Ytterbocene [Yb(C(5)MeH(4))(2)(thf)(2)] reacts with diazabutadiene 2,6-iPr(2)C(6)H(3)-N=CH-CH=N-C(6)H(3)iPr(2)-2,6 (DAD) as a one-electron reductant to afford a bis(cyclopentadienyl) Yb(III) derivative containing a DAD radical anion [Yb(C(5)MeH(4))(2)(dad(-.))]. However, ytterbocenes [YbCp*(2)(thf)(2)] (Cp*=C(5)Me(5), C(5)Me(4)H) coordinated by sterically demanding cyclopentadienyl ligands act as two-electron reductants in their reactions with DAD. These reactions occur by abstraction of one Cp* ring and result in the formation of novel Yb(III) mixed-ligand bent-sandwich complexes, [YbCp*(dad)(thf)], in which the dianion of DAD has an uncommon terminal eta(4)-coordination to the ytterbium atom. The variable-temperature magnetic measurements of complex [Yb(C(5)Me(5))(dad)(thf)] suggest the existence of redox tautomerism for this compound.

Unprecedented NC bond insertion into the η5 Yb−C9H7 bond occurs in the reaction of 2-(((2,6-diisopropylphenyl)imino)methyl)pyridine with (C9H7)2Yb(THF)2 and affords the Yb(III) derivative [Yb(η5-C9H7){N(2,6-i-Pr2C6H3)CH(C9H7)(C5H4N)}{2,6-i-Pr2C6H3NCH(C5H4N)•-}]. For the complexes Cp2Yb(THF)2 (Cp = C13H9, Cp*) coordinated by bulkier η5 ligands the same reaction results in an oxidative cleavage of the η5 Yb−Cp (Cp = C13H9, Cp*) bond and formation of [Yb{(2,6-i-Pr2C6H5NCH(C5H4N)•-}3] and [Yb(C5Me5){(2,6-i-Pr2C6H3NCH(C5H4N)•-}2], respectively.

The divalent europium bis fluorenyl complex (C 13 H 9 ) 2 Eu(THF) 2 was synthesized by the metathesis reaction of EuI 2 (THF) 2 with two equivalents of fluorenylpotassium and by the protolytic substitution of the naphthalene ligand in the (C 10 H 8 )Eu(THF) 2 complex using the reaction with fluorene. According to X ray diffraction data, the complex displays a skewed sandwich structure, in which one fluorenyl ligand is η 5 coordinated to the metal atom, whereas the η 3 coordination mode makes a great contribution to the coordination of another ligand. The (C 5 Me 4 H) 2 YbI(THF) complex was synthesized by the reaction of YbI 3 (THF) 2 with two equivalents of (C 5 Me 4 H)K. The structure of the complex was established by X ray diffraction.Since lanthanides are electropositive and met al-ligand interactions in their organic derivatives are predominantly ionic, ligands capable of forming stable organic anions that compensate the positive charge of the cations are conventionally used in these compounds. 1 This is why complexes with monoanionic η 5 coordinated carbocyclic ligands (primarily, with cyclopentadienyl ligands) are the most well studied lanthanide deriva tives. 1,2 The stability and reactivity of organic derivatives of rare earth metals are determined mainly by the high metal-carbon bond ionicity and large ionic radii. These properties, combined with a high positive charge on the metal center, are responsible for very high sensitivity of these compounds to the degree of steric saturation of their coordination sphere. In this connection, ligands used in rare earth chemistry should be rather large and provide the saturation of the coordination sphere, result ing in kinetic stability of metal complexes. Polymethylat ed cyclopentadienyl ligands meet all these requirements, and the use of these ligands in organolanthanide chemis try resulted in extensive progress in this area and allowed the synthesis of a wide range of compounds with unique reactivity.