Five- and Six-coordinate Compounds

The participation of phosphoms d orbitals in the five- and six-coordinate compounds provides increased polarizabiUty, nucleophilicity, and ionic character. In fact, compounds such as phosphoms pentachloride [10026-13-8] 5) thought to have considerable ionic character. The CJ-bond orders between the phosphoms atom and its constituents in the higher coordinate compounds maybe less than one. 

The range of cyclic systems for four-, five-, and six-coordinate compounds synthesized up to 1995 is summarized in <1996CHEC-II(8)1135>. This chapter aims to review advances in synthesis, characterization, reactivity studies, and applications of compounds containing a spiro phosphorus atom since 1995. 

Five- and six-coordinate compounds. Evidence for an increase in coordination geometry to pseudo-trigonal bipyramid and TBP in respective cyclic phosphites and phosphates containing sulfur via sulfur donor action) has been illustrated using P NMR spectroscopy and X-ray diffraction studies, e.g, 19, whereas in a cyclic phosphite (6) with a methylene group in place of the sulfur atom this was not observed." The sulfonyl-substituted oxyphosphoranes 20-24 have been examined structurally by NMR spectroscopy and X-ray diffraction. P and H NMR spectral data indicated the presence of two isomeric forms for each of the phosphoranes 20-22. 

The range of cyclic systems is shown in the tables below (Tables 1-16), m and n representing the sizes of the spiro rings. The four, five, and six coordinate compounds will normally be reviewed in... 

The chemistry of phosphorus generally resembles that of arsenic much more closely than that of nitrogen. Whereas nitrogen is only trivalent and forms covalent linkages to not more than four other atoms, arsenic, antimony and bismuth have d orbitals and, like phosphorus, can exhibit tri or penta valency and form five and six-coordinated compounds. 

Table 8.1 Mossbauer parameters of five- and six-coordinate ferric compounds with 5pe = 3/2...

The most substantial study of this sort, analysing structures of 186 four-, five- and six-coordinated tin compounds, was published by Britton and Dunitz (1981) under the memorable title Pathways for SN2 and SN3 Substitution at Sn(IV) . Organotin halides readily increase their coordination number to 5 or 6 (Davies and Smith, 1982), and a wide range of structures are stable in the solid state. 

When adequate steric hindrance on the donor atoms is provided in the tridentate ligands, high-spin five-coordinate NiX2L complexes are obtained. Significant examples are the complex obtained with the two strictly correctable ligands l,5-bis(dimethylamino)-3-thiapentane and l,5-bis(dimethylamino)-3-oxapentane.2517 These compounds give rise in solution to equilibria between five- and six-coordinate species achieved via molecular association or solvolysis. 

Figure 3 Schematic representation of observed five- and six-coordinate Cu(bipy)2(anion)2 compounds...

Chemical interactions with solvents Various workers have observed (1, 4, 7) that five- and six-coordinate organotin compounds show U9Sn signals which occur at much lower frequencies than those of the four-coordinate derivatives. For this reason, the choice of solvent in preparing samples for NMR measurements may affect the value of the observed u9Sn chemical shift. 

In the early 19th century, Gay-Lussac discovered F Si 2 NHj. Three years later Davy obtained the same compound and Davy and Berzelius synthesized other compounds of six-coordinate silicon fluorosilicic acid, HjPiFg], and its salts, the hexafluorosilicates In the second half of the last century adducts of silicon tetrahalides with tertiary amines were synthesized . In 1903 Dilthey investigated silicon diketonates containing six-coordinate silicon In the first half of our century the possibility of the presence of Si(OH)g ions in silica was widely discussed Interest in five- and six-coordinate silicon compounds increased in the second half of the present century. As a result, new classes of silicon compounds with an expanded coordination sphere were discovered and were studied by various physical and chemical methods. Specific biological activity was observed for some of the compounds and promising ways of their practical application were planned... 

A second class of chemical compounds whose reactions are catalyzed by the fluoride ion, and by CsF are organosilanes (42). The diversity of chemistry exhibited by these compounds is considerable, and with the potential for expanded valence in these compounds to five and six coordinate intermediate anions, the application of the salt/molecule technique was suggested. The gas phase ion/molecule reactions of several silane systems have been investigated through ion cyclotron resonance techniques (53,54), providing thermochemical information about the product ions in these systems. Perhaps the simplest reaction is that of F with SiFi, the intermediate anion SiFs has been stabilized under carefully controlled circumstances (55), while the 2 1 adduct SiFe is well known ( ). Less is known about the effects of alkylation on the stability of the product ions. 

Four-, five-, and six-coordinate Au compounds [AuL2] +, [AuC1L2] +, and [AuCl2L2]+ have been isolated for the ligand L = diphos. The distorted octahedral complex [Aul2(diars)2]+ is isolated from redox reactions between I2 and [Au(diars)2]+. 

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