CHELATES OF URANIUM (VI) WITH OXYGEN DONOR LIGANDS AND ASTUDY OF THEIR k STABILITY AND POLYMERIZATION

Abstract

Oxycations of the types MCn+ and MOg* are found mainly among the lighter transition elements (in their higher oxidation states) of a given period, for example, in groups IV, V and VI. They are almost non-existent with the later group metals, although exceptions, such 2+ _2+ 1—10 as Os0| and RuCg , are known. The most extensively studied and the best characterized oxycation is the dioxouraniura (VI) or uranyl ion, UCg+. It may be assumed that the uranyl ion is linear. Although such linearity may not be deduced completely unambiguously from any one experiment, there is general accord that a collinear O-U-0 structure affords better interpretation of the Raman l4 infrared16, and electronic spectra , and of x-ray diffraction intensities . It is also an established fact that the uranyl ion possesses three characteristic frequencies: the symmetric stretching frequency i)^ , lying in the range 780-900 cm"1? the asymmetric stretching frequency i)3, lying in the range 800-1000 cm"* ; and the bending vibration X)z* appearing in the neighbourhood of 200 cm"1. The rather large freque ncy intervals quoted by i^ and l)3 are indicative18 of the fact that appropriate complexation of the uranyl ion by ligand groups presumed to lie in, or nearly in, a plane 2 perpendicular to the axial O-U-0 direction, produce extremely large variations in U( and 1)-$ , HYDROLYSIS OF Ucg+ t Aqueous solutions of uranyl salts are distinctly acidic and it has also long been known that large amounts of U03 can be dissolved by solutions of uranyl salts. To account for these observations as well as for the steep rise in electrical conductance of dilute solutions in comparison with other salts of similar valence type, 19 Maclnnes Longsworth ' proposed the formation of the polyg. meric species UOgfUCg) . On the basis of cryscopic meas urements, potentiometric titrations and spectrophotometry 20 studies, Sutton proposed structures corresponding to UgOg and UgOg as well as a number of Ions containing additional hydroxo groups, viz., U308(OH)+, U30g(0H)g and U30q(0H)^ and calculated equilibrium constants for the formation of various species: 2U(|* ♦ HgO +=± U2C§+ * 2H* Km 1.U10"6 <-9 U2°5+ +»<£* +V ^==± u3°!+ +2H+ K" 5xlCf u3^+ ♦ H2° ;p=± Vb<«>* *H+ K* 2-8xlcr4 21 Sillen and coworkers have been active in advanc ing the 'core-link1 hypothesis for the formation of polynucleer complexes and have described methods for det ermining formula of polymeric olated metal species of the : I "** .1 Ml i general formula M ([0H]t M)n. In a number of cases, they found that the curves, obtained by plotting %(average number of hydroxo groups bound per mole of the metal ion) as ordinate vs. -log [H*] as abscissae for different values of total metal ion concentration (Tjj), were essentially parallel and that the horizontal spacing between the two curves was proportional to the difference between the two values of -log T«. Mathemat ically this may be expressed by the relationshipt i log Tj \ aa iloo«gf[HH+*l] /2 3 where 't' is the number of hydroxo groups bound per polynuclear link in a 'core plus links' type of complex, 2? Ahrland, Heitanen and Sillen have Interpreted their potentioraetrie data on hydrolytic reactions of UOg4, to indicate the formation of sheet-like complexes with double OH bridgest U0g([0H]2 UOg)^ 23 Recently Gustafson, et al.T employing the mathe matical treatment described by Sillen and co-workers, have determined equilibrium constants for the reactions asso ciated with the formation of various species in the hydro lysis of UOg"1*. Aplot of -log TM against -log [H*] at constant Z , obtained from the potentlometric titrations of uranyl nitrate with KOH, corresponded to slopes of feS® •« '-'•; Wf> 2.08, 2.16, 2.20, 9.P6 and 0.34 at 2 values of 0.20, 0.40, 0.60, 0.80 and 1.00 respectively. These values are somewhat higher than th* value of 9.00 which would be predicted on the basis of the formula suggested by Sillen and co-workers, i.e., polymers containing two hydroxo bridges per link. Gustafson et al.?3 have explained this behaviour on the basis of the presence of complexes such as UOg(OH)* in the system. They conc luded thit the hydrolysis of UG^4" ,in the initial stages (Z < 0.3), proceeds by the reactions and U0|+ ♦ HgO < t U0g(0H)+ +H+ 2U0g* +2H2° *==i ^(C^JgUCg"'" +2H* or 2U0g(0H)+ |SSt U0g(0H)g Uo|+ The values of Kx, Kg and Kd, defined by the eouations . [U0g(0H)+] [H*] Kl [»F] , pyq»<og>» !<3 [H*]2 1*4 ]s *d [wB*]» 4 were found to be 10"6'10, 1CT5'84 and icT6'36 respectively, It is of interest to note that the value of Kd for the fc** -11 +. K• 5 uranyl ion is considerably greater than those calculated for copper °, aluminium25, iron26"09(III), and thorium30 indicating that the monohydroxy species of uranyl ion has a relatively greater tendency to form polynuolear complexes. URANYL CHSLATES Among the uranyl chelates of oxygen donor ligands, complexes of a-hydroxy ccrboxylic acids have received 11 an extensive attention. Uranyl tartrate has been known 3^ ^the earliest investigations of uranium chemistry. Peligot32*33, Courtois34 and Itzig38 have described the preparation of solid compounds. A considerable amount of work has been done on the optical properties of uranyl complexes of aliphatic hydroxy acids. Walden36 found thpt the addition of KOH and uranyl nitrate to solutions of tartaric, malic, mandelic, and quinic acids cause a large increase in the optical rotatory powers of the solutions. 37 Bruhat has reported tfart vLTtxyl tartrate solutions exhibit cotton effect.