Covalence coordinate

Sulfur compounds exhibit a rich and multifarious variety which derives not only from the numerou.s possible oxidation states of the element (from —2 to 4-6) but also from the range of bond types utilized (covalent, coordinate,... 

In general, covalent coordination of platinum to phosphate groups has been neglected at the oligonucleotide level, although such interaction is feasible and has been observed with phosphate anions [109]. In N,N-di-methylformamide, ds-DDP seems to be able to coordinate to the phos-phodiester groups of d(TpT) and d(TpG)" [110]. In the case ofd(TpT)-,... 

A variety of anionic initiators, both ionic and covalent, have been used to polymerize lactones [Duda and Penczek, 2001 Jedlinski, 2002 Jerome and Teyssie, 1989 Penczek and Duda, 1993]. Much of the more recent activity involves the use of anionic covalent (coordination) initiators such as alkylmetal alkoxides and metal alkoxides such as R2A OR and Al(OR)3, metal carboxylates such as tin(II) 2-ethylhexanoate, metalloporpyrins (VI), and aluminox-anes such as oligomeric [A1(CH3)0] [Biela et al., 2002 Duda et al., 1990 Endo et al., 1987a,b Gross et al., 1988 Kricheldorf et al., 1990 Penczek et al., 2000a,b Sugimotoa and Inoue, 1999]. 

A Lewis acid (electrophile) shares an electron pair furnished by a Lewis base (nucleophile) to form a covalent (coordinate) bond. The Lewis concept is especially useful in explaining the acidity of an aprotic acid (no available proton), such as BFj. 

In these three ligands the and atoms are suitable for covalent coordination with the metal center (see Scheme 3.5 for labeling). Although also possesses an electron pair, this will not be the preferred position for metal coordination because normally it is involved in the formation of donor-acceptor hydrogen bonds together with one or two hydrogen atoms of the amino groups. Very few examples of the coordination of an N atom with two vicinal NH2 functions have been reported... 

Pnictide-transition metal bonds are essentially covalent coordinate, in which the pnictide provides the electrons. However, this simple picture does not account for all the structural data now available. The consensus view is that three factors are involved in the ligand contribution to the M—E bond (a) a bonding (Section 14.4.2), (b) n bonding (Section 14.4.3), and (c) steric factors (Section 14.3). The effect of the metal will not be discussed here, except insofar as individual complexes are used as examples. 

Figure 10.64 The auxiliary linkage approach to the synthesis of [2] and [3] catenanes. The auxiliary linkage may be a covalent, coordinate or noncovalent bond.

The three-dimensional network structure of diamond can be considered as constructed from the linkage of nodes (C atoms) with rods (C-C bonds) in a tetrahedral pattern. From the viewpoint of crystal engineering, in a diamondoid network the node can be any group with tetrahedral connectivity, and the linking rods (or linker) can be all kinds of bonding interactions (ionic, covalent, coordination, hydrogen bond, and weak interactions) or molecular fragment. 

Metal cations in solution are surrounded by ligands (i.e., solvent molecules, anions, or non-solvent neutral molecules). The bonding that develops is normally a result of the sharing of one or more pairs of electrons of the ligand with the metal ion, which makes a covalent coordinate bond thus, the resulting species is called coordination compound or complex. 

When each ion maintains its own primary solvation shell, the new chemical species is a solvent separated ion-pair (SSIP). If a single solvent layer is shared by ion partners, the species is a solvent shared ion-pair (SIP). If the cation and the anion are in contact and no solvent molecules are present between them, the form is contact ion-pair (CIP) or intimate ion-pair. Figure 2.1 illustrates multistep ion-pair formation equilibrium. What sets ion-pairing apart from complex formation is the absence of directional covalent coordinative bonds resulting from a Lewis base-acid interaction and a special geometrical arrangement. 

Two major areas of complexation have developed over the years with regard to synthehc macrocycles. Those with nitrogen, sulfur, phosphorus, and arsenic tend predominantly to form traditional covalent coordination complexes with transition metal ions. A notable exception to this tendency, however, is the rapidly expanding chemistry of the... 

Figure 5 shows the structures of P and P°. Both states of the P cluster contain eight iron and seven sulfur atoms, which are covalently coordinated to the MoFe protein by six cysteinyl ligands, three from the a-subunit, that is, Cys a 62, Cys 0 88, and Cys a 154, and three from the /3-subunit, that is, Cys /370, Cys /395, and Cys /3153. In both cases, Cys a62, Cys O 154, Cys /370, and Cys /3153 coordinate one iron atom each. 

Evaporating Al(OH)Pc and Ga(OH)Pc with aqueous HF led after heating to fluoroaluminium and fluorogallium phthalocyaniM polym s [Al(F)Pc] , [Ga(F)Pc] (7P)125-j28) jjjg polymers were purified by sublimation. The bond between the Pc in the staple is described as combined covalent coordinative linkage. 

The effort to obtain a clear understanding of the nature of valence and of chemical combination in general has led in recent years to the dissociation of the concept of valence into several new concepts—/onic valence, oxidation number, covalence, coordination ni/mber—corresponding to different modes of interaction of the atoms. These new concepts are discussed in later sections. 

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