STRUCTURAL AND REACTIVITY CONSEQUENCES OF THE PRESENCE OF LONE PAIRS IN MAIN-GROUP TRANSITION-METAL CLUSTER COMPOUNDS - CONVERSION OF [HAS(FE(CO)(4))(3)](2-) INTO [FE-3(CO)(9)(MU(3)-ASFE(CO)(4))(2)](2-)

The reaction of NaAsO2 with Fe(CO)(5)/KOH/MeOH produces the novel hydr ido cluster [HAs{Fe(CO)(4)}(3)](2-). An X-ray structural study was car ried out on the [PPN](+) salt at 173 K in an attempt to crystallograph ically confirm the presence of the hydrogen atom. The material crystal lizes in the trigonal space group P3 (No. 147) with a = 25.379(4) Angs trom, b = 25.379(4) Angstrom, c = 22.442(4) Angstrom, V = 12518.2(36) Angstrom(3), and Z = 6. The reactivity of this unusual cluster anion w as probed by pyrolysis, photolysis, and protonation. Addition of acid produced the previously characterized cluster As2Fe6(CO)(22). Pyrolysi s and thermolysis both yielded the dianion [Fe-3(CO)(9){mu 3-AsFe(CO)( 4)}(2)](2-). This material was crystallized as its [Et(4)N](+) salt, a nd the structure was determined by single-crystal X-ray diffraction. [ Et(4)N](2)[As2Fe5(CO)(17)] crystallizes in the triclinic space group P ($) over bar 1 (No. 2) with a = 12.466(2) Angstrom, b = 13.408(3) Ang strom, c = 15.457(3) Angstrom, a = 80.59(3)degrees, beta = 72.01(3)deg rees, gamma = 66.66(3)degrees, and V = 2253.7(8) Angstrom(3). The clus ter consists of a distorted-square-pyramidal E(2)M(3) core with the tw o main-group atoms ligated by Fe(CO)(4) fragments. Comparisons of the structure and reactivity of both [PPN](2)[HAs-{Fe(CO)(4)}(3)] and [Et( 4)N](2)[Fe-3(CO)(9){mu 3-AsFe(CO)(4)}(2)] with those of other isoelect ronic and isostructural clusters containing either group 15 or 16 elem ents has led to a correlation between the observed structure and react ivity patterns and the presence of a lone pair of electrons on the mai n-group element.