Supramolecular Influences on Metal Ion Coordination: Lead(II) under Eight-coordination

Solid‐State Luminescence and π‐Stacking in Crystalline Uranyl Dipicolinates

“…An explanation of the difference in these alkali metal arrays may be that only Rb is involved in coordinative interaction with uranyl-O [Rb-O 3.035(6) Å]. As seems to be commonly the case with heavy and apparently weakly coordinating metals, [6,21,[27][28][29][30][31] it is difficult to define a precise cutoff distance for interactions considered to be bonding but the coordination numbers of both Rb and Cs here appear to be high, involving O-donors only, with rather irregular geometry about the metals. Interestingly, the extreme M···O separations approach those of M···U, which, if they are correctly considered as contacts of closed-shell species of similar charge, would be regarded as involving negligible interactions, thus indicating that the extreme M···O contacts must also be of little significance.…”

“…Parallel to c, the complex units can be considered to form undulating columns [for example, within the ovoid entities apparent in the view down c (Figure 1)] in which there is considerable overlap of parallel dipicolinate rings of adjacent complexes of alternating chirality. This stacking [20,21] extends down b, so that there are two types of ring projections (Figure 3), describable as intra-and inter-ovoid, contributing to the formally infinite array. While projection of one ring onto its parallel neighbour in a given stack does not involve eclipsing of all atoms, there are numerous C···C contacts of Ͻ3.5 Å, associated in the case of the more strongly overlapped intra-ovoid array with an inter-centroid distance of 3.63 Å and a centroid offset of 1.35 Å.…”

Lead(II): misleading or merely hermaphroditic?

“…It is possible, nonetheless, to view the Pb atoms of Pb-succinate-Pb units as linked via Pb 2 O 2 rhombs (Fig. 2b), geometrically a feature of many Pb(II) complex structures [15,17,18,23,24], involving these two nitrate-O atoms. Further, at a limit of < 3.9 Å, it is possible to find two phenanthroline-C atoms (and their associated hydrogen atoms) approaching each Pb, thus giving each metal atom a very asymmetric environment ( Table 2) of ten Table 1 Crystal and refinement data for Bis(1,10-phenanthroline)lead(II) nitrate-succinate Chemical formula Pb 2 (phen) 4 possible donor centres or eight donors plus two acceptors, if a formal double bond of phenanthroline could be regarded as undergoing addition of the Pb via its lone pair (Fig.…”

“…It is, however, common to observe a marked asymmetry in the environment of Pb(II), the structures of the two polymorphs of PbO, litharge [19] and massicot [20], providing striking examples, such that relatively short contacts lie within one hemisphere and relatively long contacts within the other, and such a 'hemidirected' coordination sphere [7] has been taken to be a more subtle indicator of lone-pair activity (as recently discussed in [21]). There are, nonetheless, difficulties arising essentially from the fact that most information is obtained from crystal structure determinations, as in many cases there is evidence that 'intermolecular' attractions in the condensed crystalline state, such as p-p stacking of aromatic ligands [15,18], may play some role in distorting the coordination sphere. Intramolecular agostic interactions (involving ligand hydrogen atoms) provide another example of such a factor, which may also be important only in the solid state [14].…”

“…The common observation that Pb(II) is found in a very asymmetric environment of a large number (often 10 or more) of possible donor atoms [7,8], however, has often meant that this problem has been by no means easily resolved. In a particular case [12], leadoxygen separations ranging from 2.38-3.07 Å, for example, have been considered, on the basis of bondvalence calculations, to be indicative of significant bonding interactions and there are numerous examples [5,[12][13][14][15][16][17][18] of 'long' Pb···X contacts that are nonetheless much shorter than the next most distant contacts and that have thus been taken as indicative of weak attractions. Uncertainty in the definition of the primary coordination sphere donor atoms in turn generates uncertainty in the identification of any coordination sphere 'hole' that might be taken to signify the presence of a stereochemically active lone pair [7,15].…”