Free and coordinated biarsolyls.

A trace side product from the reaction of [Mo(AsC 4 Me 4 )(CO) 3 (η-C 5 H 5 )] with [Mn(THF)(CO) 2 (η 5 -C 5 H 4 Me)] was identified as the biarsolyl complex [Mn 2 {μ-(AsC 4 Me 4 ) 2 }(CO) 4 (C 5 H 4 Me) 2 ] on the basis of a strategic synthesis from [Mn(THF)(CO) 2 (η 5 -C 5 H 4 Me)] and the pre-formed and structurally characterised biarsolyl (AsC 4 Me 4 ) 2 . Crystallographic and computational data for this first example of a biarsolyl complex and free biarsolyls are discussed in the context of those for free biarsolyls.