Lanthanide complexes covalent bonding

Lanthanides form soluble complexes with many inorganic and organic substances however, the nature of the bonding in these complexes has not been completely determined. There is evidence for either ionic or covalent bond formation or a combination of both. Lanthanides are complexed by inorganic ions, but not as readily as are the transition elements. The inorganic complexes are not as important... 

It is also often assumed that A is constant along a series of lanthanide complexes with the same ligand. The mechanism providing spin density delocalization from the metal to the ligands is due to a weak covalent bonding involving the 6s metal orbital, which, in turn, can transfer unpaired spin density on nearby nuclei through spin polarization from 4f orbitals [82]. 

See for further literature dealing with the ionic /covalent bonding behavior in lanthanide complexes a) Jorgensen CK, Pappalardo R, Schmidtke H-H (1963) J Chem Phys 39 1422 b) Katzin LI, Barnett ML (1964) J Phys Chem 68 3779 c) Burns CJ, Bursten BE (1989) Comments Inorg Chem 9 61... 

Also electrons of other inner orbitals than d may of course give rise to covalent bonding, provided they are on suitable energy levels. It is thus very likely that the abnormal stability of certain actinide complexes is due to partially covalent bonds involving 5/electrons (5). On the other hand there are no signs that the 4/electrons of the lanthanides are able to participate in such bonds evidently these are situated too deep within the atom. 

Figure 2.19 ORTEP plot for Nd(L )3phen with ellipsoids drawn at the 30% probability level. Hydrogen atoms omitted for clarity [35]. (Reproduced from J. Feng, J.B. Yu, S.Y. Song, L.N. Sun, W.Q. Ean, XM. Guo, S. Dang and H.J. Zhang, Near-infrared luminescent xerogel materials covalently bonded with ternary lanthanide [Er(III), Nd(III), Yb(III), Sm(III)] complexes, Dalton Transactions, 13, 2406-2414, 2009, by permission of the Royal Society of Chemistry.)...

For the lanthanide complexes, even if the amount of covalent interaction is very small, we might have a chance to get the evidence of f-orbital participation in bonding (Figure 4). 

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