Bipyramidal complexes trigonal species

The Criegee mechanism originally proposed involves the formation of an osmium(VI)-ester complex (106) from the [4+2] cycloaddition of the Osvul cis-dioxo moiety with an alkene, followed by the hydrolysis or reduction of (106) to cis-glycol and reduced osmium species. In support of this mechanism a variety of Osvl cyclic esters such as (107) or (108) (L = quinuclidine) have recently been synthesized from Os04 and the alkene, and characterized by an X-ray crystal structure.290,343 In solution the dimeric complex (108) dissociates to give the monomeric dioxo trigonal-bipyramidal complex (109), which is similar to (106).344... 

The central atom in a square planar d8 complex is sometimes described as coordinatively unsaturated. This term is best used to denote a tendency to take up additional ligands without change in oxidation state it does not necessarily follow that a coordinatively-unsaturated species will be susceptible to oxidative addition. However, square d8 complexes do have some tendency to take up a ligand to become trigonal bipyramidal, 18-electron species. Trigonal bipyramidal d8 complexes formed by Fe(0),... 

In contrast, the major Cu species in hydrated K-X is clearly coordinated to three waters and can be assigned as Cu jj in site SII. Partial dehydration at 30-60° C converts Cujjj to the trigonal bipyramidal complex, Cujj. Rehydration at room temperature converts most of the Cujj to Cum hut traces of the former are still present. Cum becomes the sole copper species on dehydration at 400° C and rehydration at room temperature. Complete dehydration at 400° C gives the same species as formed in Na-X which is assigned to Cuq in site SI. ... 

Only one divalent ion exchanged X-zeolite has been studied in detail thus far, namely Ca-X. It shows a surprise in that Cu + is found to form a completely new species identified as Cu(Oz)3(OH)2 trigonal bipyramidal complex designated Cun (37). Its ESR spectra indicate trigonal bipyramidal symmetry and are distinctly different from those of Cujj in both resolution and dependence on the... 

Reduction of [CoCl(L)](BPh4) (L = N(dppe)3, P(dppe)3) with Na/Hg in THF results in red-brown dimeric [Co(L)]2(Hg2) species (141) in which Hg + is linearly coordinated between two Co-1 centres.500 These unstable diamagnetic trigonal-bipyramidal complexes (structure, Table 40) react with CO and C02 at ambient temperatures in an apparent redox process (equation 107).500... 

A series of air-sensitive trigonal bipyramidal and trigonal pyramidal Ni complexes, [Ni(PR3)4]+, [NiX(PR3)3], [NiX(AsR3)3] (X = halide), and [Ni(tripod)]+ and [NiX(tri-pod)] (tripod = face-capping tripodal P, As ligand), have been reported. The methods of synthesis of these species are varied, but essentially are based on the reduction of Ni or oxidation of Ni precursors. A range of four- and five-coordinate Ni complexes have been stmcturally characterized and some of these are illustrated in (42)-(46). 

Iron powder reacts with two equivalents of PPhMe2Br2 in Et20 to form a white solid, which is acutely sensitive to dioxygen. At exposure to low O2-partial pressures (< 100 ppm) it is possible to isolate an intensely purple trigonal-bipyramidal complex [FeBr3(PPhMe2)2], with the phosphines trans to each other and in the apical positions. It might have been expected that exposure of the initial white material, presumably an iron(ii) species, would have yielded a jU-oxo complex. 

Nickel(II) phosphine complexes have been reported to he efficient catalysts in carbonylation reactions. To investigate this reaction mechanism, we have studied the reaction of CO on the related Ni(II) complexes NiX2(PMes)n (n = 2,3) and [NiX(PMes)m]BFj> (m = 3,4). Pentacoordinate carbonyl nickel(II) species (without reduction of Ni(II) to Ni(0)) were isolated (1) by direct substitution of PMcs by CO in the pentacoordinate complex and (2) by addition of CO on the trans square-planar tetracoordinate complex. These compounds are trigonal-bipyramidal complexes with CO in equatorial position. The Ni-CO distance (1.73 A) is the shortest reported Ni-CO distance. Since these carbonylation reactions can be viewed as substitution of an equatorial PMes by CO in a TBP, they can be related to the substitution reactions in square-planar d metal complexes. 

As a final example of the use of isotopic substitution in the study of metal complexes, we cite the use of as a ligand by Collman, Famham, and Dolcetti ( ), who found what they termed "hybridization tautomerism" in several cobalt-nitrosyl complexes. From their infrared spectra they inferred a rapid equilibrium between a trigonal-bipyramidal Co(l) species having a linear Co-nitrosyl geometry, and a sqviare-pyramidal Co(lll) species, in which the Co-nitrosyl moiety is bent. 

Organorhodium compounds are found in oxidation states ranging fi-om +6 to -3. However, the most conunon oxidation states are +1 and +3. Rh(I) species exist as both a tetra-coordinated, square planar complex as well as a penta-coordinated trigonal bipyramidal complex. Rhodium, like platinum group metals, exhibits reversible oxidation states making a broad range of organic-catalytic transformations possible. 

The structures of the hydridorhodium complexes that are present in the catalytic system have been deduced by NMR spectroscopy. Brown showed that HRh(CO)2(PPh3)j exists as an 85 15 mixture of diequatoriakapical-equatorial isomers of HRh(CO)2(PPh3)2 (Scheme 17.11) that undergoes rapid equilibration at room temperature. This rapid equilibration of trigonal bipyramidal complexes could occur by either a Berry pseudorotation mechanism or a turnstile mechanism. In situ IR transmission spectroscopy on the catalytic system demonstrated that these two isomers are the resting state of the catalyst and were by far the predominant species present during hydroformylation of 1-octene (60-100 C, 5-20 atm, [Rh] = 1 mM, PPhj/Rh = 5). °... 

All bridged dimers feature a central Ln202 core (type I), which in the presence of strong donor molecules such as THF, DME, OPPh3, or pyridine dissociate into mononuclear species with either octahedral (type II) or trigonal bipyramidal structures (type III). Similar structural motifs are found in cyclic silsesquioxane-based rare earth siloxides. In the presence of donors, distorted octahedral (V) or distorted trigonal bipyramidal complexes (IV) are formed, which in solution through loss of donor solvent may be in equilibrium with their respective dimers (see Lanthanides Coordination Chemistry) ... 

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