BORANE CLUSTER PHOTOCHEMISTRY .3. THE PHOTOCHEMISTRY AND ORGANOMETALLIC REARRANGEMENT CHEMISTRY OF SIGMA-METALATED SMALL BORANE CLUSTER COMPOUNDS

The photochemistry of sigma-metalated pentaborane(9) complexes [2-(Fe( eta(5)-C5H5)(CO)(2))B5H8] (1) and [2,4-(Fe(eta(5)-C5H5)(Co)(2))(2)B5H7 ] (2) is described. These compounds readily undergo a clean photoinduc ed decarbonylation reaction to lose a carbon monoxide ligand upon irra diation to generate an unsaturated, 16-electron iron intermediate. The isolated products arise from the insertion of the reactive 16-electro n iron center into the basal plane of the borane cluster. Thus, the co nversion of the square-pyramidal B-5 cluster complexes, 1 and 2, into the pentagonal-pyramidal FeB5 vertex complexes, 2-(eta(5)-C5H5)-2-(CO) -2-FeB5H8 (3) and ta(5)-C5H5)(CO)(2))-2-(eta(5)-C5H5)-2-(CO-2-FeB5H7 ( 4), respectively, was observed. The complete characterization of the p roducts by H-1, B-11 NMR (including both 1D and 2D B-11-B-11 {H-1} COS Y data), infrared, and mass spectral analyses are reported. The X-ray crystal structure of 3 is also reported. Upon extended UV irradiation of either complex 2 or 4, the only products observed were the dimeric [Fe(eta(5)-C5H5)(CO)(2)](2) complex and complex 3, which apparently ar ise from the light-induced fission of a terminal sigma iron-cage bond. The formation of the dimeric [Fe(eta(5)-C5H5)(CO)(2)](2) complex in t his photolysis was expected through a radical coupling process of the initially photogenerated [Fe(eta(5)-C5H5)(CO)(2)](.) species. The othe r anticipated coupling product, [(Fe(eta(5)-C5H5)(CO))B5H7](2), was no t found, but rather the formation of complex 3 was observed, which pre sumably formed through the extraction a hydrogen atom by the intermedi ate radical [2-(Fe(eta(5)-C5H5)(CO))B5H7](.) complex. A unique rearran gement of complex 4 to a new isomer, (5)-C5H5)(CO)(2))-2-(eta(5)-C5H5) -2-(CO)-2-FeB5H7] (5) is also reported. Crystallographic data for 3: s pace group Pnma (No. 62, orthorhombic), a = 14.889(1) Angstrom, b = 9. 4560(9) Angstrom, c = 7.2838(9) Angstrom, V = 1025.5(3) Angstrom(3), Z = 4 molecules/cell, R(w) = 0.037 for 959 independent reflections.