Question

(b) In light of your answer above, rationalise the coordination numbers of the following lanthanide compounds in the solid state (the coordination geometry is given for your information and does not need to be justified). Compound Coordination number Coordination geometry LaF3 9 coordinate tricapped trigonal prism LuF3 8 coordinate bicapped trigonal prism Lal3 8 coordinate bicapped trigonal prism LuCl3 6 coordinate octahedral [2 marks] (c) Using the thermodynamic data provided (kJmol-1) explain the differences in the stability of complex formation for the group 2 (Sr2+) and Group 3 (La3+) metals with EDTA4? The ionic radii of the metal ions have also been provided. lonic Radius (A) 1.18 AG° -48.4 ΔΗ -17.2 TAS° 31.2 Sr2+/EDTA4- La3+/EDTA4- 1.03 -86.7 -12.2 74.4

Answer 1

decreases , Lu? 6 The co-ordination number of the halide of lanthanides is depends on the Ponic radia of lanthanides. In lauthanide Series the lonic radii is gradually s from left to right due to the "Lanthanide Contraction". This least has an ionic radli of 1.03 A. While the element in the most right, has the radii of 0.854°. ① Now in between Lupy & Lufy; the former has a co- ordination number (CN) of g While the later has CH 8? due to the bigger size of Laßt it can Accomodate with floride (F) ions around 2+ While tue Smalley Lowest cuit accomodate that micech Fither fore it has lower con '8' compare to the Last, ② The same is also true for Laks & Luz. Due to the Smaller site of able to hold 6 a- in the crystal lattice & Show co-ordination geometry of octahedral , while the bigger Last has the CN of 8! easily This is because, around it 3+ it only w formation constant of complex can be determined as AGO = -RT lnß formation constant ? ox, AG° = -2-303 3 RT logß AGO -2.303 RT using log ß to compare the Stability of the complex formation, or, logp: -2-303 RT In general we are

for Sr² / EDTAG- System [logo]s14 +AG" 748.46103 -2.303 RT 72-303 x 8.314x298 58.48 [log B] git = 8.48 For Last EDTA"- System = 15:19 La 3+ = 15.9 Last to the Soet [logg +86.7x103 72-303 x 8.314 x 298 or. [Log B), From the given ionis radin data it is evident that La²+ (1.03 no) is smaller compare (1.18 A°). Here the Stability of complex formation can be explain in terms of louic potential (8). The lower potential is roughly estimated by the charge to size (ionic radii) of a particular ion. Since Leat It has a charge of t3'& has small lone radius of 1.030°, the Ø for Lea't is quite larger than the of Sret, since the 8 is directly propo- -otional with the charge finversly proportional with size. This ions having high charge & small size (ionic radicus) has the larger courie potential. Higher the of higher will the force of attraction between the M ** (metulion) (EDTA4-), consequently higher will the stability of Complex formation. As a result for La3t / EDTAM - The formation constant is largos compare to So“s/ED TO4, Juica also reflects in the calculation { [43p Jeux» >[wyk)spr} & begando