Butterfly cluster compounds

Butene, catalytic oxidation, 35 168-169 n-Butene dimerization, 31 36-37 composition change, 31 25-26 But-2-en-l-ol oxidation, 41 307 Butler-Volmer equation, 40 89 Butterfly cluster compounds, 38 294-295 Butyl alcohols... 

In addition to planar (two-dimensional) faces, the surfaces of supported metal particles consist of edges between surfaces as well as steps and kinks on otherwise planar surfaces (2J). These sites are thought to be highly active for the adsorption and bond dissociation of molecules. Analogies have been drawn between the binding of small molecules (CO, H2, and CH4) to coordi-natively unsaturated sites of metal surfaces and the binding of molecules and atoms (C, N, and S) in clefts of metal clusters. The simplest examples are the four-metal butterfly cluster compounds (Fig. 4), where the cluster geometry... 

Fig. 4. Butterfly cluster compounds useful for molecular modeling of intermediates at edge and kink sites on an lr(/00) surface.

Interest in the organotin derivatives of the acids and thioacids of phosphorus arises from their potential biological action and the wide variety of structures that have been identified by X-ray diffraction. These structures are often particularly complex when the compounds are derived from the partially hydrolyzed mono- or diorganotin compounds, and words such as cubes, drums, crowns, butterflies, clusters, oxygen-capped clusters, and extended clusters have been use to describe them. References to the early work are given in Ref 351, and a recent review is available.352... 

Although it is always somewhat risky to draw conclusions about surface reactions from solution experiments, a number of such studies support the curbide/carbene mechanistic proposal. A model compound for the carbide proposal is a butterfly cluster formed from an unusual six-coordinate carbide 190... 

The crystallographically determined structure of 15 confirms the presence of a butterfly arrangement of iron atoms within which the boron atom resides in contact with all four metal atoms [Fe—B = 2.044(6), 2.047(6), 1.966(6), and 1.974(6) A] and 0.31 A above the Fe —Fe vector the internal dihedral angle of the Fe4 butterfly is 114.0°. These parameters are compared in Table II with those of related clusters. Compound 15 was first isolated as a product from the reaction of Fe2(CO)6B2H6 with Fe2(CO)9 (60), and evidence for increased iron-boron interaction is observed in a dramatic change in nB-NMR spectral shift (8) from S — 24.2 to +116.0. 

A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

Energetically, the most stable of the six possible St = 0 states comes about when Sj3 and S24 both have their maximum values of 5. The next higher states Sx = 1 (ca. 90 cm" ) and Sx = 2 (ca. 280 cm ) also derive from the coupling of S13 and S24. In a subsequent detailed report on the butterfly cluster [Fe402(02CCH3)3(salox)2L2](C104), where L is the triazacyclononane macrocycle, Chaudhuri and Haase and coworkers 164) came to conclusions very similar to those described for the bipy and [HgBpzg]" compounds, the latter characterized earlier by Lippard and co-workers 149, 165). 

The tetrahedral —> butterfly rearrangement is quite common in cluster chemistry but few examples are reversible. One example of a reversible process involves the interconversion of Pd4(CO)5(PBu 3)4 to Pd4(CO)6(PBu 3)4 (Scheme What is perhaps surprising about this reaction is that Pd4(CO)5(PBu"3)4 has a butterfly geometry whereas the cluster with the additional CO ligand has a tetrahedral palladium skeleton. This is clearly in contrast with the usual observation that tetrahedral clusters have two fewer valence electrons than butterfly clusters i.e. 60 and 62 cluster valence electrons (CVE), respectively). This anomaly is a consequence of the capacity of the palladium atom to form stable compounds with 14 and 16 valence electrons and as such Pd4(CO)s(PBu 3)4 and Pd4(CO)e(PBu"3)4 do not conform to the usual total electron counts for compounds that obey the EAN rule but have 54 and 56 electrons, respectively. 

OssC(CO) (C02R)X] (X = H or I) has been determined, and the metal core shown to exhibit the expected bridged-butterfly geometry. Another preliminary report on nucleophilic attack at a coordinated CO of a carbido carbonyl cluster compound has also appeared. The heterometallic carbido cluster anion [Co6Ni2C2 (C0)i6 ] results from the reaction of [003(00)9(001)] with [Nig (C0)i2 ] and contains two C atoms located in a cavity in the cluster and separated by only... 

Core expansion of the /.ts-oxotriiron cluster 107 is reported (Scheme 8). Interaction with Mn and Re electrophiles leads to the formation of the heterotetranuclear /X4-0X0 clusters with the butterfly metal array 108 and the ji -oxo clusters with expanded metal frameworks 111-113, " which arise by Fe20 108 and Fes face capping 111-113, respectively. Interstitial clusters (e.g., 14- and 15-type complexes Scheme I) are rare for the 0x0 cluster compounds (see below) in contrast to the related carbido and nitrido clusters. 

Typical structural motifs for S-, Sc-, and Te-containing iron cluster compounds shown in Scheme 9 (E, E = S, Se, Te cf. Scheme 1) involve the tetrahedral monocapped species with the closed iron triangle 118 and the dicapped triiron species with the open Fcs framework 121, which can also be regarded as a square-pyramidal c2,EE skeleton with an Fcafifi basal plane 121. The protonated forms of 118 (119 and 120) and the dicapped octahedral structure 122 based on an Fc4 square are also known. Butterfly 11 and interstitial structures (14 and 15 Scheme 1) are very rare for non-first-row main group elements, and a rare example of the /44-S butterfly cluster 116 is discussed in Section... 

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