Synthesis of Classical Divalent Lanthanide Complexes

Divalent lanthanide chemistry has been dominated by the most readily accessible divalent lanthanide metals samarium(II), europium(II), and ytterbium(II) (classical ) for decades, and a large number of divalent lanthanide compounds have been prepared [92], There are three routes to generate divalent organolanthanide complexes oxidative reaction of lanthanide metal, metathesis reaction of a divalent lanthanide halide, and reductive reaction of a trivalent lanthanide complex. 

The first divalent organolanthanide complex is prepared by the redox reaction of europium with cyclopentadienyl in liquid ammonia (Equation 8.27) [93]. Ytterbocene can be prepared in an analogous manner. 

Transmetallation is a more convenient method to obtaining divalent organolanthanide complexes from lanthanide metals. Reaction of lanthanide metal powder with a mercury alkyl or aryl complex affords the corresponding divalent lanthanide complex (Equation 8.28) [94]. The preparation of divalent perfluorophenyl lanthanide complexes Ln(C6F5)2(THF) (Ln = Eu, n = 5 Ln = Yb, n=4) is a typical example. In most cases, the addition of a small amount of Lnl3 leads to acceleration of the reaction [95]. 

The exploration of Lnl2(THF) c, in particular, Sml2(THF)4, which is prepared by the reaction of samarium metal with ICH2CH2I in THF [96], or the reaction of iodine with an excess of samarium metal in THF, provides appropriate starting materials for divalent organometallic complexes (Equation 8.29). 

An alternative route to divalent lanthanide complexes is a metallation reaction the acid-base reaction between a divalent organolanthanide complex with an acidic substrate 

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