Seven-co-ordinate Complexes

Seven-co-ordinate complexes. The molybdenum(n) complex [Mo(CO)sCla-(PMe8Ph)3],MeOH is believed to be the first uncharged seven-co-ordinate organometallic molecule involving only unidentate ligands to be studied by X-xsy diffraction. It does not exhibit any symmetry, and it contains an unusually long Mo-P bond, of 2.62(2) A. The other molybdenum-ligand distances are Mo-P 2,50(1) and 2.528(15) A Mo-Cl 2.49(2) and 2,56(1) A and Mo-C 1.76(5) and 1.97(5) A. 

In the complexes [Mo(CO)2Brs(dam)a] and [W(CO)3Br2(dam)2], where dam = PhjAsCHaAsPha, the co-ordination of the metal atoms is described as capped-octahedral. The dam ligands, which co-ordinate through arsenic 

In the tetrameric molecule [W(CO)aH(OH)]4 (26), of rigorous Cj symmetry, the tungsten atoms occupy the corners of a nearly regular tetrahedron, and are held together by four fia-hydroxo-groups arranged at the corners 

Congtkxes with other One-carbon-donor Ligands. The dimeric molecule [Mos(CH2SiMe3),] is isostructural with the analogous tungsten compound. It consists of two tetrahedral MofCHjSiMes), units held toother by a Mo-Mo triple bond of 2.167 A. The dimer exhibits approximate 2) symmetry. The average Mo-C(CH ) distance is 2.131 A. 

The molecular structure of [MoafQHj) ] (29) resembles that of the analogous chromium compound. The metal atoms are held togetha by a very strong Mo-Mo multiple bond of length 2.183(2) A, and also by two symmetrically bridging allyl groups with their molecular planes parallel to the 

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The seven-co-ordinate complexes [R MV(CO)e], [RnMVlCOlgPPhj] (R M = PhjSn, EtHg, or PhjPAu), and [Ph3SnV(CO)4(diphos)] have been prepared by the reaction of [VICO) ]" salts with R MC1. The substituted complexes are very prone to heterolytic cleavage in weak donor solvents, and reactions of some of these tin compounds are included in Scheme 2, which describes convenient syntheses of [HV(CO)4(diphos)], [IV(CO)4(diphos)], and [VlCOj ldiphos)]. " ... 

Figure 6-13. The templated formation of a hydrazone macrocycle. This ligand contains five nitrogen donor atoms and was designed to investigate the properties of metal complexes containing a pentagonal planar donor set. In practice, the complexes of these ligands usually acquire one or two axial ligands to give six- or seven-co-ordinate complexes.

A full account of the crystal structure determination of potassium heptacyano-vanadate(m) dihydrate has appeared.387 Apart from clearing up the uncertainty surrounding the nature of the vanadium(m) cyanide complex, this study has established the V(CN)7- ion as one of the very few examples of a seven-co-ordinate complex containing only simple unidentate ligands. The choice of this co-ordination number has been rationalized on the basis of the nine-orbital rule. The two electrons of the paramagnetic V(CN)4 ion (pef = 2.8 BM) occupy two separate orbitals,... 

The introduction of bis(p-chlorophenyl) and bis(methylthio) substituents in the 4 - and 4"-positions of quinquepyridine (35) (that is, on the backbone of the second and fourth pyridyl rings in the string) was reported to have little effect on the coordination behaviour of these ligands relative to that of the unsubstituted analogue (see earlier). Namely, with nickel(II) and copper(I) and/or copper(II), doublehelical 2 2 complexes were obtained, while cobalt(II) forms both a 1 1 and a 2 2 complex in the solid state (although, in solution only a mononuclear seven-co-ordinate complex occurs - with the solid state, double-helical structure interconverting to this form upon dissolution). 

CO)3(dppe)2Wl2 represents another seven-co-ordinate molecule which adopts a distorted capped octahedral structure, this time with a CO ligand in the capping position. Similarly the seven-co-ordinate complexes (L—L)M(CO)3X2 [M = Mo or W X = Br or I L—L = Me2As(CF3)C=C(CF3)AsMe2] have been prepared and their fluxional properties investigated. These complexes can also be used as CO carriers... 

