Coordination spheres multidentate

Multidentate thioethers are readily introduced into the coordination sphere of low-valent cobalt. As an example, the tridentate thioethers 1,3,5-trithiacyclohexane and tris(methylthio)methane both replace three facially arranged carbonyl ligands in Co3(CO)9(/i-CPh), leaving the cluster otherwise intact.169... 

The existent CF models neglect or describe very approximately the effect of mul-tiatomic structure, conjugation and multidentate character of the ligand on the CF parameters. The effect of second and more distant coordination spheres is described as electrostatic, in some models, while it was proved by ab initio calculations that it is mainly covalent by its nature [53, 54]. 

For coordination entities and the ocean, we should consider (a) the metal, whether a transition metal or not, the charge, and the size (b) the ligands, whether uni- or multidentate, perhaps whether organic or inorganic (c) the coordination number—i.e., the number of donor atoms attached to the metal. The metal and the ligands surrounding it comprise the coordination sphere, which is represented by the bracket-enclosed formula. 

Sheer congestion of donor atoms around the metal ion and concomitant inter-donor atom repulsions makes these high coordination numbers difficult to attain. They are often associated with multidentate ligands with a small bite angle such as nitrate that take up little space in the coordination sphere, either alone, as in (Ph4As)2[Eu(N03)5] or in combination with other ligands, as in Ln(bipy)2(N03)3, Ln(terpy)(N03)3(H20) (Ln = Ce-Ho), and crown ether complexes (Section 4.3.7) such as Ln(12-crown )(N03)3(Ln = Nd-Lu). Other crown ether complexes can have 11 and 12 coordination, e.g. Eu(15-crown-5)(N03)3 (Ln = Nd-Lu) and Ln(18-crown-6)(N03)3(Ln = La, Nd). 

Fig. 2. First and second order rate constants for water substitution of the inner coordination sphere of Ni2+ and Cu + for a series of hgands. The values for simple ligands are corrected to yield the first order rate constants of substitution, whereas overall constants are given for multidentate chelates (values from ref. 2)...

Sterically demanding and/or multidentate ligands are often employed to saturate the Ln coordination sphere, imparting control and thwarting common decomposition pathways, including ... 

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