Six- and seven-coordinate complexes

C. Preparation of Six- and Seven-Coordinate Complexes of Molyb-denum(II) and Tungsten(II) by Halogen Oxidation of Zero-Valent Carbonyl Complexes Containing at Least One Other Donor Ligand 49... 

A number of papers describing the synthesis and characterization of six- and seven-coordinate complexes of the types [M(CO)n(SAS)2] (M = Mo, W n = 2,3 SAS = S2CNR2, S2COEt, S2CNC4H4, S2PR2, etc.) have been published. These are not considered here because they do not contain a halo group. 

The observed five-, six- and seven-coordinate complexes of Figures... 

Iridium(V) complexes, although not common, take a variety of forms, including polynuclear and multidentate structures as well as five-, six- and seven-coordinate species, some of which are stable and have been characterized by X-ray crystallography. 

Some recent advances are the isolation of aryl isocyanide complexes of chromium(III) Cr(CNAr)6]X3, and six- and seven-coordinate alkyl isocyanide complexes of chromium(II) Cr(CNR)6j7]X2. All but the chromium(O) and the seven-coordinate chromium(II) complexes lave less than 18 electrons in the valency shell. 

Numerous complexes of the type MHg(CN)2X-nH20 (M = Na, K, Rb, Cs X = C1, Br, I, NCO, NCS, N3) have been prepared by the reactions of MX with Hg(CN)2.207,208 The compounds are double salts closely related to KHg(CN)2I, which consists of Hg(CN)2 molecules with each mercury weakly bonded to four iodines (Hg—I = 338 pm).209 Some species with bivalent cations have been established. The compounds M Hg(CN)2SCN 2-4H20 (M = Mg, Ca, Sr, Ba), formed by the reactions between Hg(CN)2 and the corresponding M(SCN)2, have been characterized by X-ray structural analysis.210 The Mg salt has a difficult structure with six- and seven-coordinated mercury (Figure 8) the other three complexes have layer structures similar to KHg(CN)2I. 

A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. Two of the older methods, namely, the reaction of the hexacarbonyls with halogens in the presence of an isocyanide (775,116) or reactions of the salt Ag4Mo(CN)8 with isocyanides (74), have given six- and seven-coordinate products. Recently, however, the discovery of the reductive or nonreductive cleavage of multiple metal - metal bonds in dinuclear group VIA compounds by isocyanides has provided a facile route to the synthesis of a variety of homoleptic and related isocyanide complexes of these metals in reasonable yields. 

This article is concerned with six- and seven-coordinate halocarbonyl complexes of molybdenum(II) and tungsten(II) (Section II), of which earlier reviews by Colton et al.2 on substituted halocarbonyls of group 6 transition metals and on steric effects in substituted halocarbonyls of molybdenum and tungsten3 have been published. Also, reviews on the structures of seven-coordinate compounds by Drew in 19774 and by Melnik and Sharrock in 1985s contain relevant material. 

SIX- AND SEVEN-COORDINATE HALOCARBONYL COMPLEXES OF MOLYBDENUM(II) AND TUNGSTEN(II)... 

X-Ray Crystal Structure Determinations of Six- and Seven-Coordinate Halocarbonyl Complexes Reported after 1984... 

Iridium(III) complexes (d6) are diamagnetic and low-spin ((f,). The coordination number of most known iridium(III) complexes is six, though a few five- and seven-coordinate complexes have been reported. Iridium(III) forms complexes with soft ligands. The [Ir(H20)6]3+ ion is unknown no solid iridium(III) complexes with coordinated H20 groups are known with the exception of a few aquachloro complexes.197... 

Support for the above conclusions was also obtained from an analysis of the Cd NMR and solid state MAS spectra of the five, six, and seven coordinate cadmium texaphyrin complexes 116, 156, and 157, respectively [70]. From the solid-state MAS results, a single tensor was observed for the five-coordinate complex 116 with a corresponding isotropic chemical shift of 194 ppm. The MAS spectrum of a complex prepared in the presence of pyridine, however, revealed two tensors, presumed to be due to a mixture of six and seven coordinate species. Based upon the isotropic chemical shifts and the symmetry of the tensors the six coordinate species was assigned to the isotropic shift at 188 ppm and the seven coordinate species assigned to the isotropic shift at 221 ppm. The MAS spectrum of the benzimidazole complex of cadmium texaphyrin consists of only a single... 

Thus far the synthesis of six- and seven-coordinate halocarbonyl complexes has been restricted to oxidation by CI2, Br2 and U. This section deals with a range of other oxidizing agents such as ICl, IBr, GeC, SnCl4, and HgXj, and organometallic complexes such as /ac-[MnCl (CO)3(S2CPCy3)]. 

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