Five-co-ordinate Complexes

Cobalt.—Pentacyanocobaltate(n) reacts with iodine cyanide in aqueous solution according to the equation  

This is an example of oxidative addition at a five-co-ordinate complex to compare with the more common examples of oxidative addition at square-planar fi complexes (see Chapter 12). Hydrogen peroxide and hydroxyl-amine react similarly with pentacyanocobaltate(ii). These reactions are all second-order and proceed by radical mechanisms. The course of 

Photochemical dealkylation of alkylcobaloximes and alkylcobalamins is associated with charge transfer between the alkyl group and cobalt cobalt(ii) and alkyl radicals are the initial products. A similar situation exists for the vitamin B12 model (6). In both cases, there are close parallels 

Substitution at [CoCmnOaCPPhg)] by the bidentate ligands ethylene-diamine, 2,2 -bipyridyl, 1,10-phenanthroline, and maleonitriledithiolate, and by the monodentate ligand triphenyl phosphite, takes place by two parallel paths. In the first, equilibrium loss of triphenylphosphine is followed by attack of the incoming ligand at the square-planar intermediate 

Platinum.—Intramolecular rearrangement and ligand exchange have been studied for methyldiphosphine adducts of platinum(ii) dithiolates.  

Five-co-ordinate Complexes.—review on five-co-ordinate cobalt(u) complexes has appeared. All of the five-co-ordinate complexes described here contain multidentate ligands in four trigonal-bipyramidal geometry is found, whereas in two the metal co-ordination is square pyramidal. 

194(9) and 2.055(10) A. The N(axiaI)-Co-N(equatorial) angle is 91.1(3)°. The Co-Br distance of 2.658(3) A is considered unusually long. The crystal structure of [Co(QP)Q]BPh4 contains complex cations in which four phosphorus atoms of thequadridentate ligand QP,tris-(o-diphenylphosphino-phenyl)phosphine (6), and a chlorine atom form a trigonal bipyramid about 

The cobalt atoms in the cations (8) of the complex iodotris-(2-diphenyI-phosphinoethyl)aminecobalt(n) iodide exhibit distorted square-pyramidal co-ordination, with a phosphorus atom at the apical position and two 

The crystal structure of bis(salicylaldehyde)ethylenedi-iminecobalt(ii) contains discrete dimers (9). Each cobalt atom is surrounded by a cis planar arrangement of two oxygen and two nitrogen atoms, the Co-0 and Co-N distances being 1.920(4) and 1.897(5) A, respectively. Square-pyramidal 

Water exchange is relatively slow at the cobalt(ii) complex of l,4,8,ll-tetramethyl-l,4,8,ll-tetra-azacyclotetradecane (LLLL), [Co(LLLL)(OH2)] +. Fourier-transform O n.m.r. gave a value of 4.2 x 10 s for the rate constant at 298 K, and activation parameters A/f = 36.5 kJ mol and A5 = - 34 J K mol. The markedly negative value for the entropy of activation was held to betoken an associative 

The structures in solution (dichloromethane) of [NiX2(PMe3)3] (X=C1, Br, I, or CN) and their ligand exchange reactions have been studied by and P n.m.r. spectroscopy. These compounds are rare amongst five-co-ordinate compounds in exhibiting stereochemical rigidity at accessible temperatures, in this case at and 

144 Lincoln and C. D. Hubbard, Proceedings of the 16th International Conference on Coordination Chemistry, Dublin, 1974, 3.30 Chem. Abs., 1976, 85, 37 623). 

The rate law for trimethylphosphine exchange indicates a dissociative mechanism. A dissociative mechanism, with rate-limiting breaking of a nickel to terminal arsenic bond, also operates for the reactions of trigonal-bipyramidal [NiX(qas)]+ [qas = tris-(u-diphenylarsinophenyl)arsine] with the nucleophiles CN, NOg, Ns , SCN , I, thiourea, and triphenylphosphine. A preliminary report on a very similar reaction, equation (20), in methanol solution, gives observed rate constants kt and Arb) for the 

Grimley and H. L. Collier, Amer. Chem. Soc., 173rd Meeting, 1977, Abstract Inorg.222. D. A. Sweigart, Inorg. Chim. Acta, 1977, 23, LI 3.  

