Alkaline-earth coordination sphere

Unlike the other alkaline earth and transition metal ions, essentially on account of its small ionic radius and consequent high electron density, Mg2+ tends to bind the smaller water molecules rather than bulkier ligands in the inner coordination sphere. Many Mg2+-binding sites in proteins have only 3, 4 or even less direct binding contacts to the protein, leaving several sites in the inner coordination sphere occupied by water, or in the phosphoryl transferases, by nucleoside di- or triphosphates. 

The other pathway leading to the formation ofoxogroups in the coordination sphere of the metal atom is provided by uncontrolled oxidation of the basic alkoxides such as alkali, alkaline earth metal, and quite probably the rare earth metal ones by oxygen dissolved in solvents and present in the atmosphere. The primary oxidation products are peroxides and hydroperoxides — M(OOR)n and M(OOH)n, whose decomposition gives water among the other... 

The carbazole compounds, (77 -Af-carbazolyl)2Ca(pyridine)4, (Tj -N-car-bazolyl)2Sr(NH3)(DME)2, and (i7 -Af-carbazolyl)2Ba(DME)3 (Fig. 9), are colorless or yellow solids that have good solubility in donor solvents such as pyridine. They are monomeric in the solid state with the carbazolyl ligands bonded only through the nitrogen atom (tj ) to the metals. The metal coordination spheres are completed by incorporation of the Lewis bases pyridine, DME, and ammonia. A smooth increase in coordination number with metal size, from 6 (Ca) to 7 (Sr) to 8 (Ba), is observed in these compounds. The reason for the Tj -Af-bondingof the carbazolyl ligand instead of multihapto bonding as occurs in the alkali metal compounds of carbazole and the fluorenyl compounds of the alkaline-earth metals (see Section III,G,2) is presently unclear. ... 

Fig. 1 includes three categories of metal ions. In the first group, the water molecules of the inner coordination sphere are so labile that substitution takes place at almost every encounter. The overall rate for the complex formation is thus diffusion controlled. It is not possible to separate one single substitution step from the overall process. The rate constants in this group (to which most of the alkali and some of the alkaline earth ions belong) are therefore only in a very trivial sense characteristic of the nature of the metal ion. 

Fig. 6. Mean OH stretching frequencies vOH (average of the various unit-cell group modes) of hydroxide ions which do not donate OH- X hydrogen bonds of alkali and alkaline earth metal hydroxides ( ), and on surfaces of solid oxides as A1203 etc. (O) vs mean M-O distances rM Q of the first OMx coordination sphere [22,86] (3556 cm"l, free ion value [1])...

The alkali and alkaline earth cations are hydrated in solution and their rates of water exchange are extremely fast, almost approaching the diffusion controlled limit. The rate determining step in the exchange process is the dissociation of a solvent molecule to leave a vacancy in the first coordination sphere, Fig. 1.14. The smaller ions with the most intense electrostatic field (proportional to charge/ionic radius) and the higher heats... 

For d-transition-metal ions, the number of water molecules in the primary coordination sphere (A-zone) is in most cases determined by the strength of orbital overlap between the metal ion and H2O molecules, crystal field stabilization effects, and cationic charge. Other species (e.g., alkaline earths, rare earths) interact with solvent molecules via ion-dipole forces with minimal orbital overlap conhibution to the bonding. Their solvation numbers are determined by a combination of coulombic attraction between cations and water molecules, steric fiictors, and van der Waals repulsion between the bound water molecules. The larger size and high charge of the lanthanides combine with the absence of directed valence effects to produce primary-sphere hydration numbers above eight for these metal ions. 

The 18-crown-6-ether (Scheme 19e) is six-coordinated only in highly ionic complexes of alkaline and alkaline-earth metals, in which it furnishes the metal ion with a hexagonal planar coordination sphere. Only the small lithium ion can fit in an octahedral conformation of the crown ether, even if significantly distorted. In a similar vein, complexes of this ligand with transition metals are found only for Cd(II) and Hg(II) ions in eight-coordinate complexes in which the crown ether occupies six equatorial sites of a hexagonal bipyranfid. 

---