Small molecule complexes electron pairs

A variety of small molecules can be inserted between the two metals in these complexes. The reaction with CH2N2 in CHjClj to insert — CH2- is second order and kinetic parameters for different X s suggest a rate-limiting transfer of an electron pair from the Pt—Pt bond to the methylene group of CH2N2 ... 

A pair of electron densities of two molecules separated by a large distance provided the most relevant chemical system for which the overlap disappears at infinite intermolecular separation. For small overlaps, occurring at intermediate separation rrl[PA Pb] / 0 but its numerical value can be expected to be small. For this reason, pairs of electron densities corresponding to molecules involved in a weakly bound intermolecular complex provide an ideal testing ground for approximations to T d[pA,pB. ... 

Several properties of transition metals make them useful in catalysis. Metal ions provide a high concentration of positive charge that is especially useful in binding small molecules. Because transition metals act as Lewis acids (electron pair acceptors), they are effective electrophiles. (Amino acid side chains are poor electrophiles because they cannot accept unshared pairs of electrons.) Because their directed valences allow them to interact with two or more ligands, metal ions help orient the substrate within the active site. As a consequence, the substrate-metal ion complex polarizes the substrate and promotes catalysis. For... 

A plausible explanation of the detrimental effects of ammonia comes from the work of Hichalske and Bunker on stress cracking in silicates [19] They observed that small molecules such as ammonia and water, in which a lone electron pair is within about 1 6 A of a labile hydrogen, are uniquely effective in promoting stress cracking in silicates The postulated mechanism comprises weakening of si-0 bonds by formation of complexes in which there are sorbate hydrogen bonds to silicate oxygens coupled with interactions between the sorbate electron pairs and silicons. Complementary molecular orbital calculations... 

H2 and alkanes were among the last remaining small molecules not to have been found to form stable metal complexes. Complexes of CO, NO, SO, 02, CS2, N2, N20, C02, and H2S had all been synthesized at least a decade or so before H2 complexes. Unlike H2, all of these contain nonbonding or n electrons for the metal to clasp, so the nature of the chemical bonding was not novel. The beauty of H2 coordination was the absolute proof it provided that the interaction of a bonding electron pair with a transition M is strong enough (with help from M - H2[Pg.59]

Although the differences between the Bronsted and Lewis bases are minimal compared with that between the acids, Bronsted bases constitute, in fact, a small group of Lewis bases. Besides anions, Lewis bases consist of molecules with lone-pair electrons as well as Tr-electron systems, n Complexes play significant roles in organic and organometallic chemistry. Many of these can be detected spectroscopically or can be isolated. 

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