Formation of ferraboranes from pentaborane(9) or BH3 · thf and an electron-rich cyclopentadienyl iron phosphine hydride

[(η5-C5R5)Fe(PMe3)2H] (R = H, Me) can be made in good yields in a simple one-pot reaction between FeCl2, PMe3, C5R5H (R = H, Me) and Na/Hg in thf. Reaction of [(η5-C5H5)Fe(PMe3)2H] with pentaborane(9) gives the known metallaborane [(η5-C5H5)-nido-2-FeB5H10] (1) in improved yield as well as the new metallaboranes [(η-C5H5)-nido-2-FeB5H8{μ-5,6-Fe(η5-C5H5)(PMe3)(μ-6,7-H)}] (2), [(η-C5H5)(PMe3)-arachno-2-FeB3H8] (3), [(η5-C5H5)2-capped-nido-2,3-Fe2B4H8] (4), [(η5-C5H5)-nido-2-FeB4H7(PMe3)] (5) and [(η5-C5H5)-nido-2-FeB5H8(PMe3)] (6). Reaction of [(η5-C5Me5)Fe(PMe3)2H] with pentaborane(9) gives predominantly [(η5-C5Me5)-nido-2-FeB5H10] (7) and [(η5-C5Me5)(PMe3)-arachno-2-FeB3H8] (8). Reaction of [(η5-C5H5)Fe(PMe3)2H] with 2 equiv. of BH3 · thf gives low yields of ferrocene and compound 3. Compound 7 thermally isomerises to the apical isomer [(η5-C5H5)-nido-2-FeB5H10] (9) in low yield. Compounds 1 and 7 deprotonate cleanly in the presence of KH at the unique B–H–B bridge to give [(η5-C5H5)-nido-2-FeB5H9−][K+] (10) and [(η5-C5Me5)-nido-2-FeB5H9−][K+] (11) respectively, whilst 6 deprotonates more slowly at one of two equivalent B–H–B bridges to give the fluxional anion [(η5-C5H5)-nido-2-FeB5H7(PMe3)−] (12).