Seven coordinate W

Many new transition metal complexes of these ligands continue to be prepared. The most interesting aspects of this chemistry are not so much the evolution of specific new ML, ML2, and ML3 compositions, but the purposes that the hgands serve or are suggested to serve. For example, poly(pyrazolyl)borate anions have been used to stabilize a 17-electron radical fragment, a seven-coordinate W complex, half-sandwich HB(pz )MCl compounds, and to mimic a variety of natural bioinorganic coordination conditions. ... 

The reaction between PySeSePy (2,2 -dipyridyl diselenide) and [W(C7H8)(CO)4] (CvHg = norbornadiene) yielded the seven-coordinate complex [Wn CsI I4N(2-Se)-A-,SV 2(CO) ,].2 Oxidative addition of 2,2 -pyridine disulfide to [L3W(CO)3] (L3 = (pyridine)3, cycloheptatriene) in dichloromethane gave the seven-coordinate [W( 72-mp)2(CO)3] (mpH = 2-mercaptopyridine). This complex reacted with NO in the presence of additional pyridine disulfide to yield the seven-coordinate tungsten I) nitrosyl complex [Wn( 72-mp)3(NO)], as well as [W(rf-mp) 2 (N O) 2] and a trace amount of [Wiv(f72-mp)4].208... 

Scheme 1. The possible intermediates formed in the polymerisation reaction of alkynes catalysed by seven-coordinate W and Mo compounds...

Another typ>e of metal carbonyl catalyst, MCl2(CO)3(AsPh3) (M=Mo, W), that induces the ring-opening polymerization of norbomene and its derivatives has been shown to polymerize tert-butylacetylene and ortlio-substituted PAs without photoinradiation or the use of CCI4. The reaction of tert-butylacetylene in the presence of seven-coordinate W (II) and Mo(II) compounds [MCl(M Cl3)(CO)3(NCR)2] (M=Mo, W M = Sn, Ge R = Me, Et) leads to the formation of high-MW polymers (M > 88,157-159... 

These observations are, indeed, consistent with an associative activation, generation of a seven-coordinated intermediate (easier in the case of Mo and W than for Cr because their larger sizes produce less steric hindrance) by attack taking place directly upon the metal atom, that is, with an A or a limiting 5 2 mechanism, accompanied by a reaction sequence involving dissociative activation similar to scheme (24) above, viz. 

A// 2 is significantly higher for Cr than for either Mo or W, and this is in agreement with the generally observed order of stability for those group VI compounds which are seven-coordinated. 

Oxidation of ris-[W(CO)2(diphos)2] with I2 leads to the unexpected one electron oxidation product [W(CO)2(diphos)2]+. This paramagnetic complex exhibits only one carbonyl stretching frequency, indicating a trans stereochemistry.307 Reaction of [W(CO)4diars] with excess bromine also results in the seven-coordinate complex [W(CO)3(diars)Br2]+, which appears to be a Win derivative.308 It is unusual that three carbonyls are retained by the metal in this oxidation state. 

Figure 12-6 Structure of Fe(DTPA)2- found in the salt Na2[Fe(DTPA)] - 2H20. The seven-coordinate pentagonal bipyramidal coordination environment of the iron atom features three N and two O ligands in the equatorial plane (dashed lines) and two axial O ligands. The axial Fe-O bond lengths are 11 to 19 pm shorter than those of the more crowded equatorial Fe-O bonds. One carboxyl group of the ligand is uncoordinated. [D. C. Flnnen, A. A. Pinkerton, W. R. Dunham, R. H. Sands, and M. O. Funk. Jr..

At elevated temperatures the N- and S-bonded isomers interconvert intramolecularly. A chelate-bonded ([Pg.193]

Hsu, H.F., W.C. Chu, C.H. Hung, and J.H. Liao. 2003. The first example of a seven-coordinate vanadium(III) thiolate complex containing the hydrazine molecule, an intermediate of nitrogen fixation. Inorg. Chem. 42 7369-7371. 

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