Density functional computations using the BP86 functional within the resolution-of-identity approximation and polarized triple-zeta basis sets are employed for the study of the three isoelectronic (CO)(4)FeL (L = CCH2 (5), BNH2 (6), NBH2 (7)) as well as (CO)(4)Fe(NBcat) (9) (cat = catecholato) complexes. In all complexes 5-7, the ligand L prefers the equatorial position of a pseudo trigonal bipyramid. The borylnitrene complex 7 has a linear Fe-N-B arrangement; its Fe-L bond dissociation energy is similar to that of the vinylidene complex 5 and only slightly lower than that of 6. Nonetheless, 7 is less stable thermodynamically than 6 by 40 kcal mol(-1). Complexes of type 7 are expected to be reactive on the basis of the following findings: the HOMO-LUMO energy gap is small, the rearrangement from 7 to 6 via an iminoborane intermediate involves barriers of 15 and 28 kcal mol(-1), and the dissociation of a CO molecule to give (CO)(3)FeNBH2 is only slightly endergonic (+7 kcal mol(-1) at 298.15 K). The catecholate bridge in 9 results in significant changes: the bipyramidal complex is no longer a minimum, but an isocycanato complex is obtained instead computationally.