_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

13
Some complexes
of the lanthanides
and actinides
13 .1 Introduction -lanthanides
The lanthanides are the fourteen elements between cerium (Z = 58) and lutetium
(Z = 71) inclusive. The Group IIIA elements yttrium (Z = 45) and lanthanum (Z
= 57). which are chemically very similar, are also included in this discussion. The
4f orbitals are being filled across the lanthanide series. These are inner orbitals.
that is, they do not extend significantly beyond the filled 5s 2 5p 6 orbitals in the
xenon core. Strong interactions between metal ( 4}) and ligand orbitals would
therefore not be expected. These elements are electropositive. forming quite ionic
compounds and they possess the common property offorming the tripositive ions
Ln'+ (4f"). There is a gradual contraction in size across the series from
La'+ (r = 1.032 Ă) ta Lu' + (0.861 Ă). Y' + (0.90 Ă) fits into the second half of
the series, near Er' + (0.89 Â).
The structures, stability and reactivity of organolanthanide compounds seem
ta be largely determined by electrostatic and steric effects. This is in marked
contrast ta d-block transition elements where interactions between metal and
ligand orbitals are important. Nothing equivalent to the 18-electron rule is tobe
expected. but rather similar chemistry regardless ofthe 4f" configuration. Size is of
crucial importance. By selecting a metal of appropriate size it may in future be
possible ta tailor the steric effect so as ta provide just the right environment to
bring about a desired reaction. Stability of complexes is often enhanced by filling
the coordination sphere with bulky ligands. up to a certain limit, so as to block
decomposition pathways.
Organolanthanide chemistry is a new. rapidly developing are a. There are
considerable experimental difficulties. The complexes are very susceptible ta
hydrolysis and to oxidation in air: the metal centres are strongly electrophilic. The
complexes are paramagnetic. except for those havingf" (La'+. Y' +) or jl 4 (Lu' +)
configurations. The use of n.m.r. spectroscopy is therefore generally limited to
compounds of these metals. Yttrium complexes are valuable in n.m.r. studies as
not only are they diamagnetic, but the only naturally occurring isotope of this
element. !:Y. has nuclear spinI= η 89 Y spectra ha ve been observed and 89 Y - 1 H
coupling appears in proton spectra.
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