Bipyramids trigonal

Berlin green, FeFe(CN)mechanism postulated for the interchange of substituents in trigonal-bipyramidal 5-co-ordinate complexes, e g. PF, and its substituted derivatives. berthoUide compound Solid phases showing a range of composition. 

Other examples are iron pentacarbonyl, Fe(CO)s, and chromium hexacarbonyl, Cr(CO)(j, which have trigonal bipyramidal and octahedral configurations respectively. 

The structure of sulphur tetrafluoride, and probably also SeF and TeF4, is trigonal bipyramidal with one position occupied by a lone pair of electrons ... 

Complexes of titanium(III) can be made from the trichloride— these are either approximately octahedral, 6-coordinate (for example TiClj.SL (L = ligand) and [TiCljfHjOj, formed when TiCls dissolves in aqueous hydrochloric acid), or 5-coordinate with a trigonal bipyramid structure. 

Iron forms the carbonyls FelCO),. Fe2(CO)g and FcjlCOlij- In iron pentacarbonyl. the iron(O) is 5-coordinated. as shown in Figure 13.5 to give a trigonal bipyramid the substance is volatile... 

Physical Properties. Sulfur tetrafluoride has the stmcture of a distorted trigonal bipyramid, the sulfur having hybrid sp d orbitals and an unshared electron pair (93). The FSF bond angles have been found to be 101° and 187°, and the bond distances 0.1646 and 0.1545 nm (94). 

Fig. 1. Berry pseudorotation about pentacoordinate ( ) phosphorus, where (Q) represent fluorine atoms, (a) Original trigonal bipyramid (b) square...

The pentahalides of phosphorus, PX, in the gas phase exhibit varying tendencies to dissociate into trihaUde and halogen. InstabiUty increases with increasing ionic radius of the halogen. The pentafluoride appears to be thermally stable. Dissociation of the pentachloride, a few percent at 100°C and 101.3 kPa (1 atm), is essentially completed at 300°C (36). The pentabromide is partially dissociated in the Hquid state and totally dissociated above ca 35°C (39). Pentaiodide does not exist. The molecules of PF and PCl in the vapor phase are trigonal bipyramids. In the crystalline state, both pentachloride and pentabromide have ionic stmctures, ie, [PClJ IPClg] and [PBr4]" PBrJ , respectively. The PX" 4 cations are tetrahedral and the PX anion is octahedral (36,37). 

Experimental results suggest that PCl is dimeric in CCl solution. The stmcture consists of two octahedra sharing edges (56). PCl is monomeric in benzene and apparendy is trigonal bipyramidal (36). SoHd PCl is ionic, consisting of [PCl cations and [PClg ] anions (36). 

Antimony Pentachloride. Antimony(V) chloride [7647-18-9], SbQ, is a colorless, hygroscopic, oily Hquid that is frequently yeUow because of the presence of dissolved chlorine it caimot be distilled at atmospheric pressure without decomposition, but the extrapolated normal boiling point is 176°C. In the soHd, Hquid, and gaseous states it consists of trigonal bipyramidal molecules with the apical chlorines being somewhat further away than the... 

Pentaphenylantimony [2170-05-0], C3QH23Sb, has attracted considerable attention because it possesses square-pyramidal rather than the expected trigonal-bipyramidal geometry, both ia the sohd state and ia solution. The cyclohexane solvate (C3H ) Sb 1/2 and penta-4-tolylantimony... 

C H Sb, however, both possess trigonal-bipyramidal geometry. In addition to compounds of the type R Sb, mixed compounds of the type R R Sb or R3R 25b, where R and R may be alkyl, alkenyl, alkynyl, or aryl groups, are known. Thus triethyl dimethyl antimony [67576-92-5], CgH2 Sb, has been prepared (197) ... 

Arsenic pentafluoride (arsenic(V) fluoride), AsF, is a colorless gas that condenses to a yellow Hquid its dielectric constant is 12.8 at 20 °C. It is formed by reaction of a mixture of bromine and antimony pentafluoride with arsenic trifluoride. The molecule is a trigonal bipyramid and is somewhat dissociated as indicated by vapor density measurements. 

Arsenic pentachloride [22441-45-8] (arsenic(V) chloride), AsQ., is produced by irradiation of a solution of AsQ. chlorine at —105°C using ultraviolet light (22). Arsenic pentachloride is a pale yellow soHd (presumably because of an entrained chlorine impurity) which melts with partial decomposition at —50° C. Raman spectra show that it is a trigonal—bipyramidal molecule both in solution and in the soHd state. 

Representative pentacarbonyls are restricted to the iron, mthenium, and osmium group. AH three pentacarbonyls possess trigonal bipyramidal stmctures as shown in stmcture (2). The pentacarbonyls of mthenium and osmium are thermally unstable. Osmium pentacarbonyl [16406-49-8] rapidly polymerizes at room temperature to form polynuclear species. The transient species Cr(CO) (1 )> Mo(CO) (1 )> W(CO) (19) have been investigated. 

The MO picture predicts that the reaction will proceed with inversion of configuration because the development of the transition state is accompanied by rehybridization of the carbon to the trigonal bipyramidal geometry. As the reaction proceeds on to product and sp hybridization is reestablished, the product is formed with inversion of configuration. 

A sterically restricted nucleophile is less reactive than a more accessible one because of nonbonded repulsions which develop in the transition state. The trigonal bipyramidal geometry of the 8 2 transition state is sterically more demanding than the tetrahedral reactant, so steric congestion increases as the transition state is approached. 

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