The sequential formation of [HFe5(CO)15B]2- (1), [HFe6(CO)17B]2- (2), and [HFe7(CO)20B]2- (3) in the facile reaction of [Fe4(CO)12HB]2- with either Fe2(CO)9 or Fe(CO)3(cco)2 (cco = cis-cyclooctene) is demonstrated. Complexes 1 and 2 were characterized spectroscopically as partially opened boride clusters whereas spectroscopic and X-ray crystallographic analyses (AsPh4 salt; monoclinic, P2(1)/c; a = 11.248(2) angstrom, b = 14.336(2) angstrom, c = 41.698(6) angstrom, beta = 90.30(1)-degrees, V = 6724(2) angstrom3, Z = 4; R, = 0.042, R2 = 0.057) of 3 show it to be a closed boride. The boron atom resides inside a trigonal prismatic array of six iron atoms, with the seventh capping one of the rectangular faces. The characteristics of the reaction suggest a cluster-building process initiated by electron transfer from [Fe4(CO)12HB]2- to Fe2(CO)9 leading to a radical-anion-mediated cluster-building process in which Fe(CO)5 is the by product.