Comparison of B-B pi-bonding in singly reduced and neutral diborane(4) derivatives: Isolation and structure of [{Li(Et(2)O)(2)}{MeO(mes)BB(mes)OMe}]

A single-crystal X-ray diffraction study of [{Li(Et(2)O)(2)}-{MeO(mes)BB(mes)OMe}] 1 (mes = C(6)H(2)Me(3)-2,4,6) allows the first structural comparison between singly reduced and neutral diborane(4) species; compound 1 features a shortened B-B bond distance of 1.636(7) Angstrom which may be compared to the 1.724(9) Angstrom observed in the neutral precursor MeO(mes)BB(mes)OMe 2 and a distance of 1.62-1.64 Angstrom observed in related doubly reduced [R(2)BBR(2)](2-) species; the B-B bond in 1, which has a formal pi-bond order of 0.5, is thus similar in length to that found in the doubly reduced diborane(4) dianions that possess a formal B-B pi-bond order of unity.