Five-coordinate complex

X-ray crystallographic studies on a tetradentate N2S2-macrocyclic complex of palladium, [Pd(tetraL)]Cl2 2H20, reveal the existence of two five-coordinate isomers, (4) and (5), one with a Pd-Cl distance of 3.20 A, and the other with a Pd-Cl distance of 3.68 This supports previous solution studies, which indicate that as Cl approaches [Pd(tetraL)], the square folds back about the N-Pd-N axis to form a trigonal bipyramid.  

A variable-temperature and variable-pressure NMR spectroscopic study in CH3CN has been made of solvent exchange at the five-coordinate nickel(II) complex, The activation volume of 2.3 1.3 cm moP suggests a dissociative mechanism. UV-visible spectra of 6 are compatible with a 5 4 coordinate equilibrium, so it is reasonable that the solvent exchange in the five-coordinate species proceeds dissociatively via this four-coordinate complex. 

The relationship between five-coordinate complexes (or intermediates) and isomerization has been illuminated by a study of Pt(II) phosphole (8) complexes.Equilibria (9) [L = (8) with R = Me, n-Bu, t-Bu, Ph, or CH2Ph X = Cl, Br, or I] were examined by P and Pt NMR spectroscopy 

The complex [PtCl2(PPh3)2] is commonly encountered in synthetic studies, but the very low solubility of both its cis and trans forms makes it difficult to study. Davies and Uma have applied cyclic voltammetry to its investigation, and find that the isomers can be distinguished by their Pt(II) — Pt(0) reduction potentials in dilute benzene-acetonitrile solution. 

In the triphenylphosphine-catalyzed isomerization, the presence of [PtCl(PPh3)3]Cl could also be detected, suggesting a consecutive displacement mechanism, as in equation (10). No isomerization took place from 

As in previous volumes, most information in this section relates to structural studies of five-coordinate d ions, and the possible relationship of these to reaction intermediates of square-planar complexes. A lot of activity 

A series of complexes (10) has been synthesized and examined. A significant finding is that only when R2 are alkyl groups are the platinum atoms 

This field of study is represented by a variety of metals but most of the complexes are labile or organometallic and they are reviewed elsewhere in this book. This section is therefore to be regarded merely as a convenient cross-reference. Coates et have continued studies on five-coordinate Cu(II) 

Substitution Reactions of Inert Metal Complexes— Coordination Numbers 6 and Above 

This review follows on from the previous report by the present authors on this area/ It covers the literature up to approximately December 1980, with a few references beyond that date. There has been the customary varied activity in the area of substitution kinetics covered in this chapter. In this first section we indicate briefly regions in which we feel that the most useful and significant progress has been made in the time covered by this volume. 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly 

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The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

Five-coordinate complexes are far more common than was once supposed and are now known for all configurations from d to d. Two limiting stereochemistries may be distinguished (Fig. 19.4). One of the first authenticated examples of 5-coordination was [VO(acac)2] which has the square-pyramidal 4 structure with the =0 occupying the unique apical site. However, many of the complexes with this coordination number have structures intermediate between the... 

Spectra of 3d five coordinate complexes. M. Ciampolini, Struct. Bonding (Berlin), 1969, 6, 52-93... 

Similar to the four- and five-coordinate complexes 120-126, for RCo (dioxime-BR2)2L 127 and Fe(dioxime-BR2)LL 128 different conformations are possible in solution and in the solid state, in which the substituents of the boron atoms may adopt cis- or trans-configurations and in which the alkyl group R may have a parallel or an antiparallel orientation with respect to the BR2 substituents [173-180]. 

The products obtained from the Pt(PR3)2X2-CNCH3 reactions were dependent on the nature of the platinum species. Five-coordinate adducts, [Pt(PR3)2(CNCH3)2X]X, were isolated for the iodo and bromo complexes (R=Ph), although the latter was unstable and slowly lost isocyanide. The observation of five-coordination here is somewhat unusual, but since this report, it was also observed in a different situation (85), mentioned above. The more common observation was the isolation of four-coordinate species, implying the low stability of most five-coordinate complexes. Data on these reactions are summarized below [Eqs. (33, 34)]. 

Treichel, Knebel, and Hess provided further data on these systems by studying reactions of [Pt(PRj)2(CNCH3)2] with various halide ions and with pseudohalides. A series of five-coordinate complexes were obtained from reactions with iodide ion (PRj = PPhj, PPh2Me, PPhMe2, PEtj), and a study was carried out to measure the stability of these complexes with respect to ligand loss 155). Stability constants for several of these complexes were obtained from spectroscopic data. Other reactants (Cl, Br, CN, SCN) generally yielded the appropriate [Pt(PRj)2(CNCH3)X] species, as expected. 

Reactions of the hydrido(hydroxo) complex 2 with several substrates were examined (Scheme 6-14) [6]. The reactions are fairly complicated and several different types of reachons are observed depending on the substrate. Methyl acrylate and small Lewis bases such as CO, P(OMe)3, BuNC coordinate to the five-coordinated complex 2 affording the corresponding six-coordinate complexes. In reactions with the unsaturated bonds in dimethylacetylenedicarboxylate, carbon dioxide, phenylisocyanate indications for the addition across the O-H bond but not across the Os-OH bond were obtained. In reactions with olefins such as methyl vinyl ketone or allyl alcohol, elimination of a water molecule was observed to afford a hydrido metalla-cyclic compound or a hydrido (ethyl) complex. No OH insertion product was obtained. 

In conclusion, it might be said that the method gives useful results for crystals with distinct molecules. It shows that the large QS in the five-coordinated complexes XFe(R2C tc)2 is primarily caused by covalency effects and is almost entirely due to the valence iron electrons. 

As expected, the related isopropyl ligand systems [TpPrl] (32) and [TpPr12] (j2) are noticeably less sterically demanding than their (-butyl counterparts. Thus, neither ligand prevents the formation of five-coordinate complexes of the types [Tp JMfXlL or [TpPr 2]M(X)L... 

The five-coordinate complexes Ir(CO)(PPh3)2L, where HL = /3-diketone, A-benzoyl-A-phenyl-hydroxylamine, salicylaldehyde, 8-hydroxyquinoline, 2-hydroxybenzophenone, 2-hydroxy-8-methoxybenzophenone, were prepared from [Ir(CO)(PPh3)2Cl].632 The resulting compounds all underwent oxidative addition reactions with Br2. Reaction of [(cod)2IrCl]2 with N-substituted 3-hydroxy-2-methyl-4-pyridine gives the bichelated complex (389). 33... 

Reaction of [Ir(coe)(N(SiMe3CH2PPh2)2] with excess 1,3-butadiene yields the structurally characterized, five-coordinate complex [Ir(C4H6)[N(SiMe2CH2PPh2)2], which contains the P2N ligand coordinated in a quasi-facial manner, and the 1,3-butadiene bound in a s-cis-ri4-tt mode.691... 

A template synthesis employing Ni(OAc)2, 2,5-dihydroxy-2,5-dimethyl-1,4-dithiane, and 3,3 -iminobis(propylamine) gave the water-soluble five-coordinate complex [Ni(495)], the crystal structure of which shows trigonal bipyramidal coordination of Ni11 with the central amine and terminal thiolates in plane and the two imino nitrogens in axial positions. Solvatochromism of the complex is interpreted in terms of S" H bonding, which may be of relevance to the catalytic cycle in hydrogenases.1341... 

Five-coordinate complexes of Pd11 are so distinctive that they are grouped together and not classified according to the ligand atoms. 

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