Carbon atom trigonal pyramidal

Some other aspects of these structures may also be discussed. In cementite the carbon atoms, with ligancy 6, coordinate six iron-atoms about themselves at the corners of a trigonal pyramid. The arrangement of iron atoms is such that octahedral coordination about carbon would be an alternative possibility, and we may ask why the carbon atom with ligancy 6 prefers the trigonal prism as its coordination polyhedron, and also why the tin atom in AuSn assumes this coordination... 

The same kind of sp3 hybridization that describes the bonds to carbon in the tetrahedral methane molecule also describes bonds to nitrogen in the trigonal pyramidal ammonia molecule, to oxygen in the bent water molecule, and to all other atoms that VSEPR theory predicts to have a tetrahedral arrangement of four charge clouds. 

Five-Center Four-Electron Bonding Structures. The potential for the existence of 5-center 4-electron (5c-Ae) bonding structures 558 have recently been surveyed by Tantillo and Hoffmann980 [calculations at the B3LYP/6-31G(d) level], A cation with three anthracenyl units joined around the C -H-C -H--C core with two approximately trigonal pyramidal carbon atoms and one five-coordinate trigonal bipyramidal carbon was found to have 5c-Ae bonding. The anticipated existence of... 

Fig. 8.8. Structures of some carbonyl complexes with M-M bonds, showing (a) a simple unbridged single bond, (b) a tetrahedral M4 cluster with both terminal and bridging CO ligands, (c) a square planar M4 cluster with two GeCH3 groups completing an octahedron, (d) a trigonal-bipyramidal M5 cluster with no bridging CO ligands and (e) a square pyramidal Ms cluster with a carbon atom at the centre of the basal plane.

There are four electron pairs around carbon in CH3 they are arranged in a tetrahedral fashion. The atoms of this species are in a trigonal pyramidal arrangement. 

Fig. SI. The unusual hydride/carbide cluster HOssC(CO) 1310P(0Me)0P(0Me)2 ], formed by the pyrolysis of Os3(CO)iiiP(OMe)3] (Ref. 226 a). The carbon atom is situated in a site which appears from this viewpoint to be square pyramidal, but is actually mid-way between square pyramidal and trigonal bi-pyramidal...

Most of the diorgano alkylidene tellurium compounds thus far reported derive their alkylidene group from 5,5-dimethyl-l,3-dioxocyclohexane. These diorgano cyclohexyl-idene tellurium compounds are colorless, crystalline substances that are stable at 20° and readily soluble in benzene, carbon tetrachloride, and dichloromethane Single crystal X-ray dilfraction showed that the tellurium atom in bis[4-methoxyphenyl] 5,5-dimethyl-l,3-dioxo-2,2-cyclohexylidene tellurium occupies the apex of a trigonal pyramid. ... 

If the square pyramidal metal carbonyl carbides Fe5(CO)i5C ° and Os5(CO)i5C are treated in a similar manner to I xyi ( ()) i T that is, as clusters in which all four of the core carbon atom s valence shell electrons are used for skeletal bonding, then they are seen to have the expected nido shapes of systems with five skeletal atoms (the metal atoms) held together by seven skeletal bond pairs. By contrast, if these carbide carbon atoms had occupied polyhedral vertex sites, with a lone pair of electrons in an exo-oriented sp hybrid orbital, then the number of skeletal bond pairs would have been reduced by one and the number of skeletal atoms would have increased by one. The five metal atoms and the carbide carbon atom would have had to be accommodated in some way on a trigonal bipyramidal skeleton. Clearly, the assumption that all four valence shell electrons from the carbide carbon atom are involved in the skeletal bonding is vindicated. 

The high coordination number of the carbon atom in these compounds has aroused some interest, but six-coordinate carbon is well known in metal carbides such as FcsC, in which the carbon atoms lie in the interstices of almost regular trigonal pyramidal prisms of iron atoms (382). 

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