ISSN:
0570-0833
Schlagwort(e):
Cluster compounds
;
Transition metals
;
Metal-metal interactions
;
Molybdenum
;
Tungsten
;
Chemistry
;
General Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant dπ orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled dπ orbitals. Both ligands readily form bridges that span M—M bonds. In solution fluxional processes that involve bridge-terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)3], [CpW(CO)2], [Co(CO)3], and CH are related by the isolobal analogy. Thus the compounds [(RO)3W ≡ W(OR)3], [Cp(CO)2W≡W(CO)2Cp], hypothetical [(CO)3Co≡Co(CO)3], and HC≡CH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding - e.g., [W3(μ3-CR)(OR′)9] versus [Co3(μ3-CR)(CO)9] and [W4(C)(NMe)(OiPr)12] versus [Fe4(C)(CO)13] - but differ with respect to M—M bonding. The carbonyl clusters use eg-type orbitals for M—M bonding whereas the alkoxide clusters employ t2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or M—M bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M2(OR)6] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.
Zusätzliches Material:
6 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/anie.198904321
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