Divalent oxygen ligands

This Group IIA (or Group 2) element (atomic symbol, Ca atomic number, 20 atomic weight, 40.078 electronic configuration = ls 2s 2p 3s 3p 4s ) loses both As electrons to form a divalent cation of 0.99A ionic radius. Ionic calcium combines readily with oxygen ligands (chiefly water, phosphates, polyphosphates, and carbox-ylates) to form stable metal ion complexes. Ca under-... 

Impressive coordination ability of divalent phenolic ligands (Fig. 22, Table 11) was demonstrated in homoleptic catecholato derivatives [152]. Surprisingly, Ce(IV) was not reduced by the catechol dianion. In addition, the low solubility of the complex Na4[Ce(C6H402)4] 21H20 even permits its direct synthesis and crystallization from aqueous solutions (Eq. 14a, Table 11) [153]. The crystal structure consists of discrete 8-coordinate [Ce(cat)4)]4 dodecahedra. Each sodium is coordinated to two nonequivalent oxygens from two catecholato... 

Fig. 7. Schematic drawing showing oxalate, ATP, and the two divalent cations bound at the active site of pyruvate kinase. M is the divalent cation at site 1 and M2 is the divalent cation at site II. Four oxygen ligands for M have been identified by EPR experiments with O labeling. Ligands X and X are contributed from the protein. The a,/3,y-tridentate coordination scheme for M2 was determined for Mn(II) at this site in one of the hybrid complexes formed in the presence of Cd(ll). Ligands Z, Z, and Z" are likely water molecules given the facile binding of the aquo-Cr "ATP species at this site (Si).

Also in the divalent state, Pd and Pt show the class-b characteristic of preferring CN and ligands with nitrogen or heavy donor atoms rather than oxygen or fluorine. Platinum(IV) by contrast is more nearly class-a in character and is frequently reduced to Pt by P- and Aj-donor ligands. The organometallic chemistry of these metals is rich and varied and that involving unsaturated hydrocarbons is the most familiar of its type. 

Thus it has so far proved possible to isolate stable derivatives of monomeric metaphosphoric acid and of metathio- and metaselenophosphoric acid, which, understandably, generally bear tert-butyl and/or trimethylsilyl substituents u. Specifically, we know aminobisiminophosphoranes (3, Z = NR2, X = Y = NR)2,3,4), aminoiminothio (or seleno)phosphoranes (5, Z = NR2, X = NR, Y = S or Se)5), and aminoiminomethylenephosphoranes (1, R = NR2, X = NR)6>. Conspicuously, no stable phosphorus(V) three-coordinate compounds have been synthesized with oxygen as divalent ligand. 

Figure 3 Schematic illustration of a hybrid hydrogel system—genetically engineered coiled-coil protein domains used to crosslink synthetic water-soluble polymers. Divalent transition metal ions are shown to form complexes with nitrogen-oxygen-donor ligands on the synthetic polymer side chains and the terminal histidine residues in the coiled coils.

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