Lanthanide elements coordination numbers

Scandium is more like an element of the first transition series (or like aluminum) than like the rare earths. For example, it characteristically has a coordination number of 6 (6-coordinate radius, 0.89 A compared to 1.00-1.17 A for the lanthanides), although coordination numbers 8 and 9 are known. Examples of CN 8 are Na5[Sc(C03)4] -6H20 and the tropolonate, [SeT4] . There is but one example of CN 9, namely the tricapped trigonal prismatic [Sc(H20)9]3+ ion found in Sc(CF3S03)3-9H20.38 The aqua ion in solution, [Sc(H20)6]3+, is appreciably hydrolyzed and OH-bridged di- and trinuclear species are formed. 

As with other transition elements, the lanthanides can be induced to form complexes with exceptionally low coordination numbers by use of the very bulky ligand, N(SiMe3)2 ... 

Symbol Ho atomic number 67 atomic weight 164.93 a lanthanide series rare earth element electron configuration [Xe]4/ii6s2 valence state +3 metallic radius (coordination number 12) 1.767A atomic volume 18.78 cc/mol ionic radius Ho3+ 0.894A one naturally occurring isotope. Ho-165. 

Symbol Lu atomic number 71 atomic weight 174.97 a lanthanide series element an /-block inner-transition metal electron configuration [Xe]4/i45di6s2 valence -1-3 atomic radius (coordination number 12) 1.7349A ionic radius (Lu3+) 0.85A two naturally-occurring isotopes Lu-176 (97.1%) and Lu-175(2.59%) Lu-172 is radioactive with a half-life of 4xl0i° years (beta-emission) several artificial isotopes known, that have mass numbers 155, 156, 167—174, 177—180. 

Symbol Tm atomic number 69 atomic weight 168.93 a lanthanide series element a rare earth metal electron configuration iXe]4/i36s2 valence +2, -i-3 atomic radius 1.73 A ionic radius, Tm " " 1.09 A for coordination number 7 one stable, natural isotope Tm-169 (100%) thirty radioisotopes in the mass range 146-168, 170-176 ty, 1.92 years. 

Most transition metals of the three d-series in all their valency states exhibit ionic radii within the limits of 0.55 and 0.86 A, favourable to octahedral coordination. In fact higher coordination numbers are observed only in fluorides of the largest transition ions, above all in compounds of the lanthanide and actinide series. Therefore fluorides of those elements, though sometimes isostructural with compounds of the d-series, will not be discussed here. For information the books and reviews written by Spedding and Daane (291), Katz and Seaborg (181) and Kaiz and Sheft (182) may be consulted. 

The use of unsubstituted or 4-methyl phenols resulted in the formation of cluster compounds [58]. However, 2,6-di(fcrt-butyl) substituted aryloxide ligands allowed the isolation of mononuclear 3-coordinate homoleptie complexes of the lanthanide elements, the coordination mode of which was first demonstrated with the N(SiMe3)2 ligand [59], The 2,6-substitution pattern is very effective because the alkyl groups are directed towards the metal center and impose a steric coordination number onto the metal which is comparable to the Cp ligand (Cp 2.49 OC6H3rBu2-2,6 2.41) [60],... 

Despite there being an obvious trend to enlargement of the Ln-OR bond lengths by increasing the coordination number at the metal center, the Ln-OR contacts seem to be particularly sensitive to the type of additional counterions as illustrated for early (Nd) and late (Y) lanthanide elements (Tables 15, 16). 

LaCrC>3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide and T is a period 4 transition element. In the cubic unit cell R occupies the cube corners, T the cube centre and O the face-centre positions. The coordination numbers of T and R are 6 and 8 respectively. LaCrC>3 loses chromium at high temperatures, leaving an excess of O2- ions. The excess charge is neutralized by the formation of Cr4+ which results in p-type semiconductivity with hole hopping via the localized 3d states of the Cr3+ and Cr4+ ions. The concentration of Cr4+ can be enhanced by the substitution of strontium for lanthanum. A 1 mol.% addition of SrO causes the conductivity to increase by a factor of approximately 10 (see Section 2.6.2). 

It should be noted the coordination number, N for trivalent lanthanides does not bear the same relevance and context as the transition elements, Fe(3d), Pd(4d) and Pt(5d). The 4/" electron population does not influence N in moving along the lanthanides series while the d electron population has considerable influence on N in the transition metals. Taking nickel as an example we have compounds of Ni(IV), Ni(III), Ni(II) and Ni(0). The majority of the compounds are of Ni(II) such as Ni(H20) +, NiiNH- ) 4" which have N = 6 and an octahedral disposition. The compound KNiF3 is a cubic perovskite with N = 6 and also paramagnetic. When Ni(II) forms diamagnetic complexes N = 4 with a square planar disposition. Tetrahedral NiCl - with N = 4 tetragonal-pyramidal Ni(CN)j- with N = 5 are also known. 

The coordination number, N of lanthanides varies significantly and no intrinsic property of 4/ group atoms predetermines a high propensity for a given N value and symmetry as is common with transition elements like 3d3 Cr(III), 3d6 Co(III), 3d8 Ni(II), 4d6 Rh(III), and 5d6 Ir(III) and Pt(IV). In the case of lanthanides, N takes on values from 3 to 16 as shown by examples given below. 

Cyclopentadienyl (and substituted variants thereof) has been the most versatile ligand used in organolanthanide chemistry. Unlike the situation with the d-block metals, where a maximum of two pentahapto- () -)cyclopentadienyls can coordinate, up to three -cyclopentadienyls can be found for the lanthanides, in keeping with the higher coordination numbers found for the f-block elements. In terms of the space occupied, a -cyclopentadienyl takes up three sites in the coordination sphere. Three types of compound can be obtained, depending upon the stoichiometry of the reaction mixture ... 

The analytical chemistry of the transition elements see Transition Metals), that is, those with partly filled shells of d (see (f Configuration) or f electrons see f-Block Metals), should include that of the first transition period (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and that of the second transition series (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag). The third transition series embraces Hf, Ta, W, Re, Os, Ir, Pt, and An, and although it formally begins with lanthanum, for historical reasons this element is usually included with the lanthanoids (rare-earth elements) see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry Rare Earth Elements). The actinoid elements see Actinides Inorganic Coordination Chemistry) are all radioactive see Radioactive Decay) and those with atomic number see Atomic Number) greater than uranium (Z = 92) are artificial the analytical chemistry of these elements is too specialized to consider here. 

Higher coordination numbers of 8 -F 1 are adopted in the LT-YF3 type by the trifluorides of the larger ions TP+, bP+ and the smaller rare-earth ions Sm to Ln. The tysonite or LaF3 type with CN 9 + 3 is found for the trifluorides of the larger 4f and the 5f elements (see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry). 

Lanthanide elements have atomic numbers ranging from 57 to 71. With the inclusion of scandium (Sc) and yttrium (Y), a total of 17 elements are referred to as the rare earth elements. A mixture of rare earths was discovered in 1794 by J. Gadolin and ytterbium was separated from this mixture in 1878 by Mariganac, while the last rare earth element promethium (Pm) was separated by a nuclear reaction in 1974. Therefore, a period of more than 100 years separates the discovery of all the rare earth elements. In the latter part of the last century scientists started to focus on the applications of rare earth elements. Numerous interesting and important properties were found with respect to their magnetic, optical, and electronic behavior. This is the reason that many countries list all rare earth elements, except promethium (Pm), as strategic materials. Rare earth coordination chemistry, therefore, developed quickly as a result of this increased activity. 

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