Octahedral Chelate Complexes

Octahedral Chelate Complexes.— 2T-Ray analysis of (-l-) g,-c -dicyanobis-(ethylenediamine)cobalt(iii) chloride monohydrate establishes that the absolute configuration of the [Co(CN)2(en)J+ complex cation is a (AA), in conformity with the previous spectroscopic assignment. Both chelate rings are of the ob conformation. The mean Co-N and Co-C distances are 1.97 and 1.87 A, respectively and the Co-C-N units do not deviate significantly from linearity. In a preliminary note the absolute configurations of both complex ions in (—)6s -[Co(N02)2(en)J(+)68 -[Co(mal)jen], where mal = malonato, CHa(C08)2 , are said to be A and all en chelate rings to exhibit the ob conformation.  

Three X-iay analyses of racemic tris(ethy]enediamine)cobalt(iii) salts have been described. In [Co(en)3]a(HP04)3,9H20 and [Co(en)3]j[CdCle]Clg,- 

2HjO equal numbers of the complex cations adopt the enantiomorphic A(SSS) and A(AAA) configurations, the mean Co-N distances being respectively 1.967 and 1.972 A. Equal numbers of the A (ASS) and A (SAA) configurations of the cation are found in [Co(en)3]2[CuaCyCU,2HjO, where the mean Co-N distance is 1.969 A.  

It has been established by X-ray analysis that in potassium (-b)58 -tris-(l,10-phenanthroline)nickel(ii) (—)5g,-trisoxalatocobaltate(m) dihydrate both complex ions have the A configuration. The mean Co-O distance is 1.93 A. The complex anion in sodium (- -)s4,-bismaIonatoethylenediamine-cobaltate(iii) has the A configuration the six-membered malonato chelate rings adopt boat conformations.  

The C-N(amine) and C-N(amidine) distances of 1.32 and 1.31 A, both involving the same carbon atom, suggest appreciable multiple-bond character. The optically inactive s-fac-form of [Co(dien)2]Bra contains cations which have the expected structure (18). The cobalt atoms lie on crystallographic centres of symmetry and the cations as a whole have approximate Ca, symmetry. The respective Co-N(central) and Co-N-(terminal) distances are 1.95(1) and 1.97(1) A. Both of the two independent five-membered rings have an unsymmetrical skew conformation. 

---

In early studies of these reactions, the turnover efficiency was not always high, and stoichiometric amounts of the promoters were often necessary to obtain reasonable chemical yields (Scheme 105) (256). This problem was first solved by using chiral alkoxy Ti(IV) complexes and molecular sieves 4A for reaction between the structurally elaborated a,/3-unsaturated acid derivatives and 1,3-dienes (257). Use of alkylated benzenes as solvents might be helpiul. The A1 complex formed from tri-methylaluminum and a C2 chiral 1,2-bis-sulfonamide has proven to be an extremely efficient catalyst for this type of reaction (258). This cycloaddition is useful for preparing optically active prostaglandin intermediates. Cationic bis(oxazoline)-Fe(III) catalysts that form octahedral chelate complexes with dienophiles promote enantioselective reaction with cyclopentadiene (259). The Mg complexes are equally effective. 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

Figure 11. Sokolov s strategy for constructing a trefoil knot on an octahedral tris-chelate complex [39].

Transition metal hydroxyoxime complexes have been reviewed very recently.2507 Their use in both analytical chemistry and extraction metallurgy is well known. The square planar structure of the bis chelate complex NiL (347) with the deprotonated 2-hydroxybenzaldoxime (HL) is typical of this series of nickel complexes.2508 Their bis adducts, NiLJ, with bases such as py, substituted pyridines and cyclomethyleneimines, are six-coordinate.2509 The acyl oxime (H2L) complexes are similar to the aforementioned complexes being either square planar bis chelates Ni(HL)2 (348) or octahedral bis adducts, Ni(HL)2B2.2507 When the acyl oxime acts as a dibasic ligand L, the corresponding (NiL) complexes are insoluble and involve extensive polymerization. 

Macrocycles with 14-16-membered chelate rings all encircle both high-spin and low-spin nickel(II) in the solid compounds. With anions which have coordination tendency, e.g. halides, pseudohalides and in some cases NOJ, trans octahedral paramagnetic complexes are obtained which are often blue or violet. With anions such as C104, PF6 and BF4 (and I, in some cases) which show little tendency to coordinate, square planar diamagnetic complexes are obtained which are generally yellow. 

Chelate complexes of ethylenediamine provide many of the examples on which the theories of coordination chemistry have been founded. Co111 complexes of this ligand were studied by Alfred Werner and his students186 and the separation of or-CoCl(en)2(NH3)2+ into its optical enantiomers187 was a key factor in establishing octahedral stereochemistry. 

Chelate complexes, however, can have holes in the electron distribution merely because of the extent of the distortion of the coordination sphere from octahedral. The best way to see this is not from the occupation numbers themselves, but from the electron density function... 

X-ray results are available for numerous tris chelate complexes, however, we will be primarily concerned with the three classes mentioned above. The most striking feature of the structural results is the adherence to D3 symmetry even in complexes which are severely distorted from the octahedral or trigonal antiprismatic D3(jn) limit. Muetterties and Guggenberger75) have recently pointed out that with the exception of about six tris(dithiolate) complexes, 18, that are close to the D3h (trigonal prismatic) limit, all structurally established tris chelates have D3 or near... 

Most of the tris bidentate chelate complexes listed in Table 11 do not have octahedral geometry because the chelate bite angle a is usually less than 90°. This is contrary to the Oh symmetry commonly assumed for six coordinate complexes. 

---