Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry

Abstract

AbstractThe lanthanides are a block of elements with atomic numbers 57–71, which exhibit subtle gradations in their chemical properties, since the ionic radii gradually decrease across the series (the ‘lanthanide contraction’). In reactivity, they resemble the group 2 elements, but are much more diverse; although the aqueous chemistry is essentially that of the +3 oxidation state, the +2 and +4 states are significant for certain metals. The 4f orbitals are well shielded and play little part in lanthanide chemistry, so lanthanides have virtually no chemistry in the zerovalent state and do not form multiple bonds to atoms like N and O. As with the transition metals, lanthanide ions usually have a partly filled subshell (4f), but, in contrast, crystal‐field splittings are weak compared with spin‐orbit coupling, and thus spectroscopic properties of lanthanide complexes strongly resemble those of the free ions; sharp f‐f‐transitions in both excitation and emission spectra confer valuable properties upon certain ions. Lanthanides form a full range of binary compounds with nonmetals, especially the group 16 and 17 elements. The halides LnX3, in particular, demonstrate both the high coordination numbers and the lanthanide contraction. The complex chemistry in aqueous solution is dominated by ‘hard’ donors, especially oxygen‐based ligands (lanthanide ions have very high hydration energies), but ligands with other donor atoms, such as nitrogen, form stable complexes in nonaqueous solvents. The lanthanide aqua ion has a coordination number of 9, decreasing to 8 for the later metals; higher coordination numbers of up to 12 are possible with ligands with a small ‘bite’, such as nitrate. Many complexes exist of oxygen‐donor ligands such as THF and phosphine oxides, with formulae of the type [La(THF)nCl3] (n, e.g. 3, 4) and [Ln(R3PO)n(NO3)3] (n, e.g. 2, 3). Diketonate complexes are important; stable adducts with molecules like polyethers are often volatile enough to be MOCVD precursors. Bi‐ and tridentate N‐donor ligands like bipy and terpy form high coordination number complexes that often exhibit subtle variations in structure with choice of lanthanide and solvent. Low‐coordination numbers are achieved using very bulky amide, alkoxide, and alkyl ligands, when values as low as 2 and 3 are possible. Polyaminepolycarboxylic acids such as EDTA and DTPA form very stable complexes that are highly important in separating mixtures of the lanthanides and also, in the case of [Gd(DTPA)(H2O)]3−, as a contrast enhancing agent in MRI, one of an increasing number of applications that rely upon their electronic and magnetic properties. Yttrium has very similar properties to the later lanthanides, but scandium is significantly smaller. Although the first 3d metal, scandium is not a transition metal, but has significantly different properties, partly on account of its slightly greater size, forming complexes with higher coordination numbers (e.g. the aqua ion [Sc(H2O)7]3+). Its developing chemistry is that of a slightly smaller version of lutetium.