Metallaheteroborane chemistry: Part 16. Contrasting metal to heteroborane bonding modes in isoelectronic {MC2B9} and {MAs2B9} clusters. Synthesis and characterisation of [9-{Fe(CO)(2)(eta(5)-C5H5)}-nido-7,8-C2B9H12], [7-{Fe(CO)(2)(eta(5)- C5H5)}-nido-7,8-As2B9H10] and [7-{M(CO)(2)(eta(7)-C7H7)}- nido-7,8-As2B9H10], where M is Mo or W

Structural details of previously reported [9-{Fe(CO)(2)(eta(5)-C5H5)}-nido-7,8-C2B9H12] 1 and [7-{Fe(CO)(2)(eta(5)-C5H5)}nido- 7,8-As2B9H10] 2, and of the new compounds [7-{M(CO)(2)(eta(7)-C7H7)}-nido-7,8-As2B9H10], where M is Mo 3 or W 4, were established by H-1 and B-11 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)(eta(5)-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) Angstrom and Fe(1) - H(9) - B(9) 113(1) degrees. There was considerable disorder in the diarsenaborane cages and (eta(n)-CnHn) sections of the molecules 2 and 3. The principal interatomic distances of interest are Fe(1) - As(7) 2.334(1) Angstrom and Mo(1) - As(7) 2.598(1) Angstrom in 2 and 3, respectively. The Fe-57 Mossbauer spectrum of 1, obtained at 78 K, comprised a well-resolved doublet with measured parameters delta = 0.113 mm s(-1) and DeltaE(Q) = 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-(eta(5)-C5H5)-closo- 3,1, 2-FeC2B9H11] 5 in 85% yield.