Intermediate-Valence Tautomerism in Decamethylytterbocene Complexes of Methyl-Substituted Bipyridines

Reversible Sigma C–C Bond Formation Between Phenanthroline Ligands Activated by (C5Me5)2Yb

“…The 3,8-Me 2 phen adduct is somewhat more soluble in toluene, but all of the neutral adducts are much less soluble than the 2,2'-bipyridine adducts described in earlier papers. 4,5,14,28 The solid-state and solution-state physical properties of the adducts are quite different and these properties are described in the separate sections that follow. The neutral phen adduct may also be sublimed at 190 °C under reduced pressure to yield dark purple crystalline material.…”

“…This feature is indicative of the f 13 configuration. Another peak at 8939 eV, indicating the f 14 configuration as shown in Figure 3 by data on the intermediate valent Cp* 2 Yb(4-Me-bipy) compound, 4 8, is not clearly visible in the phen adduct spectra. These spectra were fit with methods described previously, 5 giving estimates of n f as shown in Table 2.…”

“…4 Whether a monomeric ytterbocene diimine complex has a triplet or singlet ground state is largely governed by kinetic exchange, that is, by mixing of excited state configurations into the ground state. 38,39 The interaction between the half-occupied ligand orbital and a halfoccupied 4f-orbital stabilizes the singlet state while interactions between the half-occupied ligand orbital and the empty metal based orbitals on the Cp* 2 Yb fragment, especially the 5d-orbitals, stabilize the triplet state.…”

“…This similarity is in contrast to what was observed for the various bipyridine adducts of Cp* 2 Yb described in earlier work in which the neutral bipy adducts have substantially lower  eff values relative to their cationic derivatives. 4,5,28 Figure 1. Plot of the effective magnetic moment,  eff per Yb, as a function of temperature for 1-6 in the 2-300 K temperature range.…”

“…In the Cp* 2 Yb(x,x'-bipy) adducts, the changes in the C(2)-C(2') provide qualitative insights into the ground state electronic structure of these adducts. 4,5 In these charge-transfer complexes, the SOMO of the bipyridine radical-anion has b 1 symmetry (in C 2v symmetry) and the C(2)-C(2') distance, represented by A in Scheme 1, shortens relative to the equivalent distance in the free bipyridine ligand, since these C-p-orbitals are a bonding combination. A related analysis of the C-N and C-C distances in the phenanthroline adducts is not as Table 7 are in the range given for Csp 3 -Csp 3 bond lengths of 1.507 Å ( = 0.015 Å).…”