Does lanthanum involve its empty 4f sub-shell in any form of chemical bonding?
Here are some quotes that give the context for my question.
1. In this extract, the author firmly rules out this possibility:
"Two features are particularly striking in the chemistry of the 4f—elements, Viz. (a) the uniformity of the +3 oxidation state and the small number of other oxidation states, and (b) the irregularity of structures and occurrence of high coordination numbers. In the compounds described here, coordination numbers of six, seven, eight, and nine have been mentioned, and in other compounds ten- and even twelve-coordinate metal ions are found, eg. in La2(SO4)3.9H20 and (NH4)2Ce(NO3)6. It is tempting to think that the f-orbitals must be involved in the bonding, since the maximum number of hybrid orbitals which can be constructed from an s-p-d -set is nine, and to obtain eight bonding orbitals directed towards the corners of a cube (as in C602) requires at least one f-orbital. There is, however, no evidence to suggest that the f-orbitals are involved at all in the bonding, and even ligand-ﬁeld effects are extremely small. All the compounds appear to be essentially ionic, with very little covalency involving even the 6s- or 6p-orbitals. The curious structures and high coordination numbers are similar to those found with other large cations (e.g. salts of Ba2+ or Pb2+) and are presumably a result of the optimisation of electrostatic forces. The large internuclear distances necessitated by the radii of the cations will cause the electrostatic energy per pair of ions to be relatively small, despite the high cationic charge, and many such pairs must be formed to achieve a suﬂiciently large lattice energy. Similarly, the large radii diminish considerably the polarising effect of the +3 charge which would otherwise be expected to lead to considerable covalency." (p. 151)
Parish RV 1977, The metallic elements, Longman, London, p. 151
2. In this passage, the authors note this is a contentious area:
"To involve this combination in bonding one could invoke metal f orbitals. There might be some justification for doing so in the actinide or lanthanide series, but we would not like to enter this contentious area at this time."
Burdett JK, Hoffmann R & Fay RC 1978, "Eight-coordination", Inorg. Chem. 17, 9, 2553-2568
3. In this passage, the authors speculate on the possibility:
"The one case in which contributions to the bonding from the f orbitals is possible is in complexes of the heavier elements in which the coordination number is high. Use of the s orbital, together with all the p and d orbitals or one valency shell, permits a maximum coordination number of nine in a covalent species. Thus, higher coordination numbers imply either bond orders less than unity or else use of the f orbitals In addition, certain shapes (such as a regular cube) or lower coordination number also demand use or f orbitals on symmetry grounds. These higher coordination numbers have only become clearly established recently, but their occurrence in lanthanide or actinide element complexes suggest the possibility of f orbital participation. Examples include the 10-coordinate complexes mentioned above, LaEDTA(H2O)4 and Ce(NO3)52- or 10-coordinate La2(CO3]3.8H2O; 11-coordinate Th(NO3)4.5H2O (coordination by four bidentate nitrate groups and three of the water molecules); and the 12-coordinate lanthanum atoms in La2(SO4)3.9H2O-with twelve sulfate O atoms around one type of La atom position."
- MacKay KM, MacKay RA &Henderson W 2002, Introduction to modern inorganic chemistry, 6th ed., Nelson Thornes, Cheltenham. p. 256
4. Finally, Gschneider (deceased) spends quite a bit of time discussing 4f hybridization among the lanthanides, yet limits himself to physical properties i.e. crystal structure sequence; melting points; and solid-solution behavior; and has nothing to say about chemical properties.
GschneidnerJr. KA 2016, Chapter 282 - Systematics, Handbook on the physics and chemistry of rare earths, vol. 50, pp. 1-18
If 4f subshell involvement in La had been demonstrated I’d have expected this to be quite a news item. Despite looking I haven’t been able to find any conclusive evidence for this in the literature. In contrast, there is plenty of literature on 5f involvement in the light actinides.
This leads me to conclude that Parish's (1977) assessment, which strikes me as being especially articulate, is still right.
Can anybody shed any further light on this question?
thank you, René Vernon