D-Transition series

Attention will be restricted here to fluorine compounds of the three d-transition series elements, in which the metal ion is octahedrally coordinated. Octahedral coordination, however, is found in nearly all these cases, which is quite reasonable considering the sizes of the ions in question. A close-packed octahedron of fluoride ions of radius 1.33 A adapts a size of its octahedral interstice appropriate to a sphere of radius 0.55 A. Cations having this size and larger ones meet the conditions of a contact between cations and anions. Thus stability is predicted for octahedral coordination until such contacts of ions become possible for coordination numbers higher than 6. For a coordination of 8 fluoride ions this is only the case if the radii of the cations are as large as 0.86 A (square antiprism) or 0.97 A (cube) resp. 

Fig. 1. Oxidation numbers for - d-transition series , - f-transition series (lanthanides and actinides) (non-common or uncertain oxidation numbers have been put between brackets)...

Of course, the two hypotheses are not really independent and only the simphcity of the language employed allows us to state them in such a clear-cut form. Nevertheless, they embody a problem for chemists which is analogous to the one encountered in the chemistry of d-transition series, and which has kept researchers busy for a very long time. 

Table la. Potential parameters for the d-transition series taken from Andersen and Jepsen (Ref. 9). Cl is the band centre and pi is the band mass. S is the Wigner-Seitz radius in atomic units... 

In the d-transition series the conduction bands at the Fermi level are predominantly narrow d-bands superimposed upon a broad s-p band structure. In the actinides narrow f-bands at the Fermi level are superimposed upon an spd band structure, the d-bands now being broad conduction band states. The energy bands of the rare earths and heavy actinides are so narrow that it is energetically favourable for Mott-localized states to form. The reason is that polar states are present in bands and this costs... 

Several ABO3 oxides, where A is a large cation like Ba and B is a small cation of the d-transition series, are known to exhibit polytypism. These oxides consist of close-packed, ordered, AO3 layers which are stacked one over the other and the B cations... 

For lanthanide systems, as for the d-transition series, a purely electrostatic calculation is not expected to yield realistic values for the Bq parameters. Thus for PrCl3 Hutchings and Ray (37) found that only Bq of the calculated quantities was in reasonable agreement with the experimentally determined results, the calculated Bq and Bq values being respectively much larger and much smaller than the experimental results. Moreover, for [Ce(Cot)2]-, using the crystallographic results (23) a = 2.742 A and 9 = 40.85°, and the Ce3+ values of Watson and Freeman (38), and for U(Cot)2 with a = 2.647 A and 9 = 43.40° (22) and the of Lenander (39), similar conclusions follow (see (26)). 

The dodecahedron requires a larger radius ratio than the antiprism (i.e., 0.665 versus 0.645), and it is tempting to say that it is favored over the antiprism as a coordination type for the larger (with respect to the d-transition series) actinide elements. The lack of a sufficiently large number of well-determined structures makes such a suggestion tenuous at this time. However, there is some support for this thesis among the one dimensional polymers, as explained in the following section. 

From a commercial point of view, the insertion compound should be inexpensive, environmentally benign, and lightweight. This implies that the ion should be preferably from the 3>d transition series. 

Copper differs in its chemistry from the earlier members of the first transition series. The outer electronic configuration contains a completely-filled set of d-orbitals and. as expected, copper forms compounds where it has the oxidation state -)-l. losing the outer (4s) electron and retaining all the 3d electrons. However, like the transition metals preceding it, it also shows the oxidation state +2 oxidation states other than -l-l and - -2 are unimportant. 

Table 24.3 lists representative examples of the compounds of these elements in their various oxidation states. The wide range of the oxidation states is particularly noteworthy. It arises from the fact that, in moving across the transition series, the number of d electrons has increased and, in this mid-region, the d orbitals have not yet sunk energetically into the inert electron core. The number of d electrons available for bonding is consequently maximized, and not... 

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

The electronic spectra of the hexafluoro-complexes of the first row transition series. G. C. Allen and K. D. Warren, Struct. Bonding (Berlin), 1971, 9, 50-138 (137). 

J) -CHj, 318 (74) NH3, 345 (II) HjO, 380 ( 75). This shows that both organo-ligands exert a very strong ligand field comparable with that of CN and establishes their position in the spectrochemical series. It is a pity that the second d-d transitions ( T2g-(- A,g), which would establish the position of these ligands in the nephelauxetic series, have not yet been reported. Two bands have, however, been reported for the binuclear complex [(NC)5Co(CFjCFj)Co(CN)5] - at 320 and 276 nm (I2I). 

Hence, a high-lying d band ( 2jt- d is small) is favorable for a stronger interaction and consequently a lower barrier for chemisorption. This explains why CO carmot be dissociated on Cu and why the reactivity increases on going to the left in the transition series. However, there is more to it. 

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