High nuclearity ruthenium carbonyl cluster chemistry. 5. Local density functional, electronic spectroscopy, magnetic susceptibility, and electron paramagnetic resonance studies on (carbido)decaruthenium carbonyl clusters

Abstract

Electronic spectra of 'giant tetrahedral' decaruthenium cluster anions are consistent with a HOMO-LUMO gap of ≤ 1.2 eV, with intense transitions assigned to M-M* → CO 2π* at high energy and weak transitions assigned to M-CO π → M-CO σ* at low energy; the former are relatively insensitive to increasing phosphine substitution or cluster core charge, whereas the latter show some ligand dependence. The first diffuse reflectance UV-vis-NIR spectra of metal carbonyl clusters have been obtained; spectra of [Ru2(μ-H)(μ-NC5H4)2(CO)4(NC5H5)2][Ru10(μ-H)(μ6-C)(CO)2 3(PPh3)] and [ppn]2[Ru10(μ6-C)(CO)24] contain broad absorptions extending to a λ(onset) of 1300 nm with absorption maxima corresponding to those of the solution spectra superimposed. Local density functional (LDF) calculations support the optical spectra assignments and predict a triply degenerate HOMO approximately 1.3 eV below the LUMO. Magnetic susceptibility data for [Ru2(μ-H)(ν-NC5H4)2(CO)4(NC5H5)2][Ru10(μ-H)(μ6-C)(CO)2 3(PPh3)] show a temperature independent susceptibility χ(m) of + 1384 + 10 x 10-6 cm3 mol-1 (diamagnetic correction including decaruthenium valence electrons), arising from Van Vleck paramagnetism; unlike related decaosmium clusters and lower nuclearity ruthenium clusters, no temperature dependent component of the susceptibility exists. Electron paramagnetic resonance (EPR) investigations on three decaruthenium cluster anions reveal a temperature-independent paramagnetic (TIP) signal which does not derive from solid-state packing effects, the nature of the cation, the presence of impurities such as colloids, particles, or oxides, or incipient metallic character ('mesometallic' behavior); this EPR signal, the first reported TIP resonance from a carbonyl cluster, is assigned to the presence of radical decaruthenium clusters due to oxidation of the cluster anions. Unlike high-nuclearity clusters examined previously by EPR, no temperature-dependent response was found for [Ru2(μ-H)(μ-NC5H4)2(CO)4(NC5H5)2][Ru10(μ-H)(μ6-C)(CO)2 4]. The electronic spectroscopy, magnetic susceptibility data, EPR studies, and LDF calculations are consistent with these high-nuclearity carbonyl clusters having 'molecular' rather than 'mesometallic' character.