Electronic spectra of "giant tetrahedral" decaruthenium cluster anions are consistent with a HOMO-LUMO gap of less than or equal to 1.2 eV, with intense transitions assigned to M-M* --> CO 2 pi* at high energy and weak transitions assigned to M-CO pi --> M-CO sigma* 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-MR spectra of metal carbonyl clusters have been obtained; spectra of [Ru-2(mu-H)(mu-NC5H4)(2)(CO)(4)(NC5H5)(2)][Ru-10(mu-H)(mu(6)-C)(Co)(23)(PPh(3))] and [ppn](2)[Ru-10(mu(6)-C)(CO)(24)] contain broad absorptions extending to a lambda(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 [Ru-2(mu-H)(mu-NC5H4)(2)(CO)(4)(NC5H5)(2)][Ru-10(mu-H)(mu(6)-C)(CO23(PPh(3))] show a temperature independent susceptibility chi(m) of +1384 +/- 10 x 10(-6) cm(3) 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 [Ru-2(mu-H)(mu-NC5H4)(2)(CO)(4)(NC5H5)(2)][Ru-10(mu-H)(mu(6)-C)(CO)(24)]. 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.