Trigonal bipyramids five-coordinate compounds

Phosphorus ligand compounds of Mn ", except for alkyl and hydride species (Section 41.4.6), are confined to a report of [MnCl2(diphos)2]Cl (see Section 41.3.4 and Table 17) and now the unexpected, dark-green, phosphine-iodo compound [MnIj(PMe3)2], which has a trigonal bipyramidal five-coordinate structure, with axial phosphines (at Mn—P = 2.43-2.44 A). Undoubtedly the latter compound is but the tip of the iceberg , and we can expect to see a considerable further development of this area of Mn " chemistry although it is pertinent to recall that the phosphine alkyls of Mn " disproportionate. 

Unlike with nitrogen, the five-coordinate compounds Ph5E are known. The P and As compounds have the normal trigonal bipyramidal geometry but Ph5Sb is unexpectedly square pyramidal (2). 

The five-coordination compounds show some more exotic possibilities. PF5 has a trigonal bipyramidal shape with inequivalent axial and equatorial positions. The lone pair in SF4 chooses an equatorial position since it can do less damage there— it make a 90° angle with two F atoms, whereas an axial position would make 90° angles with three F atoms. This leaves SF4 with a shape resembling a distorted. seesaw. The two lone pairs in CIF3 are most stable when located in two equatorial positions, separated by 120". This leaves Cll 3 in a distorted tee shape. The eomplex that forms between 1 and l> in iK iieous solution is a linear ion. 

Tetraorganoantimony(V) alkoxides and triorganoantimony(V) dialkoxides are monomeric, five-coordinate compounds, e.g. PluSbOMe and Ph3Sb(OMe)2 [420]. This is not surprising, because the trigonal bipyramidal geometry found in these compounds is a preferred coordination mode of antimony(V). Despite this, tri-phenyl(4-nitrocatecholato)antimony(V) was found to dimerize via Sb- - -O second-... 

The metal-carbon triple bond chemistry of ruthenium and osmium described in this article bears a close resemblance to the metal-carbon double bond chemistry of these elements as exemplified by the methylene complexes [26]. In both systems two structural classes are found, five coordinate (trigonal bipyramidal, formally zero oxidation state) and six coordinate (octahedral, formally +2 oxidation state). In both systems the five coordinate compounds exhibit multiple metal-carbon bonds which are rather non-polar and typically undergo addition reactions with electrophilic reagents. On the other hand the six coordinate compounds, both M=C and M=C, begin to show electrophilic character at the carbon centres especially in cationic complexes. Further development of the carbyne chemistry of ruthenium and osmium will depend upon the discovery of new synthetic methods allowing the preparation of a broader range of compounds with widely differing carbyne substituents. 

Coordination number 5 also has two predominate geometries, square pyramidal and trigonal bipyramidal, that differ only slightly in energy. In fact, such five-coordinate compounds are often examples offluxional compounds, those that exist in... 

The infrared spectrum of Fe(CO)5 has shown that the compound has a trigonal bipyramid structure [ ]. The assignment of the observed frequencies of the gas was successfully made on the basis of a trigonal bipyramid model (Fig. 5). Recently the stereochemistry of five-coordinate compounds of type LFe(CO)4 or L2Fe(CO)3, where L is PhaP, MeNC, EtNC, PhNC, BuNC, or (PhNC)5Co has been examined. By means of the infrared spectra and comparison with the selection rules, the configurations were... 

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