Capping rule

HOs6(CO)18] and H2Os6(CO)18 marks one of the highlights of the early period of osmium carbonyl cluster chemistry [162]. While both the dianionic cluster and the monoanionic system have the expected octahedral metal core, a capped square based pyramidal structure was found for H2Os6(CO)18. This turned out to be the archetypal example for the capping rule , a concept which proved to be very successful in the analysis of the structures of metal carbonyl clusters of the iron and cobalt triads [166],... 

Donnelly, D., Overington, J. P, Blundell, T. L. The prediction and orientation of a-helices from sequence alignments the combined use of environment-dependent substitution tables, Fourier transform methods and helix capping rules Prot. Engng. 1994 7, 645-653. 

A final word about the capping rule concerns a limitation. The [Os8(CO)22]2 cluster has the bicapped Structure shown in (16-XX). However, two other isomeric bicapped Mg structures, (16-XXI) and (16-XXII), are possible and the capping rule per se cannot predict which one to expect, nor can any other rule presently known. The mww-capped structure (16-XXI) is known for [Re8C(CO)24]2 , but no example of (16-XXII) has yet been reported. It may be that when the steric requirements of the CO groups are considered, this cis-capped arrangement is, for that reason, not possible. 

A short piece of Teflon or of rubber tubing (or a rubber cap) fitted tightly over one end of a stirring rod of convenient size gives the so-called policeman it is used for detaching particles of a precipitate adhering to the side of a vessel which cannot be removed by a stream of water from a wash bottle it should not, as a rule, be employed for stirring, nor should it be allowed to remain in a solution. 

U.S. EPA promulgated MACT standards for most HWCs on September 30, 1999. These emission standards created a technology-based national cap for HAP emission from the combustion of hazardous waste in these devices. A number of parties, representing both industrial and environmental communities, requested judicial review of this rule, and challenged its emission standards and several implementation provisions. On July 24,2001, the United States Court of Appeals for the District of Columbia Circuit vacated the emission standards however, it allowed EPA to promulgate interim standards that were in place since February 13, 2002. U.S. EPA issued the new Final Rule and standards on April 20, 2004. Today s standards30 31 shown in Tables 23.5 and 23.6 result from the above judiciary and regulatory actions. 

In order to apply the Jemmis-Schleyer interstitial electron rule, the closo B H 2 dianions (their isoelectronic analogues are treated similarly) are dissected conceptually into two BH caps and one or two constituent (BH) rings. The BH- caps contribute three interstitial electrons each but the rings (which, formally, have zero electrons in the n MOs), contribute none. Hence, six electrons, described as interstitial, link the bonding symmetry-adapted cap and ring orbitals together perfectly. 

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