σ-Arsolido complexes†

The σ-stannyl complexes [M(SnⁿBu₃)(CO)_n(η⁵-C₅H₅)] (n = 3, M = Mo, W; n = 2, M = Fe) serve as mild reagents for the installation of σ-arsolyl ligands in transmetallation reactions with As-chloro-arsoles ClAsC₄R₄ (R = Me, Ph) to afford [M(σ-AsC₄R₄)(CO)_n(η⁵-C₅H₅)]. The reaction of [Cr(SnⁿBu₃)(CO)₃(η⁵-C₅H₅)] with ClAsC₄Ph₄ most likely proceeds in a similar manner but is immediately followed by rapid formation of (AsC₄Ph₄)₂ and [Cr₂(CO)₆(η⁵-C₅H₅)₂]. The reaction of [Mo(SnⁿBu₃)(CO)₃(η⁵-C₅H₅)] with ClAsC₄(SiMe₃)-2,5-Me₂-3,4 is accompanied by monodesilylation to afford [Mo{σ-AsC₄(SiMe₃)-2-Me₂-3,4}(CO)₃(η⁵-C₅H₅)]. The slow reaction of [Fe(SnⁿBu₃)(CO)₂(η⁵-C₅H₅)] with ClAsC₄Me₄ produced only traces of [Fe(σ-AsC₄Me₄)(CO)₂(η⁵-C₅H₅)] due to competition with the Diels–Alder type dimerisation of the haloarsole. Although attempts to decarbonylate the σ-arsolyl complexes were unsuccessful, computational analysis suggests that the trigonal ‘XL’ arsolenium coordination mode is viable.