The Mossbauer effect spectra for a series of small [Fe(eta(5)-C5H5)(CO)(x)] substituted metallaborane complexes are reported, where x = 1 or 2. The pentaborane cage in compounds [Fe(eta(5)-C5H5)(CO)(2)B5H7P(C6H5)(2)] (1), [Fe(eta(5)-C5H5)(CO)(2)B5H8] (2), and [(Fe(eta(5)-C5H5)(CO)(2))(2)B5H7] (3) was found to act as a significantly better donor ligand than the ligands in a comparison group of previously reported [Fe(eta(5)-C5H5)(CO)LX] complexes, where L = CO or PPh(3) and X = halide, pseudohalide, or alkyl ligands. These metallaborane complexes were found to most resemble their silyl analogues in Mossbauer spectral parameters and the electronic distribution around the iron centers. In addition, the Mossbauer data showed that the [mu-2,3-(P(C6H5)(2)B5H7](-) ligand was a superior donor to the corresponding unsubstituted [B5H8](-) ligand. The Mossbauer spectral results for the metallaborane complexes studied were found to be in general agreement with the anticipated donor and accepting bonding considerations for the cage ligands based upon their infrared and B-11 NMR spectra and X-ray structural features. The Mossbauer data for the [Fe(eta(5)-C5H5)(CO)B4H6(P(C6H5)(2))] (4) and [Fe(eta(5)-C5H5)(CO)B3H7(P(C6H5)(2))] (5) complexes, in comparison with compound 1, showed that as the borane cage becomes progressively smaller, it becomes a poorer donor ligand. A qualitative relationship was found between the observed Mossbauer isomer shift data and the number of boron cage vertices for the structurally related [Fe(eta(5)-C5H5)(CO)(x)ByHzP(C6H5)(2)] complexes, where x = 1 or 2, y = 3-5, and z = 6 or 7. The X-ray crystallographic data for compounds 1, 2, 5, and [Fe(eta(5)-C5H5)(CO)B5H8] (6) were also found to agree with the trends observed in the Mossbauer spectra which showed that the s-electron density on the iron nucleus increases in the order 5 < 6 < 2 < 1. The X-ray crystal structure of complex 2 is also reported. Crystallographic data for 2 : space group P2(1)/c (No. 14, monoclinic), a = 6.084(3) Angstrom, b = 15.045(8) Angstrom, c = 13.449(7) Angstrom, beta = 99.69(5)degrees, V = 1213(1) Angstrom(3), Z = 4 molecules/cell.