Hexacoordinate isomers

On the basis of the work of Ramirez (10), Trippett (11) and Dillon (12) the more stable hexacoordinate isomers are of cis-structure and consequently we assigned the structures of 2, jj as trans and those of 3, as cis. The elimination step (c) leading to the phosphorane also involves the less stable isomer. ... 

FIGURE 9.3 Possible Hexacoordinate Isomers for [Co(NH3)4Cl2]+ considered by Werner. Only the octahedral structure allows for only two isomers. 

Hexacoordinate, hypervalent sulfur fluorides have an octahedral geometry that is symmetrical for SF6, which appears as a sharp singlet at +56 ppm, but which has magnetically nonequivalent (axial and equatorial, ab4 system) fluorines for compounds of the structure R-SF5. Compounds with the general structure R-SF4-X can exist as cis- and trans-isomers, the former having three types of fluorine, and the latter only one (Scheme 7.17). 

O-as- S p i ro p h os p h or a n e s 69, which display anti-apicophilicty, exhibit enhanced reactivity toward nucleophiles compared to 0-/ra r-spirophosphoranes 70 for example 69 produces hexacoordinate 157 on treatment with MeLi at 0°C while 70 is unreactive under the same conditions (Scheme 16). Compound 69 (R = Bn) is more reactive toward bases than 70 (R = Bn) with KHMDS deprotonating 69 (R = Bn) in THF at 0 °C but not 70 (R= Bn). -BuLi was sufficiently basic to deprotonate both 69 and 70 <2002JA13154>. Further differences are observed in the stability and stereochemistry of the brominated products formed between the reaction of the anion with BrCF2CF2Br. The effect of the bromide substituent in the O-m-derivative is such that it accelerates pseudorotation to the O-trans-isomer (Scheme 17). 

Typical values at 4.2 K. Isomer shifts are taken relative to the standard value for Fe metal at 298 K. b Fe—(O, N) hexacoordinate or pentacoordinate sites. c Fe-S tetrahedral sulfur ligation. 

PhCCSiF3 are in equilibrium with the pentacoordinated 1 1 complex 843. Only at low temperatures, a second equivalent of pyridine slowly coordinates to the silicon to selectively form one of three conceivable isomers, namely, the hexacoordinated 1 2 complex 844 having the two pyridine molecules in /raor-position (Scheme 118).832... 

In the presence of the bidentate ligands, 2,2-bipyridyl and 1,10-phenanthroline, phenylethynyltrifluorosilane 842 is in equilibrium with the two possible isomers of the hexacoordinated 1 1 complexes 845 and 846, and 847 and 848, respectively. These have been also investigated by X-ray crystallography (Figure 6).833... 

There is no present basis for deciding which or how many of the several isomers of Figure 4c might be involved but we would conclude that heptacoordinated Cr(III) is stable enough to require activation to revert it to hexacoordination. 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

It is notable that the existence of configurational isomers in solution has been established for these hexacoordinate organoantimony compounds, since among hexacoordinate organotin compounds, the structures of which have been extensively studied, there are few reports of such isomers.4... 

In solution, the establishment of cis/trans-D equilibria in hexacoordinate tin complexes is generally fast on the H NMR time scale. A rare observation of different isomers of neutral octahedral diorganotin compounds in solution was described1002. This is somewhat surprising, but may be traced to the usually poor solubility of hexacoordinate organotin compounds, making low-temperature NMR studies difficult if not impossible. 

The 119Sn NMR spectrum of the adduct PhSnCl3 (EtO)2POCH2CH2PO(OEt)2 at —90 °C consists of a triplet at —557 ppm and a doublet of doublets at —551 ppm with approximately 5 1 relative intensities. These resonances were attributed to isomers of the hexacoordinate complex301,1056. The first isomer has two equivalent phosphorus atoms in the tin coordination sphere and consists of cis-fac octahedra in accordance with the preferred cis -bridging behavior of the ethylene-diphosphonate ligand. The second isomer with nonequivalent phosphorus atoms consists of cis-mer octahedra125. 

Density functional B3LYP calculation performed on [H2N(CH2)3]2SnF2 isomers of the model compound showed that the all-trans isomer is energetically preferred. However, the LUMO of the cis isomer is of lower energy than the LUMO of the former, which suggests that the fluoride attack at tin in solution leading to hexacoordinate anionic species (Section IX.B) can take place at the cis isomer971b. 

In contrast to the existence of numerous stable ionic triorganosilylonium complexes, there is no well-defined pentacoordinate molecular 1 1 adduct of acyclic structure of a triorganohalosilane with an uncharged nucleophile. This implies a higher thermodynamic stability of the ionic silylonium complexes than their molecular isomers for the open chain triorganosilyl structure [Eq. (53)]. Stable penta- or hexacoordinate 2 1 nucleophile-silane open chain complexes with three organic groups bound to... 

An initial report that the crystal structure of [Co(NH3)e]-[FeCle] is isomorphous with that of ]Co(NH3)6][TlCl6] has been shown to be incorrect. However, the Mossbauer chemical isomer shifts for [Co(en)3][FeCl6] and [Co(NH3)e][FeCl6] of 0.60 + 0.10 and 0.59 0.10 mm./second, respectively, agree with the values found for other hexacoordinate iron(III) compounds and differ considerably from the value of 0.40 mm./... 

In the known molecular structure of a hexacoordinate silicon compound with the 8-oxoquinolinato ligand published by Klebe et al. [2] (Fig. 2 (left)), the methyl group and the chloro ligand are located in cis position to each other. In the case of compound 3 we expected to obtain also the cis isomer. 

Summary Treatment of Si(NCO)4 or Si(NCS)4 with 4-aminopent-3-en-2-ones yielded novel neutral hexacoordinate silicon(IV) complexes with an S/O2N4 framework, compounds 3-6. These silicon(IV) complexes were characterized in the solid state by single-crystal X-ray diffraction and Si VACP/MAS NMR spectroscopy. Compounds 3-5 crystallized as the (OC-6-12)-isomer, and 6 was isolated as the rranj-isomer. 

---