STRUCTURES, BONDING, INFRARED-SPECTROSCOPY, AND 2-ELECTRON REDUCTION POTENTIALS OF THE COORDINATED METALLOPNICTANES FE-3(CO)(9)(MU(3)-EMLN)(2) (E = P, AS, SB MLN = CR(CO)(5), MNCP(CO)(2))

The triiron clusters Fe-3(CO)(9)(mu(3)-EMLn)(2) (1-E, MLn = MnCp(CO)(2 ), E = P, As, Sb; 2-E, MLn = Cr(CO)5, E = P, As, Sb) can be considered cluster analogues of organopnictane (ER3) ligands, in which the tripl y bridging E ligands are coordinated to 16-electron capping metal grou ps, MLn. Structural parameters, infrared CO stretching frequencies, an d reduction potentials for this metallopnictane series are reported. A nalysis of structural, spectroscopic, and electrochemical data reveal systematic variations as a function of the capping heteroatom and the metal fragment coordinated to the heteroatom. The covalent radius of t he capping heteroatom dictates the structure of the Fe-3(CO)(9)(mu(3)- E)(2) bonding framework, and both Fe-E and Fe-Fe distances increase li nearly with increasing heteroatom covalent radius. The electronegativi ty of the capping heteroatom (E) influences the frequencies of the Fes -core carbonyl stretching modes (nu(CO)(Sb) < nu(CO)(As) < nu(CO)(P)), while the two-electron reduction potentials for the series 1 and seri es 2 clusters (E-1/2(Sb) > E-1/2(As) > E-1/2(P)) correlate with the co valent radius of E. Clusters with capping Cr(CO)5 groups are reduced a t potentials similar to 400 mV more positive than the MnCp(CO)(2)-capp ed analogues. On the basis of the frequencies of the CO stretching mod es for the capping MLn group and the E-MLn distance, the metallopnicta nce ligands are classified as intermediate between organopnictanes and halopnictanes in terms of their net electronic impact (the sum of sig ma-donating and pi-accepting properties). Results of Fenske-Hall molec ular orbital calculations provide a qualitative description of the Fe- 3-E-2 bonding as a function of heteroatom (E). Trends in Mulliken popu lations and a Walsh analysis suggest the Fe-E bonds are strongest for the most electronegative E, phosphorus. The Fe-Fe overlap populations are nearly constant with E, despite the increase in Fe-Fe distance att endant upon incorporation of larger heteroatoms. Two-electron reductio n of the metallopnictane Fes-core changes the donor and acceptor chara cteristics of the metallopnictane ligand, as demonstrated by a structu re determination for [(PhCH2)Me3N](2)(1-P). The Mn-P distance in (1-P) (2-) is 0.1 Angstrom longer than in the neutral analogue, 1-P. Single- crystal X-ray structure determinations for 1-As, 1-Sb, 2-P, 2-As, 2-Sb , and [(PhCH2)Me3N](2)(1-P) are reported.