Theoretical, Spectroscopic, and Electrochemical Studies of Tetracobalt, Tetrarhodium, and Tetrairidium Dodecacarbonyl and Tris(diphenylphosphino)methane-Substituted Derivatives

Optical spectra and cyclic voltammograms of M₄(CO)₁₂and M₄(CO)₉[(PPh₂)₃CH] clusters have been measured, where M = Co, Rh, and Ir. For the M₄(CO)₁₂complexes SCF-Xα-DV calculations were performed to examine bonding trends and to aid in assignment of their spectra. All cluster complexes exhibit a metal-localized σ→σ* transition, at 375 nm in Co₄(CO)₁₂, 303 nm in Rh₄(CO)₁₂, and 321 nm in Ir₄(CO)₁₂. All clusters are calculated to contain similar LUMO's with 7a₂and 27e in the C₃ν clusters (M = Co, Rh) correlating with the 9t₁LUMO in T_dsymmetry Ir₄(CO)₁₂. The energy ordering for this transition, Co < Ir < Rh, is reversed from what might be expected because of increased metal-metal bonding on descending a metal triad; however, the order agrees with calculated band positions (Co, 393 nm; Ir, 350 nm; Rh, 274 nm). The inverted order for Rh and Ir is attributed to the structural change C_3v→T_don progressing from Rh₄(CO)₁₂to Ir₄(CO)₁₂where three bridging carbonyls become terminal. Coordination of the (PPh₂)₃CH ligand decreases the energy of the σ → σ* transition by 350–6000 cm^-1in these clusters. The Xa calculations also accurately model the splitting between metal d band and carbonyl peaks in the UPS spectra of M₄(CO)₁₂clusters. Calculated splittings within the d band are 2.5-3 eV for M = Co, 3.0-3.5 eV for M = Rh, and 6 eV for M = Ir. This reflects the increasing metal-metal interactions on descending the triad. Calculated charges for the M₄core in the M₄(CO)₁₂clusters show the Rh₄(+5.39) and Ir₄(+5.40) cores to be much more electron deficient than Co₄(+3.11). In a previous study it was found that cluster core charges correlated well with redox potentials as ligands are changed around a common metal core. A similar correlation does not hold for a change in metal core and is attributed to changing covalent metal-metal interactions that are not reflected in the values of core charges.