Isolable group 2 metal bonded dimers bonding

A versatile application of Co(allyl)3 has been found in its reaction with butadiene, which effects catalytic dimerization under very mild conditions. Individual reaction steps are reproduced in Scheme 19. These reactions lead to the isolated and structurally characterized complex (13) as the true catalyst. Scheme 19 is in fact a collection of all the classical steps involved in homogeneous transition metal bond-forming reactions. The first step consists of replacement of two allyl groups by butadiene, which is reductively eliminated as diallyl, similar to what has been found with cod and with other two-or four-electron ligands. 

When the terminal carbonyl group on the Fe atom is replaced by a ligand such as PPhj, P(OPh)3, or PMcs, no isolable insertion products can be obtained . Other mixed transition metal insertion products such as CpCr(CO)3SnBr2CpMo(CO)3 and CpFe(CO)2SnCl2Co(CO)4 are prepared by reacting the trihalotin derivative of one of the metals with the metal-metal-bonded dimer of the other, but not by a direct insertion reaction ... 

Isolable Group 2 Metal(l)-Metal(l) Bonded Dimers... 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

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