Structures, bonding, infrared spectroscopy, and two-electron reduction potentials of the coordinated metallopnictanes Fe3(CO)9(μ3-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 triply bridging E ligands are coordinated to 16-electron capping metal groups, MLn. Structural parameters, infrared CO stretching frequencies, and reduction potentials for this metallopnictane series are reported. Analysis 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 the 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 linearly with increasing heteroatom covalent radius. The electronegativity 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 series 2 clusters (E-1/2(Sb) > E-1/2(As) > E-1/2(P)) correlate with the covalent radius of E. Clusters with capping Cr(CO)5 groups are reduced at potentials similar to 400 mV more positive than the MnCp(CO)(2)-capped analogues. On the basis of the frequencies of the CO stretching modes for the capping MLn group and the E-MLn distance, the metallopnictance ligands are classified as intermediate between organopnictanes and halopnictanes in terms of their net electronic impact (the sum of sigma-donating and pi-accepting properties). Results of Fenske-Hall molecular orbital calculations provide a qualitative description of the Fe-3-E-2 bonding as a function of heteroatom (E). Trends in Mulliken populations 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 attendant upon incorporation of larger heteroatoms. Two-electron reduction of the metallopnictane Fes-core changes the donor and acceptor characteristics of the metallopnictane ligand, as demonstrated by a structure 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.