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Date: 21-7-2020
881
Date: 14-5-2020
1108
Date: 12-2-2018
1320
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Complexes of group 2 metals with ligands other than water
In this section we consider complexes formed in aqueous solution in which the metal centre is coordinated by O- and N-donor ligands to give cationic species. Two important ligands are [EDTA]4- and [P3O10]5-. Both form water-soluble complexes with Mg2+ and the heavier metal ions, and are sequestering agents used in water-softening to remove Mg2+and Ca2+ ions. Macrocyclic ligands, including crown ethers and cryptands form stable complexes with Mg2+, Ca2+, Sr2+ and Ba2+. In an analogous manner to that noted for group 1 cations, selectivity corresponding to matching of cation and ligand-cavity sizes is observed. Thus, values of the stability constants for complexation with cryptand-222 (cavity radius 140 pm) in water follow the sequence Ba2+> Sr2+ » Ca2+> Mg2+. An important class of macrocyclic ligands are the porphyrins and the parent compound is shown in Figure 1.1a; deprotonation of the twoNHgroups of a porphyrin gives a dianionic porphyrinato ligand. Chlorophylls, the pigments in green plants involved in photosynthesis, are porphyrinato derivatives containing Mg2+ coordinated within a square planar array of the four N-donor atoms. The structure of chlorophyll a is shown in Figure 1.1b; the extensive conjugation in the ring system means that the molecule absorbs light in the visible region (λ max 660 nm) and this initiates a complicated series of reactions involving other systems containing Mn or Fe. Note that it is the ligand (not Mg2+)that is involved in these redox reactions.
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دور النظارات المطلية في حماية العين
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العلماء يفسرون أخيرا السبب وراء ارتفاع جبل إيفرست القياسي
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اختتام المراسم التأبينية التي أهدي ثوابها إلى أرواح شهداء المق*ا*و*مة
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