Clusters homoleptic

Elements of this group, like metals of groups 8 and 9, form mononuclear and cluster homoleptic complexes in which metals adopt a zero oxidation state. The following homoleptic complexes of these elements are known [M(CNR)4] (M = Ni, Pd, Pt), [Ni(CNR)4], [Ni4(CNR),], [Ni4(CNR)4] , [NigiCNR). ], [Ni4(CNR)i4](C104)s, and [Ni3(CNR),i](C104)6. Linear structures were assigned for the last two compounds.Monomeric platinum and palladium compounds... 

Pr )2(PPh3)7](PFg)2-3249 and [Au, i(PPlioIV1 e)i(l]3, 3250 The structure of the mixed-ligands clusters corresponds to a C3v metal skeletal framework and the homoleptic to a centered bicapped square antiprism (Figure 32). Other stoichiometries are known as, for example, [AunCl2(PPh3)8]Cl.3251... 

Whereas a large number of metal-sulfur clusters are present in nature, carbonyl clusters are exclusively products of chemical synthesis. They have been widely used in industrial catalytic processes17 and some of these processes are triggered by the redox aptitude of these species.lc g As for the metal-sulfur clusters, we will briefly discuss their structures and their propensity to donate/accept electrons in order of increasing nuclearity. We will consider only homonuclear and homoleptic metal-carbonyl derivatives. However, it is noted that heteronuclear derivatives are gaining considerable interest due to the synergistic effect of metal-metal bonds possessing a polar character.lc,ld... 

In earlier work, no molecular structural data were available on mononuclear M amides, but this has changed markedly in the last three (and particularly last two) decades. Another significant development has been the use of the title compounds, and especially the homoleptic dimethylamides M(NMe2)3, as reagents, or synthons for the preparation of bridging binuclear imides 7 (Section 10.3) or cluster imides (e.g. 3 7) (Section 10.4), or Bi[N(SiMe3)2]3 as a precursor for atomic layer deposition.Heteroleptic amidometal chlorides have featured as precursors to cationic metal(III) amides (Section 10.2.4). 

The oxidative addition reaction of [Ru6C(CO)ifJ2 4 and pentamethyl-cyclopentadiene in the presence of [Fefrj-CsHs)]4 in dichloromethane at room temperature affords HRufiC(CO)l4(i7-CsMe5) 105 in 10% yield together with larger quantities of the homoleptic cluster Ru6C(CO)17 2.29... 

Ligand displacement from labile precursors has led to the homoleptic clusters Ni4(CNR)7, Ni4(CNR)6 and Ni8(CNBu )12 (27,22), Pt3(CNR)6 (23), and Pt7(CNXylyl),2 (20), which represent rare examples of polynuclear homoleptic metal(O)-isocyanide clusters. 

Naturally, the ideal source of starting materials for homoleptic metal isocyanide compounds is via metal carbonyl complexes, but previously only with the two carbonyls Ni(CO)4 (24) and Co2(CO)g (25) has direct substitution of all carbonyl groups been effected. Recently, however, remarkable discoveries by Coville and co-workers (26-31) on the transition-metal-catalyzed substitution of carbonyl groups in monomeric and cluster compounds have shown that Fe(CNR)s, Mo(CNR)6, and Ir4(CO)5(CNR)7 (32) can be prepared in high yield by stepwise substitution from the parent carbonyl. 

To date, only nickel, palladium, and platinum have formed homoleptic metal(O) isocyanide clusters. This may be due to two factors, either the greater tendency to nucleation in the group or simply the ready access to the highly labile metal(O) precursors M(COD)2 [M = Ni (137), Pt (138)]. 

This catalytic labilization of carbonyl groups has been extended to the replacement of carbonyls in metal carbonyl clusters. Metal cluster complexes are at present the subject of extensive studies, partly because of their possible relevance as models for chemisorbed metal surfaces and because of their catalytic activity. The majority of these clusters contain carbonyl ligands, and these have been prepared from the vast number of metal carbonyl precursors generally available by a variety of synthetic methods usually without recourse to designed or rational procedures. In metal isocyanide chemistry, however, suitable precursors are lacking, and as a consequence, there are few routes to homoleptic metal-isocyanide clusters, and few isocyanide clusters are known (see Section IV,A). 

Some remarkable examples of positive cooperativity in helicate formation have also been reported by Ken Raymond from the University of California, Berkley, USA. Raymond s systems use the deprotonated 1,2-dihydroxy benzene dianion moiety as a basic ligating motif. Based on the identity and length of spacer units between two such binding domains, a large array of triple helices and coordination clusters (cf. Section 10.5.3) self-assemble selectively. In one example, a mixture of three such ligands in the presence of an appropriate ratio of Ga3+ results in the selective formation of three individual, homoleptic triple helices (Scheme 10.26), 23... 

The first reports of LA ROP using yttrium complexes focused on homoleptic alkoxide complexes, such as cluster complexes of the form Ln5(p-0)(0R)i3 [27]. A patent, and preprint, published by DuPont described the application of a homoleptic yttrium alkoxide, Y(OCH2CH2NMe2)3, formed in situ by reaction of yttrium fm-Ao-propoxide with ALV-dimethy I am i noethanol. The complex showed a very high rate (kob = 0.5 s 1, [Y]0 = 3 mM) and reasonable polymerization control [28]. 

B. von Ahsen, M. Berkei, G. Henkel, H. Willner, and F. Aubke, The Synthesis, Vibrational Spectra, and Molecular Structure of [Ir(CO)6][SbF6]3-4HF - The First Structurally Characterized Salt with a Tripositive, Homoleptic Metal Carbonyl Cation and the First Example of a Tetrahedral Hydrogen-Bonded (HF)4 Cluster, J. Am. Chem. Soc. 124, 8371-8379 (2002). 

G. Longoni, C. Femoni, M. C. Iapalucci, and P. Zanello, Electron-sink Features of Homoleptic Transition-metal Carbonyl Clusters, in Metal Clusters in Chemistry (Eds. P. Braunstein, L. A. Oro, and P. R. Raithby, Wiley-YCH, Weinheim, 1999, Yol. 2, Chap. 3.9). 

Average bond length and angle parameters for the homoleptic tris(dithiolene) complexes are less sensitive to the identity of the transition metal or dithiolene ligand than those of bis(dithiolene) structures. Ranges of values are summarized in Fig. 22. Average M—S bond lengths cluster between 2.263 and 2.543 A, with values for two [Fe(mnt)3]2 units (2.263 and 2.269 A) (18), Co CAlCO),, 3... 

Some compounds and their reactions have been noted in Section 18-G-2. The homoleptic carbonyls are clusters such as M4(CO)i2 and M6(CO)i8. The hydride, HRh(CO)4, is very much less stable than HCo(CO)4 and has been made only under ca. 1400 atm pressure since it readily loses H2 to give clusters. Both Rh and Ir give anions [M(CO)4] and [M(CO)3]4 as R3NH+ salts as well as various cluster anions such as [Ir8(CO)22]2 and [Rh5(CO)i5] . Hydrido and other substituted polynuclear carbonyls are known. 

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