The d oxidative addition may seem unfamiliar because there are many more examples 47) of d d and d d processes. However, ruthenocene, which is sl (P ruthenium (II) complex with a delocalized electronic structure, undergoes two-electron oxidative addition by I2 and Br2 to give the Ru(IV) complexes Ru(cp)2r and Ru(cp)2Br (48). X-ray studies of Ru(cp)2r show that it is eflFectively a seven-co-ordinate complex (48). 

Heats and enthalpies of oxidation of cis- and trans-3-t-butylthietan 1-oxides to the 1,1-dioxides by perlauric acid at 298 K have been determined. The structures of cis-2,4-diphenylthietan l-oxide and the seven-co-ordinate complex of 3,3-dimethylthietan 1-oxide with the shift reagent Eu(dpm)3 have been established by X-ray analysis. The mass spectrum of thietan 1-oxide shows fragmentation to CsHs and... 

The kinetics and stoicheiometry of the oxidation of oxovanadium(iv) by peroxide have been studied and continuous-flow e.s.r. techniques used to investigate the intermediate which appears to be a vanadium(v) complex with a paramagnetic ligand, formulated as either [OVOO,aq] or [V02(H02). aq]. Six- and seven-co-ordinate vanadium(v) has been identified by. Y-ray crystallography in the peroxo-complexes (NH4)[V0(02)2-Nh ]36o (NH4)4[0 V0(02)2 2] respcctively. and seven-co-ordination... 

S-, Se-, and Te-donor Ligands. The TViV -dialkyldithiocarbamato-complexes [V0(R2NCS2)3] (where R = Me or Et, or R2N = 1-pyrrolidine) have been prepared. Their i.r. and mass spectra suggest that the compounds consist of discrete, seven-co-ordinate molecules.The [VOScj] " ion has been identified spectrophotometrically in the course of the reaction between an aqueous solution of [V04] and HjSe gas. CU3VX4 (X = S, Se, or Te) have been prepared and shown to have a sulvanite-type structure. ... 

The spectra of [Fe(edtaH)H20], [Fe(edta)H20] , and [Fe(H0-edta)(H20)2] have been examined in the solid state and in solution. All the complexes are found to be seven-co-ordinate in water, methanol, formamide. and glycerol... 

Figure 6-29. The condensation of 6.31 with glyoxal in the presence of nickel(n) acetate gives the nickel(ii) complex of 6.30. In the product, the nickel(n) ion is actually seven co-ordinate with two additional axial solvent molecules which are not shown.

Figure 6-40. The complex 6.41 undergoes an intramolecular nucleophilic attack at a second imine to generate a new macrocycle with the correct cavity size for cobalt(n). The lower structure shows the complex cation as it is found in the solid state. The cobalt ion is actually seven-co-ordinate, with axial water and methanol ligands (omitted for clarity).

Rate equations of this type are normally associated with the formation of associative intermediates or the involvement of deprotonated ligand forms in reactions with two (or more) competitive pathways. However, in the case of octahedral metal complexes of ligands such as 2,2 -bipyridine or 1,10-phenanthroline, such mechanisms do not appear to be likely. Associative mechanisms would involve seven-co-ordinate intermediates, which are likely to be sterically strained and electronically disfavoured on ligand field grounds. Furthermore, this type of ligand does not appear to contain any strongly acidic protons which are likely to be involved in reactions with aqueous hydroxide ion (but see later). 

Accordingly, the search for unambiguous data leads to studies of kinetically inert octahedral metal complexes or pyridine, bpy and phen, which are not expected to form seven co-ordinate products. There have been numerous studies of [M(bpy)3]"+ and [M(phen)3]"+ complexes, but it is probably fair to state that no definitive evidence for activation towards attack of hydroxide at the ligand has been presented. There is no doubt, however, that sufficient kinetic anomalies exist for further study to be worthwhile, and the importance of [M(bpy)3]"+ and [M(phen)3]"+ complexes in photocatalytic systems should justify this. A possible chemical consequence of the reaction of a nucleophile with a co-ordinated heterocycle is seen in the reaction of (2,2 - bipyrimidinejruthenium(11) and hydroxide ... 

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