Six-co-ordinate Complexes

Three dynamic intramolecular rearrangements have been identified in the cyclic-triene complexes (36a) and (36b) by variable-temperature C n.m.r. methods. The spectrum of (36b) at -120 °C exhibits six signals of equal intensity for 

Above this other changes are observed which imply that local scrambling of CO groups in the allyl-bonded Fe(CO)a group takes place followed at higher temperatures by the onset of local CO scrambling in the other Fe(CO)a moiety. An alternative explanation can account for the first set of changes (—120 to —88 °C) but this was eliminated by similar studies on a substituted derivative [Fe2(CO)6(PEts)-(CioHia)]. The above conclusions apply equally to complex (36a). 

The isomerization to the 1,3-form of cyclohepta-1,4-diene co-ordinated to iron was the subject of earlier kinetic investigations and the preliminary communication has been followed up by a full report. Fluxional behaviour in diazepineiron tricarbonyls (37) has been shown to require proton transfer via an intermolecular 

Leppard, H. J. Hansen, K. Bachmann, and W. von Philipsborn,/. Organometallic Chem., 

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An orbital correlation diagram has been presented for linear and bent M—N—O systems in five-co-ordinate tetragonal molecules, and the diagram can be extended to six-co-ordinate complexes with only minor modifications. From this study, it is concluded that the antibonding nature of the level to be filled under symmetry, and not its composition, lead to distortion and bending of the M—N—O unit, and that v(NO) is not a good guide to the linear... 

The nitronates PhCHNOj and MejCNOj form the six-co-ordinate complexes Mn(nitronate)L2 (L = H20orpy L2 = tmen)- and o-hydroxy-l-phenalenone(L) forms the octahedral complex Mn(L — H)2,2H20. Co-ordination of Mn by penicillin and cephalosporin antibiotics has been studied by n.m.r. line-broadening. ... 

A C n.m.r. study of diamagnetic complexes of propane-l,3-diamine-NNiV N -tetra-acetate ion (pdta ), including [CoL] , detected no complexes in which L was quinquedentate, due to steric compression of the glycinate chelate arms and not to decreased ring strain in the six-co-ordinate complexes as previously reported. ... 

R2S = thioxan or Et2S) in solution was also examined. Five- and six-co-ordinated complexes [Pt(thioxan) ] (n = 5 or 6) are formed in solution by treatment of [Pt(thioxan)J with excess thioxan the complexes for n = 6 were isolated as nitrate and picrate salts. ... 

Optically Active Complexes. The c.d. spectra of nineteen optically active chromium(iii) complexes have been recorded in both the spin-allowed and spin-forbidden d-d transition regions, and discussed in detail. The rotatory strengths of the d-d transitions of trigonally distorted six-co-ordinate complexes of chromium(iii) with N- and O-donor atoms have been calculated using a MO model. The results obtained suggest that neither the Piper nor the Liehr MO model provides adequate representations of the source of d-d optical activity in such systems. ... 

Full details of the crystal structure of Rha(H20XAsMe3)2C4(CF3)4 (see Vol. 2, p. 322) have now appeared. Other rhodiacyclopentadiene systems (108) and (109) have been prepared from Rh(CO)ClL2 and hexafluorobut-2-yne, and some of their reactions are reported. The six-co-ordinate complexes (108) are readily decarbony-lated and reformed. In the presence of carbon monoxide, Rh(CO)CIL2 [L = AsPhs,... 

Bromo(bispentafiuorophenyl)thallium reacts with the sodium or thallium(i) salts of the j8-diketones, (MeCO)gCH2, CFa-CO-CHa-COMe, (CF, CO)3-CH2,PhCOCHj-COMe,and (PhCO)sCH, to form four-co-ordinate dionato complexes, e.g. (C,F5)sTl[(MeCO),CH], and with the sodium salt of 8-hydroxyquinoline to form a dinuclear complex with bridging ligands. Reaction of these with triphenylphosphine oxide and triphenylarsine oxide, and with 2,2 -bipyridyl and phenanthrene, yields a number of five- and six-co-ordinate complexes. ... 

Carbonyl Complexes. In all of these the metal atom is six- or seven-coordinate. In the six-co-ordinate complexes the co-ordination polyhedron around the metal atom is the octahedron. 

Protonation of cA-[M(CO)2(P P)al (M=Mo, P—P=dmpe or dppe M=W, P—P=dmpe) gives the seven-co-ordinate hydrido-cations [MH(CO)2(P—Pla]" which on deprotonation with KOBu produce isomeric /r j-[M(CO)2(P—P)2l- These transformations constitute the use of seven-co-ordinate intermediates in the isomerization of six-co-ordinate complexes. The rra/w-complexes [M(CO)2(P—P)2] revert under first-order kinetics to the more stable cw-isomers with rates (W> Mo) which are unaffected by excess base or one atmosphere of carbon monoxide and these data are consistent with earlier results supporting a chelate arm-off arm-on mechanism with a five-co-ordinate intermediate (see Vol. 5). 

Substitution and CO exchange in six-co-ordinate complexes cis-[MBr(CO)4L] (M = Mn or Re, L=PPh3, P(OR)2, or py] occur via a five-co-ordinate intermediate which is subject to rapid intramolecular rearrangement during its lifetime. The species [MX(CO)5] (M = Mn or Re, X=H or I) are stereochemically rigid on the n.m.r. time-scale over a range of temperatures, as are similar iron-group complexes... 

Six-co-ordinate Complexes. Graham has extended his work in this area by studying stereochemical non-rigidity in cir-[Fe(XR3)a(C 0)J (X=Si, R=Me, Et, Pr , Bu , Ph, or Cl). The distorted solid-state structures are best described as pseudo-bicapped tetrahedral. In solution all are fluxional with AG ca. 10—11 kcal mol" except the complex with XR, = SnCls which is rigid on the n.m.r. time-scale and has AG > 16.1 kcal mol . The rearrangement is non-dissociative and probably involves a cis-trans-cis interchange even though the trans-form is undetected in solution. 

Bis(dimethylphenylphosphine)(ethylene)(carbonyl)dichlororuthenium is the first example of a six-co-ordinate complex containing ethylene as a ligand which has been studied by X-ray crystallography (18). The ethylenic double bond is closely parallel to the Ru—P bonds the Ru—C(ethylene) distance is 2.214(4) A and C=C is 1.376(10) A. The analysis indicates that the ethylene is possibly non-planar with the hydrogen atoms bent away from the metal. 

Stereoisomerism is commonly shown by octahedral (six co-ordinate) complexes associated with bidentate ligands. An example is the complex containing nickel as the transition metal and 1,2-diaminoethane (NH2CH2CH2NH2) as the bidentate ligand (Figure 24.9). The two isomers are stereoisomers because the two different molecules are mirror images of each other and cannot be superimposed. 

The iridium complex [(Ph2PMe)2lr(CO)Cl] forms a six-co-ordinate complex... 

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