The topics of four- and five-co-ordinate complexes are conveniently linked by a study of the kinetics of ligand dissociation and intramolecular rearrangement for a series of cations [MLs]+(M = Co, Rh, or Ir ) and [HMLJ+(M = NF, Pd , or Pt ), where L = PEtg or a trialkyl phosphite. The relevance to four-co-ordinate complexes is of course that these five-co-ordinate species can be regarded as models for transition states for associative substitution at four-co-ordinate species. Several quadridentate arsenic ligands are effective in stabilizing five-co-ordinate 

Stable in non-polar solvents but are kinetically fairly labile. A variable-temperature n.m.r. study of solutions of such complexes has shown that exchange processes are complicated, with three different bimolecular mechanisms involved. The stereochemical characteristics of this ligand are important in elucidating the possible reaction pathways. The kinetics of reaction of the [NiCl(qas)]+ cation (18), where qas is the tripodal ligand (19), with cyanide or thiocyanate in methanol have been 

Sweigart has published three more papers on substitution kinetics of five-coordinate complexes of the [M(dithio-ligand)2L] type. Reactions of the tri-n-butyl-phosphine adducts of bis-(0-dialkyldithiophosphato)nickel(n) complexes with bidentate nitrogen or phosphorus nucleophiles go by the D [5 nl(lim)] mechanism. It now appears that the substitution mechanism operating for this type of complex depends on the charge on the complex, i.e. on the formal oxidation state of the metal. Reactions 

Reactions of iron and cobalt dithioline complexes, [M(mnt)jX] mnt = [SjCa-(CN)al, X = a phosphine or phosphite, with unidentate nucleophiles (L) give five-co-ordinate products, [M(mnt)aL] , and with bidentate nucleophiles (L—L) give six-co-ordinate products, [M(mnt)2(L—L)]. In each case the iron complexes react more rapidly than those of cobalt and the mechanism involves both dissociative and, unusually for five-co-ordinate substrates, associative pathways. The kinetic equations are necessarily complicated but the experimental results are consistent with the predictions in limiting cases. Kinetic studies have been carried out on the ligand-exchange reactions  

Rearrangement in a five-co-ordinate species, from a trigonal bipyramid to a square pyramid, has been effected by increasing pressure on the [Cr(en)3] + salt of [NKCN),] -. 

This chapter is concerned with substitution reactions of inert metal ions of co ordination number six or higher. Where a metal ion is sometimes inert and sometimes labile e.g. iron(n) which is inert in its low-spin complexes such as [Fe(bipy)3] + (ttg ) but labile when high-spin (e.g. [FeCHgO) ) only references to the inert 

Although cobalt(m) and chromimn(m) continue to be kinetically the most widely investigated inert metal ions, some progress has been made in recent years towards our understanding of the rarer metals. The continuing efforts of Taube and his associates are particularly noteworthy in the field of ruthenium chemistry, and several interesting publications from them dealing with aspects of ruthenium(n) and ruthenium(in) complexes have appeared recently. - The photochemical studies of 

In recent years interest in the substitution reactions of cobalt(m) complexes has 

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N-, P-, As-, and Sh-donor Ligands. [Cr(CO) (bipy)] has been reduced by sodium in THF. Electronic and e.s.r. spectra of the resulting complex suggest that it involves co-ordination of the uni-negative ion of bipy. The complex loses CO to afford either a five-co-ordinate complex or a dimeric species. ... 

Phosphorus and arsenic ligands. Five-co-ordinate complexes [Ir(PPh3)(L L3)] of the tripod-like ligand (49 L, = P. As) have been obtained by refluxing... 

This five-co-ordinate complex then undergoes a rapid reaction with the incoming ligand (water or some other species) to generate a new six-co-ordinate amido complex, which becomes protonated to generate the observed product of the reaction (Fig. 5-43). 

A related mechanism is almost certainly observed in the almost instantaneous metha-nolysis of some pivalamide derivatives in the presence of copper(n) chloride. For example, ligand 5.34 forms a five co-ordinate complex with copper(n) chloride. When... 

It has now emerged that these changes are due to the reversible addition of the nucleophile to the complex, but the site of addition in the products is at the metal. In other words, a five co-ordinate complex has been formed instead of the species in which the hydroxide is covalently attached to the pyridine ring (Fig. 8-23). It is, of course, impossible to distin-... 

The new five-co-ordinate complex [RuCl2(CO)(PCy3)2] (PCy3 = tricyclohexyl-phosphine) has been prepared (equation 4).17 Its i.r. spectrum suggested the structure... 

Pt(C2F4)(C2H4)2 [<5 -123-6, /(Pt-F) 248 Hz] has also been confirmed crystallographically. (166) Another series of five-co-ordinate complexes, containing tripodal ligands MeC(CH2APh2)3 (A = P, As), is exemplified... 

Polymeric cobalt(ii) compounds of (87) and (88) have been isolated and their properties compared with those of the monomeric analogues of the ligands (89) and (90). The ligand (91) forms a five-co-ordinate complex with cobalt(ii), one of the ethereal oxygen atoms remaining unco-ordinated. ... 

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