Alkaline-earth bond distances

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

Similar trends are observed for other members of the series, such as lithium silanides and germanides where consistently shorter Li-Ge bonds than Li-Si are observed.213 Similarly, heavy alkaline earth metal silanides and germanides display the same trend - slightly shorter M-Ge bonds than M-Si distances.218-220... 

If we examine the distances listed in Table 7.2 some interesting facts emerge. For a given metal A. the A—P distance is constant as we might expect for an ionic alkaline earth metal-phosphide bond. Furthermore, these distances increase calcium < strontium < barium in increments of about 15 pm os do the ionic radii of Ca2+. St7, and Ba- (Table 4.4). However, the B—P distances vary somewhat more with no periodic trends (Mn. Cu larger Ni, Fe, Co smaller). Most interesting, however, is the huger variability in the P—P distance from about 380 pm (Mn. Fe) to 225 pm (Cu). As it Luros Out, the lower limit of 225 pm (Cu) is a typical value for a P— P bond (Table E.l,... 

In 2001, Schumann and co-workers reported a similar set of metallocene complexes with l,3-di- o-propyl-4,5-dimethylimidazoly-2-ylidene.22 X-ray crystallography and NMR studies confirmed a similar trend between metal-carbene bond strength and alkaline earth metal as that found by Arduengo and co-workers. Additionally they showed that as the steric bulk of the cyclopentadienyl ligand increases the metal-carbene bond distance is elongated. 

A = Alkali metal AE = Alkaline-earth metal a-P = Amorphous phosphorus CN = Mean coordination number (i(M-P) = Distance between M and P atom Distances between P atoms E = Element = Band gap M = Metal PBO = Pauling bond order (P) = Formal charge of a P atom (M) = Formal charge of a M atom R = Zr, Ftf, rare earth metal or actinoid metal RE = Rare earth element / cov = Covalent radius ... 

The formula and bond distances of the metallic carbonyls are given below, the latter being obtained from electron diffraction data. (Compounds of the alkali and alkaline earth metals with formulae of the type K (GO)g have been shown to be phenolates of hexahydroxybenzene.)... 

The cation to ligand distances in Ca2+ and Ba2+ salts depend on the ionic radii and exhibit the same relationship as in the alkaline salts (Fig. 1). Thus, there is no possibility of strong cation-ligand bonds in these alkaline-earth salts. 

In ab initio calculations, there are many different possible choices of theoretical levels and basis sets. Since HF calculations fail to reproduce the bent structure and C(Cp)-E bond distances in alkaline earth and Group 14 metallocenes (30,39,40), it is probably safe to assume that electron correlation must be included in geometry optimizations. Thus, electron correlation is required to reproduce the donor-acceptor nature of the main group-Cp interaction as well as the structural consequence of the main group lone pairs. For instance, previous DFT calculations (which include electron correlation) predicted reasonable C(Cp)-E distances and the bent structures for heavier alkaline-earth metallocenes... 

---