Pseudo-octahedral

Figure 13.10 Schcmalic representation of the pseudo-octahedral structures of (SbCls(OPCl3)j and (SbF5(OSO)j,...

The structural relationships in Bi203 are more complex. At room temperature the stable fonn is monoclinic o -Bi203 which has a polymeric layer structure featuring distorted, 5-coordinate Bi in pseudo-octahedral iBiOs units. Above 717°C this transforms to the cubic -form which has a defect fluorite structure (Cap2, p. 118) with randomly distributed oxygen vacancies, i.e. [Bi203D]. The )3-form and several oxygen-rich forms (in which some of the vacant sites are filled... 

Se3Bri3- < > SeCls , TeClj-, TeCle ", etc.< > The anion structures are much as expected with the Se species featuring square planar (pseudo-octahedral) units, and the trinuclear Se " anions as in the tellurium analogue above. See also p. 776. There are, in addition, a fascinating series of bromoselenate(II) dianions based on fused planar SeBr4 units, e.g. Se3Brg ", Se4Bri4 ,... 

The rhodium(III) triaryls have pseudo-octahedral structures therefore, in the air-stable trimesityl rhodium, the three mesityl groups are arranged in /ac-positions, with ort/io-methyls blocking the other coordination sites (Figure 2.107). 

Isothermal a—time curves for the decomposition at 363—464 K of the pseudo octahedral compounds NiL2(NCS)2 (L = py, 3-picoline or quinoline) to NiL(NCS)2 and volatilized L, obeyed the contracting volume equation [eqn. (7), n = 3]. E values decreased in the sequence L = py > 3-picoline > quinoline and this order was ascribed to the effect of increasing ligand volumes [1128]. 

Figures 2,3,5, and 6 show anionic compounds 5 to 22 and 25 to 30, which have been described in the literature since 1997. In these adducts, as in examples 1 to 4, the P(VI) derivatives have carbon or oxygen atoms in the immediate proximity of the central (pseudo-)octahedral atom. This is probably due to the accessibility of the ligand precursors, the ease of their manipulation and, more importantly, to the sheer strength of the resulting P-C and P-0 bonds. They all present tris(bidendate) structures in which the three chelating rings can be identical (Fig. 2 and most of Fig. 6) or of two different types (Fig. 3). The ligands can be monooxo (Fig. 6) or dioxo (Fig. 2 and Fig. 3). These differences in composition have, of course, consequences for the making of the derivatives.

Most of the anionic compounds that have been reported contain six oxygen atoms at the periphery of the pseudo-octahedral phosphorus. Two different strategies have been used for the preparation of the moieties depending upon the homogeneous (three times the same chelate) or heterogeneous (two different types) distribution of the bidentate ligands. 

Many types of phosphorus-phosphorus bonds are known, but it is rare to find such bonds in hexacoordinated phosphorus compounds (with the exception of 57). Cavell reported in 1998 the reaction of PCI5 with phenylbis(o-(trimethyl-siloxy)phenyl)phosphane, yielding the corresponding bischelate 61 in decent yield (52%) [99]. The octahedral nature of the central phosphorus atom was unambiguously determined by X-ray structural analysis. Two short axial bonds (2.202 A) lie perpendicular to the pseudo-octahedral equatorial plane. 

Chirality is an important part of today s chemistry and, in this respect, the pseudo-octahedral geometry of hexacoordinated phosphorus derivatives is interesting as it suffices to coordinate to the central atom three identical sym-... 

As just mentioned, phosphorus porphyrins have unique photochemical properties. Their photophysics is also interesting. Emitter-quencher assemblies based on porphyrin building blocks have attracted attention due to their potential to serve as models in photosynthetic research (see [90] for an example) or for the development of photoswitches that could be used for the fabrication of molecular electronic/optical devices. In this context, Maiya and coworkers constructed a P(VI) porphyrin system 59b with two switchable azobenzene groups positioned in the apical positions of the pseudo-octahedral phosphorus atom [92]. Photoswitch ability (luminescence on/off) was demonstrated as... 

The role of coordinated ethylene is evidenced by the recent ab initio calculation performed by Espelid and Borve [121-123], who have shown that ethylene may coordinate in two different ways to the reduced Cr(II) species, either as a molecular complex or covalently bound to chromium. At longer Cr-C distances (2.36-2.38 A) an ethylene-chromium zr-complex forms, in which the four d electrons of chromium remain high-spin coupled and the coordination interaction is characterized by donation from ethylene to chromium. Cr(II) species in a pseudo-tetrahedral geometry may adsorb up to two equivalents of ethylene. In the case of a pseudo-octahedral Cr(II) site a third ethylene molecule can also be present. The monoethylene complex on the pseudo-tetrahedral Cr(II) site was also found to undergo a transformation to covalently bound complex, characterized by shorter Cr-C distances (about... 

The P-N chelate (91) (dapdmp) exhibits a variety of coordination geometries in complexes with divalent Co. Pseudotetrahedral Co(dapdmp)X2 (X = C1, Br, I, NCS), low-spin five-coordinate [Co(dapdmp)2X]+ (X = C1, Br, I), planar [Co(dapdmp)2]2+ and pseudo-octahedral Co(dapdmp) (N03)2 were all identified.390 The tetradentate P2N2 Schiff base complex (92) is formed by reacting the free ligand with CoI2. The iodo complex is low spin and square pyramidal.391... 

Examples in organometallic systems are known. Reaction of thiuram disulfides, (R2NCS2)2, with Co(Cp)(CO)2 produces dithiocarbamato pseudo-octahedral cobalt(III) complexes Co(Cp)(dtc)2 with one chelated and one monodentate dtc, also accessible via Co(Cp)I(dtc).1050 Fluxional behavior, including monodentate chelate exchange, was observed for some complexes in temperature-dependent NMR studies. The Co(Cp)I(dtc) complex was defined in a crystal structure. 

The [Ni(NCS)f,]4 ion is almost perfectly octahedral, with Ni—N distances of around 209.5 pm and N—Ni—N angles around 89.5°. The Ni—N—C and N—C—S entities are practically linear.438,439 In [Ni(NCS)2L2] where L is a R-substituted pyridine, stereochemistry and spin state depend on the type and positions of R.431 While for 2-Me- and 2-Et-pyridine square planar complexes are observed, other pyridins lead to coordination polymers with pseudo-octahedral Ni11 due to N,S-bridging thiocyanate. Ni11 thiocyanato complexes have been studied quite intensively as hosts for inclusion compounds.440"442... 

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