Six-coordinate lanthanide complexes

The structure of a genuine six-coordinate lanthanide complex namely, Er(NCS)6 in C4H903NEr(NCS)6 was determined by Martin and coworkers [28]. Earlier spectroscopic studies predicted the octahedral disposition of this complex [29]. The geometrical... 

Generally, six-coordinate lanthanide complexes with /3-diketones are coordinatively unsaturated, and thus particularly difficult to synthesize because water or other solvent molecules are particularly prone to occupy extra coordination positions. However, the anionic ligands of adopt a bidentate coordination mode to produce a six-coordinate... 

The first structural study of an authentic six-coordinate lanthanide complex was carried out in 1968 by Martin et al, (1968) although the spectroscopic work of Ryan and Jdrgensen (1966) on the LnX " species which are probably octahedral had been published two years earlier. The number of six-coordinate complexes studied by X-ray diffraction is quite small and this coordination number is stUl often inferred from the chemical formula. 

Many factors affect coordination numbers and coordination polyhedra. It was recognized during 1962-1975 that the chemistry of yttrium and lanthanides is dominated by large coordination numbers. Although the structure of Nd(Br0.3>3 9H2O had been determined as early as 1939 by Helmholtz [6] and shown that the central neodymium ion was bound to nine water molecules in a face-centred trigonal prismatic structure, the commonly held opinion was that the rare earth ions formed six-coordinate, octahedral complexes [7]. This notion was based on the known octahedral, six-coordination known at that time for the most... 

With the exception of thorium and protactinium, all of the early actinides possess a stable +3 ion in aqueous solution, although higher oxidation states are more stable under aerobic conditions. Trivalent compounds of the early actinides are structurally similar to those of their trivalent lanthanide counterparts, but their reaction chemistry can differ significantly, due to the enhanced ability of the actinides to act as reductants. Examples of trivalent coordination compounds of thorium and protactinium are rare. The early actinides possess large ionic radii (effective ionic radii = 1.00-1.06 A in six-coordinate metal complexes),and can therefore support large coordination numbers in chemical compounds 12-coordinate metal centers are common, and coordination numbers as high as 14 have been observed. 

High coordination numbers are common in the f block. The didentate chelate ligand 2,4-dioxopentan-3-ido (acac ) forms six-coordinate octahedral complexes with first-row d-block metal ions (like Cr(III), shown at left), but eight or nine coordinate complexes with f-block lanthanide ions (like Eu(III), shown at right). 

The structures and dynamic magnetic behaviours of 31-Dy4 and 31-Tb4 have been presented in Figure 3.22. Four lanthanide ions are aggregated in the grid-like metal core by a central p4-S and eight peripheral i2-S atoms from ethanethiol ligands [38]. The individual lanthanide centres occupy distorted six-coordinate [LnNS5] octahedral coordination environments. Ac susceptibilities measurements reveal pronounced temperature dependence with a series of maxima below 28 K, typical for SMM behaviour, in complex 31-Dy4. Furthermore, an... 

Vicentini and coworkers have reported the complexes of DPPA with lanthanide perchlorates 224), hexafluorophosphates 225), chlorides and nitrates 226). The anions in the perchlorate and hexafluorophosphate complexes are noncoordinated and hence the complexes are six coordinated. Conductance data for the nitrate complexes indicate that the coordination interaction between the lanthanide ion and the nitrate ion decreases along the lanthanide series 226). 

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 lanthanide elements, in their complexes with jS-diketones, tend to adopt interesting, higher coordination geometries. These compounds frequently crystallize as hydrates from which water removal without decomposition of the compound is difficult. Some structural information is summarized in Table 1. 2,2,6,6-Tetramethylheptanedionate chelates of the lighter lanthanides (La to Dy) can be obtained in nonsolvated form, crystallize in the monoclinic system and contain dimer units whereas the heavier analogues (Ho to Lu) tend to be orthorhombic with isolated six-coordinate monomers.128... 

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