Metallaheteroborane chemistry: Part 16. Contrasting metal to heteroborane bonding modes in isoelectronic {MC2B9} and {MAs2B9} clusters. Synthesis and characterisation of [9-{Fe(CO)2(η5-C5H5)}-nido-7,8-C2B9H12], [7-{Fe(CO)2(η5-C5H5)}-nido-7,8-As2B9H10] and [7-{M(CO)2(η7-C7H7)}-nido-7,8-As2B9H10], whe

Structural details of previously reported [9-{Fe(CO)2(η5-C5H5)}-nido-7,8-C2B9H12] 1 and [7-{Fe(CO)2(η5-C5H5)}-nido-7,8-As2B9H10] 2, and of the new compounds [7-{M(CO)2(η7-C7H7)}-nido-7,8-As2B9H10], where M is Mo 3 or W 4, were established by 1H and 11B NMR spectroscopy and, for 1, 2 and 3, by single-crystal X-ray diffraction methods. The NMR data showed that the {C2B9} and {As2B9} cages were of the nido type, with each having a single bridging hydrogen atom on the open face of its cage. The X-ray diffraction study of 1 confirmed that the {Fe(CO)2(η5-C5H5)} unit is attached exo to the cage by an unusual interaction of the iron atom with a B–H bond. X-Ray diffraction studies of 2 and 3 showed that the transition-element atoms bonded to arsenic donor sites. The dimensions in the {Fe–H–B} region are Fe(1)–B(9) 2.377(1), Fe(1)–H(9) 1.59(2), B(9)–H(9) 1.25(2) A and Fe(1)–H(9)–B(9) 113(1)°. There was considerable disorder in the diarsenaborane cages and (ηn-CnHn) sections of the molecules 2 and 3. The principal interatomic distances of interest are Fe(1)–As(7) 2.334(1) A and Mo(1)–As(7) 2.598(1) A in 2 and 3, respectively. The 57Fe Mossbauer spectrum of 1, obtained at 78 K, comprised a well-resolved doublet with measured parameters δ = 0.113 mm s−1 and ΔEQ = 1.94 mm s−1. The bonding in the {Fe–H–B} region of 1, when compared with bonding in other ferraboranes, and with models that have been described for agostic hydrogen M–H–B and M–H–C interactions, is shown to be unique. Photolysis of 1 in tetrahydrofuran solution produces the known “mixed-sandwich” paramagnetic species [3-(η5-C5H5)-closo-3,1, 2-FeC2B9H11] 5 in 85% yield.

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