination has been identified, with some ambiguity, from IR data alone. In most of the cases, either the criterion suggested by Addison et al. (287), Curtis and Curtis (288) or Lever et al. (289) has been used for this purpose. Ionic nitrate groups have been identified in the complexes of BuL (60), HMPA (223), and O-PhenNO (178) with lanthanide nitrates. Both unidentate and bidentate nitrate groups are present in the complexes of lanthanide nitrates with MP (232), OMPA (234), CMP (240), TMSO (264), DMF (42), and BuL (60). Complexes of PyO, 2-MePyO, 2,6-DMePyO, and 2,4,6-TMePyO which have the general formula Ln(L)3(N03)3 -x H20 contain only bidentate nitrate groups (152,170). 

The nitro group has various coordination modes [7,78-81], Synthesis of complexes of metal nitrates is carried out, in the majority of cases, by the method of immediate interaction of these salts with ligands in aqueous-organic (mainly, alcohols) media. Also, the interactions of metal oxides, ligands, and nitric acid are applied, for example (4.18) [82,83], The structure of mixed-ligand complex 848, containing chelate NOT - group, was proved by x-ray diffraction [82,83],... 

Using excess of HMPA, a green compound U(N03)4(hmpa)4 is obtained. Its IR spectrum shows two different modes of nitrate coordination, and no ionic nitrate. 

In nitrate complexes, the nitrate ligand is found in the different coordination modes shown in Scheme 33 ... 

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... 

Figure 4 Some coordination modes of the (a) nitrate anion, (b) nitrite anion, and (c) tetrahedral anions...

Structure In rare earth nitrates, the nitrate groups usually have one of the coordination modes shown in Figure 1.26 when coordinated to the central ions. 

N02 N03 Nitrite Nitrate O 0 0 N120° 0 0 N—0 distance varies from 113 to 123 pm and bond angle varies from 116° to 132° depending on cation versatile ligand (see Chapter 9) Forms compounds with nearly all metals as ligand, has a variety of coordination modes... 

Co(Bpph2) (pz )Bpph2 ]. 2C6H14 (Fig. 4.11) have been reported. The scorpionate ligands are coordinated in the classical k3-coordination mode in all complexes. In the nitrate derivative, the coordination sphere is completed by three oxygen donor atoms from NO3 and THF whereas in the heteroleptic complex, Bpph2 acts as a k3-A 2, H-donor.64... 

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