Uranium coordination number

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... 

The tetraallylic complexes of thorium and uranium can be isolated as red sohds and are stable to 0° and — 20°C, respectively, whereas the uranium homoalkyls defy isolation at temperatures as low as — 80°C. The greater stability of the thorium complex has been attributed partially to the great instability of Th(III) as compared with U(III) (96). The tc mode of bonding and the resultant increase in coordination number to eight of the allyl complexes significantly increases their thermal stability relative to the uranium homoalkyls. 

The ground state geometries in these complexes are often determined by a subtle balance of the energy released by an increase in coordination number and the energy lost by intramolecular repulsion. The question of versus bonding in the tris(cyclopentadienyl)uranium aUyls is one molecular example. The structures of the lanthanide tiicyclopentadienides represent another example in extended crystalline arrays. 

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. 

The literature is most prolific on neutral An amides, but neutral U and amides are also well documented. There are only few publications on amides,one being on three alkali metal amidouranate(VI)s. ° There is a paper on a compound (18) which couid be classified as of U° and two on mixed valence U /U amides (see Section 5.4 for these types). There are a number of articles on monocationic amidol/ tetraphenylborates, three on dicationic analogues, three on a cationic U salt and four on alkali metal amidoactinate(IV)s and three on corresponding U salts. For crystalline uranium amides containing only monodentate ct-bonded ligands, metal coordination numbers of 3-7 are... 

In the addition mechanism, the coordination number of uranium is increased from six to seven, whereas in the substitution mechanism, there is no increase in the coordination number. The absence of any synergistic effect for uranium(IV) and thorium(IV),127 which give coordination-saturated complexes M(HA2)4 with organophosphoric acids in which the metal ions are eight coordinate, is in accord with the addition mechanism. 

The stereochemistry of uranium alkoxides is very diverse, and it is possible to conclude that the geometry of the metal atom is independent on its oxidation state and the coordination numbers are in general lower than those observed for the inorganic compounds. In the series of monomeric U(IV) molecules — e.g., [U(OC6H3Bu 2-2,6)4] [1669], [UI2(OC6H3Bu 2-2,6)2THF] [79],... 

Question A How are the nitrates likely to be bound in U(N03)4(hmpa)2 Suggest a coordination number and coordination geometry for uranium. 

Question B Suggest a credible structure for the compound U(N03)4(hmpa)4, indicating the coordination number of uranium. 

It will be noted that the transitions are often broader than those found in the spectra of lanthanide complexes - and indeed the later actinides, see Section 12.2.4. The 5f energy levels are more sensitive to coordination number than are the corresponding levels in the lanthanides since there are bigger crystal-field effects, one sees pronounced differences between the spectra of 6-coordinate [UCle] and of U" +(aq) (Figure 12.5), leading to the conclusion that the uranium(iv) aqua ion was not six coordinate (most recent EX-AFS results suggest a value of 9 or 10, see Table 11.1). Figure 12.6 displays another example of the difference in spectra between similar complexes of different coordination number. 

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