Reaction of the edge-bridged double cubane cluster [(Tp)(2)M2Fe6S8(PEt3)(4)] (1; Tp = hydrotris(pyrazolyl)borate(1-)) with hydrosulfide affords the clusters [(Tp)(2)M2Fe6S9(SH)(2)](3-,4-) (M = Mo (2), V), which have been established as the first structural (topological) analogues of the P-N cluster of nitrogenase. The synthetic reaction is an example of core conversion, resulting in the transformation M2Fe6(mu(3)-S)(6)(mu(4)-S)(2)(C-i) -> M2Fe6(mu(2)-S)(2)(mu(3)-S)(6)(mu(6)-S) (C-2v), the reaction pathway of which is unknown. The most prominent structural feature of P-N-type clusters is the mu(6)-S atom, which bridges six iron atoms in two MFe3S3 cuboidal halves of the cluster. The initial issue in core conversion is the origin of the mu(6)-S atom. Utilizing SeH- as a surrogate reactant for SH- in the system 1/SeH-/L- in acetonitrile, a series of selenide clusters [(Tp)(2)Mo2Fe6S8SeL2](3-) (L- = SH- (4), SeH- (5), EtS- (6), CN- (7)) was prepared. The electrospray mass spectra of 4 and 6 revealed inclusion of one Se atom in each cluster, and H-1 NMR spectra and crystallographic refinements of 4-7 indicated that this atom was disordered over the two mu(2)-S/Se positions. The clusters {[(Tp)(2)Mo2Fe6S9](mu(2)-S)}(2)(5-) (8) and {[(Tp)(2)Mo2Fe6S8Se](mu(2)-Se)}25- ( 9) were prepared from 2 and 5, respectively, and shown to be isostructural. They consist of two P-N-type cluster units bridged by two mu(2)-S or mu(2)-Se atoms. It is concluded that, in the preparation of 2, the probable structural fate of the attacking nucleophile is as a mu(2)-S atom, and that the mu(3)-S and mu(6)-S atoms of the product cluster derive from precursor cluster 1. Cluster fragmentation during P-N-type cluster synthesis is unlikely.