Dimetal interactions

Quadruple dimetal interactions provide an interesting test ... 

Metals of the second and third transition series are well known to be characterized by multiple dimetal interactions of orders 3, 3f, and 4 [29]. The large reported errors in the measured diatomic dissociation energies for some of these metals are interpreted as due to spectroscopic activation, producing equiUbrium mixtures of compounds of poorly resolved bond order. It is noted that in all such cases, an average over two bond orders reproduces the experimental data rather well. 

The interaction of butadiynediyl dimetal complexes [Fp -C -CsC-M, Fp =FeCp (CO)2, M= Fp, Rp, SiMea, Rp= RuCp(CO)2] with diiron nonacarbonyl, Fe2(CO)9, results in the formation of a mixture of products, as is also observed in the case of their interaction with organic acetylenes. Interesting polymetallic complexes, propargylidene-ketene compounds, zwitterionic cluster compounds, and pa-p -propargylidene-cyclobutene compoimds were isolated from the reaction mixtures and successfully characterized. The product distributions were found to be dependent on the metal fragment (M) at the other end of the C4 rod. The results of the reaction are described... 

The interaction of a-orbitals of M—C and M —C bonds a-a conjugation) in the dimet-alla compounds, RsMM Rs, was evaluated by quantum-chemical calculations. Thus, the HOMO of MesSnSnMes is localized mainly on Ihe Sn atoms bul il also includes contributions of C and H atoms, whose magnitude exceeds 30% . Calculations of H3MM H3 (M and M = C, Si, Ge, Sn, Pb) show the necessity to take into account both the bonding and antibonding orbitals of M—H, M —H and M—M . Orbital inleraclions are enhanced as the atomic numbers of M and M increase . 

As we have noted in a previous section of this chapter, cumulenic resonance forms present one extreme description for dimetal complexes linked by all carbon bridges. They are frequently encountered in some oxidation state (mostly the dioxidised one) and also contribute to intermediate ones between the purely cumulenic and oligoynediyl or the cumulenic- and the alkynyl-bridged dicarbyne forms (see Schemes 6.6-6.8). Purely cumulenic wires are encountered in ot,a)-di- or -tetraferrocenyl substituted cumulenes, but no evidence other than a splitting of the ferrocenyl-based redox waves has been presented to support the presence of electronic interactions between the fer-rocenyl end groups across the cumulenic ligands. Based on the results of ex-... 

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A few other examples of chain-forming coordination polymers are mentioned. A polymer 31 was obtained under inert conditions by the reaction of 4,4, 5,5 -tetramethyl-2,2 -bisphosphinine with naphthalene/Na in dimethoxy-ethane [85]. The bisphosphinine is reduced two-fold. In the deep purple oxygen-sensitive crystals the chain is held together by electrostatic interactions through alternation between three and one Na(I) cations. The reaction of pentaphosphaferrocene with CuX (X = Cl, Br, I) was used to prepare yellow-brown crystals of one- and two-dimensional poljmiers in which the phospha-ferrocene units are connected by CuX units [86]. One-, two- and three-dimensional tubes were prepared by employing dimetal building units [87]. For example, a one-dimensional tube was obtained from a Rh2 compound with malonic acid and linkers such as tra 5-l,2-bis(4-pyridyl)ethylene. 

Axial interaction of an aryl unit with the [Ru-Ru] bond tends to increase the metal-metal distances. Petrukhina et al. isolated two complexes by codeposition of 2 and 8 with [2.2]paracyclophane to yield (17) and (18), respectively (Scheme 11) [68]. A sandwich structure with the aromatic moiety entrapped between two dimetal imits is observed. The [Ru-Ru] distance increases from 2.627(9) A in 8 to 2.656(3) A in 18 on axial coordination of the arene moiety. Similarly, a change of [Ru-Ru] distance from 2.673(1) A in 2 to 2.678(3) A in 17 was also observed. The inter-centroid distances between the two rings in [2.2]paracyclophane group are shorter (2.974(4) A in 17 and 2.982(5) A in 18) compared to the free [2.2] paracyclophane ligand (3.09 A). This supports the hypothesis that coordination of aryl group to the electrophilic ruthenium centers allows the aromatic decks to move closer which also increases the [Ru-Ru] bond distances. 

A family of mono-(I) and bis-terphenyl (II) quadruply bonded complexes (see Fig. 2) have been synthesised by reaction of the corresponding dimetal tetracarboxlate with one or two equivalents of the lithium salt of the terphenyl ligand.The crystal structures of these compounds show short M-M bond distances of around 2.09 A for the molybdenum complexes and 2.213(1) A for the tungsten complex trans-W2(02CCF3)2(C6H3-2,6- C6H3-2,6-Me2 2)2- Theoretical calculations show that the interaction... 

Fulvalene dimetal complexes have been profitably employed in studies of metal-metal interactions across a spacer ligand with two nominally aromatic components. Work on the oxidation of biferrocene, 1, bis(fulvalene)diiron, 2, and their analogues occupy a central focus in these investigations.[1] The ferrocenyl moiety... 

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