Seven-ligand compounds

The electrochemistry of a number of such six-coordinate compounds [MnXL]+ and seven-coordinate compounds [MX2L] (with L = (203), R,R = Me and X = halide, water, triphenylphosphine oxide, imidazole, 1-methylimidazole or pyridine) has been investigated.551 The redox behaviour of these compounds was of interest because it was considered that the potentially -acceptor macrocycle (203 R = R = Me) may promote the formation of Mn° or Mn1 species or may yield a metal-stabilized ligand radical with the manganese remaining in its divalent state. For a number of macrocyclic ligand systems, it has been demonstrated that the redox behaviour can be quite dependent on axial ligation it was also of interest to study whether this was the case for the present systems. 

The most common geometries encountered are capped octahedral and capped trigonal prismatic. Many of the known seven-coordinate compounds involve /1-diketonate ligands, particularly adducts of the type Ln(diketonate)3.L (L = Lewis base e.g. H2O, py). Several... 

Neutral complexes of the thiocyanate ligands can be isolated as Lewis base adducts. Thorium and uranium form tetra(hydrate) complexes An(NCS)4(H20)4. The compound U(NCS)4(piper-azine) has been reported. The neutral adducts Th(NCS)4(L)4 and U(NCS)4(L)3 (L = N,N-diisopropylpropionamide) have been structurally characterized. The smaller size of the uranium ion is reflected in the isolation of a seven-coordinate compound, while an eight-coordinate thorium compound is formed. If more sterically demanding carbamide ligands are employed (L = N,N-diisopropylbutyramide, L = 7V,VV-dicyclohexylacetamide), seven-coordinate thorium complexes, Th(NCS)4(L)3 and Th(NCS)2Cl2(L )3, can be isolated. " ... 

In 1985, the seven-coordinate compound Ti((02CCH2)2NCH2)2)(H20) containing the hexacoor-dinate ethylenediaminetetraacetate ligand (EDTA) was structurally characterized. Similarly, the... 

A derivative of triazacyclononane was used to prepare the titanium salt [Ti((OC6H3-But)NCH2CH2)3)][BPh4].872 Most recently, the ligand (TV-ethoxo)ethylenediaminetriacetate (EEDTA) was used to prepare the seven-coordinate compound (Ti(EEDTA) (H20))(H20)2.873... 

The binuclear tungsten(II) and molybdenum(II) compounds react with phosphorous and nitrogen atom donor ligands and after chloride bridge splitting give mononuclear seven-coordinate compounds. 

In the reaction of a bimetallic tungsten-tin compound with acetonitrile we isolated and characterised structurally a mononuclear seven-coordinate compound with two mutually cis nitrile ligands and a trichlorostannyl moiety coordinated to tungsten through a direct tungsten-tin bond and a chloride bridge (Eq. 4) [39]. 

A similar seven-coordinate compound [WCl(GeCl3)(CO)3(NCMe)2] was obtained in reactions of an analogous heterobimetallic tungsten-germanium complex with nitriles, but the isolated and X-ray investigated product contained two anionic ligands, a chloride and a trichlorogermanium moiety in the trans position [35]. 

The NMR characterisation of fBA adducts formed in the reaction of tungsten and molybdenum seven-coordinate compounds leaves little doubt concerning their formulation. However, we were unable to separate such compounds and to prepare a crystalline sample. To obtain a more stable terminal alkyne compound, we introduced two phosphine ligands into the coordination sphere of tungsten(II). In this way a cationic complex of tungsten(II), [WCl(CO)( BuC=CH)(NCMe)(PPh3)2]2[SnCl6], was prepared [45]. The crystal structure of this compound consists of two independent cations and a... 

Seven-coordinate bimetallic compounds are also active unicomponent catalysts in the ring-opening metathesis polymerisation (ROMP) of cyclic olefins. The first step in this reaction is the coordination of olefin to metal. However, only in the reaction with a chelating ligand such as norbornadiene (NBD) was it possible to isolate and characterise the reaction product. In the reaction with NBD chloride bridge splitting in the binuclear compound of tungsten(II) by a diene molecule occurs very easily and monomeric seven-coordinate compounds are formed (Eq. 6) [48]. 

Seven-coordinated Zr is also found in the mixed ligand compound [Zr3Fe(/t4-0)(/z-OPr">6(OPr )4(acac)3]. The /Z4-0X0 ligand is tetrahedrally coordinated and the Fe(m) atom is five-coordinated (distorted TBP). However, with Co(n) the hexanuclear [Zr2Co4(/Z6-0)(M-OFT )8(OPr )2(acac)4] was obtained. This has a (i -oxo ligand encapsulated in an octahedron (trans-Zr2) of metal atoms in the familiar cubic M Oi9 core framework with all metals octahedrally coordinated. ... 

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