Higher nuclearity clusters

The UV-visible spectra of Ni and Nia have also been identified in argon matrices (93) Ni absorbed at 377, 529, and 4l0 nm, with vi-bronic structure on the first two bands, and with spacing of—330 cm , and Nis absorbed at 420 and 480 nm, the latter band showing vibrational spacing of -200 cm" . Higher-nuclearity clusters were observed, but not characterized. After prolonged warm-up of these matrices, nickel colloid was formed (93). 

Professor Cotton s studies have considerably clarified our understanding of di- and tri-nuclear metal-metal bonded compounds. For higher nuclearity clusters, rationalisation of structures, bonding, reactivity etc., must be much more tenuous because of the increased number of variables (metal-metal, metal-ligand, steric effects) now present. However, I would briefly like to present a few trends which seem to be emerging in this area. 

Addition of either nucleophilic or electrophilic metallic species can result in the capping of triangular- or square-metal faces in carbonyl clusters. These redox reactions provide high yield syntheses of higher nuclearity clusters and somewhat resemble surface reconstruction on metals. With a few examples,... 

The pyrolysis of Os3(CO)12 to higher nuclearity clusters is the subject of continuous investigation. This is because although Os6(CO) 8 is the major... 

The synthetically useful dianions [M3(CO)u]2- were first isolated by Shore and co-workers as the Ca2+ (M=Ru) and the K+ (M=Os) salts by the reduction of M3(CO)12 using alkali metal benzophenone solutions in THF.1 [Ru3(CO) J2 reacts with Ru3(CO) 2 to form the higher nuclearity clusters [Ru4(CO)13]2- and [Ru6(CO)i8]2- but the triruthenium anion can be obtained in high purity by slowly adding triruthenium dodecacarbonyl to an excess of reducing solution using vacuum-line techniques.2 Vacuum-line syntheses of both dianions have been described in detail.1... 

REACTIONS OF Ru5C(CO)15 AND Ru6C(CO)17 THAT RESULT IN HIGHER NUCLEARITY CLUSTERS... 

Metal carbonyl anions react with main group halides and oxides to yield a number of main-group transition-metal carbonyl complexes in good yields. These complexes serve as starting materials for a number of higher nuclearity cluster complexes. 

All the above complexes, 9-12, are isolated from reactions involving gallium or indium metal and [Mn2(CO)10] or [Re2(CO)10] in an autoclave, although 11 has also been obtained by thermolysis of 6. Some details on the reactivity of 11 toward phosphonium halides (23) have been reported and indicate formation of the halo-indium complexes [PPh4]2[Mn2(CO)8 /j-lnX2 2]. A higher nuclearity cluster has also been obtained in the reaction of [Re2(CO)10] with indium metal, viz. [Re4(CO)12 /i3-InRe(CO)5 4], 13 (24,24a). This consists of a central tetrahedral Re4(CO)12 core with each face capped by an InRe(CO)s fragment as shown in Fig. 4. 

In the related V clusters a similar core structure is seen but with shorter Fe4S3 bond distances. The ten atom Fe4( 43-S)3( 4-S)3 core is contained in the FeMoco structure, Figure 3, with the same bond connectivity and similar spatial arrangement and, as such, probably provides the best starting point for development of higher nuclearity clusters related to the nitrogenase cofactors. 

The 31P H NMR spectra of a number of heteronuclear gold cluster compounds are found to be deceptively simple and NMR studies have been used as a probe of the behavior of these species in solution. This is especially true of the higher nuclearity clusters, which often exhibit spectra that are much simpler than would be predicted on the basis of their solid-state structures. For example, [Pt(H)(PPh3)(AuPPh3)7]2+, which adopts the solid-state structure illustrated in Fig. 9 (137) in which the phosphine ligands occupy several different chemical environments within the molecule, shows only two resonances in the 31P 1H NMR spectrum. These are in a ratio of 7 1 and exhibit satellites due to coupling to the central platinum nucleus as Fig. 10 illustrates. 

The free or polymer-bound bis(arene)metal complex can also react with metal atoms. Francis et al. (44) first published evidence that the siloxane-bound ic-complexes are converted to dimers and higher nuclearity clusters by additional metal atoms. Their experiments were conducted on quiescent thin liquid films of polymer applied to the optical window of a cryotip (see above, Small Scale Syntheses). Low nuclearity polymer-encapsulated molecules of Tin, Vn, Crn and Mon (n = 2-5) were inferred from quantitative studies of the metal atom aggregation process. The initial reaction appears to occur as follows ... 

Low-valent palladium and platinum form numerous clusters, usually based on the M3 triangle, as in the carbonyl phosphine complexes 18-H-I. Higher nuclearity clusters can be built up from edge-sharing triangles, e.g., the butterfly structure 18-H-II. 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

For alkynes bonded to higher nuclearity clusters no overall molecular orbital treatment encompassing all the variations in geometry has appeared yet. However, there are a small number of examples of specific alkyne-substituted clusters which have been analyzed by one type of molecular orbital treatment or another, and a number of these have been mentioned in Section III,G because photoelectron spectroscopy has been used as an aid to assignments. CNDO calculations (397) on Fe3(CO)9(EtCCEt) (390) and M3(CO)9(/i-H)(CCR) (M = Ru, Os) (391) and Fenske-Hall calculations (398) on Co4(CO)10(PhCCH) (389) indicate that there is net back donation into alkyne n orbitals, which increases as the number of metal atoms to which the ligand is bonded increases. The normally accepted view of considering the interaction... 

When four monodentate ligands, including solvent molecules, are bound to the central metal ion, a trans disposition of the bridging cyanides is the preferred orientation [Fig. 18(c)]. The only exception to this trend is found in the structure of cw- [(pcq)Fe (CN)3l2[Mn (Me0H)2(H20)2] (119) [Fig. 18(kinetic products, given the presence of terminal cyanides or labile solvent molecules in their structures. The availability of these potential coordination sites renders such trinuclear complexes convenient scaffolds for the assembly of higher nuclearity clusters or extended structures (Section V). 

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