Uranyl complexes coordination numbers

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

Hexavalent. The majority of An(VI) coordination chemistry with N-donors has been explored with the uranyl cation, 50i. Stable adducts with the hgands discussed in the tri- and tetravalent complexes have been described, for example, U02X2L (X = halide, OR, NO3, RCO2). The coordination numbers observed for these complexes are typically 6, 7, or 8 with octahedral, pentagonal bipyramidal, or hexagonal bipyramidal geometries, respectively. Neutral and anionic thiocyanates have also been isolated, for example [U02(NCS)j2- yH20(x = 2 5). 

Ethers. Complexes with dialkylether ligands (as well as tetrahydrofuran) have been reported for uranyl chlorides, nitrates, thiocyanides, perchlorates, and fteto-diketonate ligands. The formulation of the complexes would suggest uranium coordination numbers ranging from six... 

It is assumed CMPO eoordination with uranyl is the same as that for the trivalent and tetravalent actinides. Monodentate eoordination occurs via the phosphoryl oxygen for the nitrate eomplexes and bidentate coordination occurs through both the phosphoryl and earbonyl oxygen atoms for the chloride complex, leading to a coordination number of eight for both kinds of eomplexes." ... 

It is worthy to note that the Chemyaev-Schelokov and Suglobov rows can be used not only in classification of experimental data but also in prediction of the interplay between composition and stmcture for many different uranyl complexes. However, as to our knowledge, there were no attempts made in understanding chemical factors that determine coordination number of uranium in uranyl complexes (i.e. 5, 6, 7, or 8). However, this question is of primary importance for us and was considered in detail in [13, 14]. In these works, we have used a new stereoatomic model of crystal stmctures which we shall briefly outline in the following section. 

The U02 - XOs - H2O (X= C or N) systems have been chosen for analysis for two reasons. First, they are well studied experimentally, since carbonate and nitrate uranyl complexes are important from technological standpoints. Second, the isoelectronic anions XOs have the same planar structure (symmetry Dsh) and form bidentate coordination around uranyl ions (the type). However, their electron-donor characteristics are different El = 3.1 and 3.4 for the NOs and COs ions, respectively. From this viewpoint, it would be of interest to understand how the difference in the electron-donor properties influences complex formation in the U02 - XOs " - H2O systems. As in the case of aqua-complexes, we shall use the 18-electron rule to obtain answers to the following questions (a) what is the composition of stable complexes in aqueous solutions containing carbonate or nitrate uranyl complexes (b) what is the coordination number of U(VI) in these complexes. 

The idea that expanded porphyrins, as "bigger porphyrins", could serve as ligands for cations that are too large to be accommodated as 1 1 in-plane adducts by the tetrapyrrolic porphyrins is, of course, not new. Indeed, it has its antecedents in the early work of Marks and Day and in even earlier studies carried out by Woodward and coworkers. In the case of the latter group, for instance, it became apparent sometime between 1966, the lime of the initial disclosure of sapphyrin 3 (a disclosure, incidentally, that represented the first documented report of an expanded porphyrin), and 1983, the time of the follow-up full paper, that the pentapyrrolic sapphyrins, with an inner core of ca. 5.5 A diameter, should be well-suited for the complexation of uranyl ion, U02 (in-plane ionic radius 0.81-0.86 A depending on coordination number ). 

The Fourier transform moduli of EXAFS data of three silica samples that sorbed U(VI) appeared to be very similar (Fig. 3-5). The uranyl U=0 distance of 1.78 0.01 A is typical of U(VI) compounds. Shells for hydroxide coordinated in the equatorial plane appeared in the range 2.22 to 2.31 A, while bond distances for coordinated water molecules were observed from 2.43 to 2.51 A. The presence of a U-Si shell (between 3.1 and 3.3 A) in all samples investigated suggests the formation of inner-sphere uranyl surface complexes, and there was no evidence for a U-U shell, which would indicate surface precipitation or polynuclear uranyl species. The short U-Si bond length of about 3.2 A and coordination number of 1 at pH around 3 suggests bidentate complexation to a single silica atom. At pH 5, the coordination number of 2 for the Si shell suggests the coordination of the uranyl complex to two Si atoms (Sylwester et al., 2000). 

The actinide nitrates whose structures are known are listed in Table 20.9, and a review of actinide complexes has been published by Casellato et al [413]. These are limited mainly to tetravalent Th and to uranyl complexes. In the Th(iv) and Pu(iv) compounds the NO3 ions are bidentate, which allows for large coordination numbers compared to the usual eight-fold coordination of these ions with monodentate ligands. In the uranyl complexes the two close actinyl oxygen atoms limit the available space for other bonded atoms, and the maximum number of equatorial oxygen atoms is six, even for bidentate NO3 ions. In Cs2lJ02(N03)4 all four NO3 ions are attached to the U atom even though space allows only two of them to be bidentate. 

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