Ligand Effects on the Electronic Structure, Spectra, and Electrochemistry of Tetracobalt Carbonyl Clusters

Self-consistent field Xa calculations using the discrete variational method (SCF-Xα-DV) have been performed for Co₄(CO)₁₂(I) and the C_3v symmetry model clusters (η-C₆H₆)Co₄(CO)₉(II), Co₄(CO)₉[(PH₂)₃CH] (III), and (η-C₆H₆)Co₄(CO)₆[(PH₂)₃CH] (IV) to explore the effect of phosphine and arene ligands on cluster electronic structures. Density of states plots have been used to simplify analysis of bonding trends in these complex clusters. The cluster core charge varies from 3.11, to 2.58, to 2.41, to 1.80 e along the I-IV series. Charge on the apical Co is least positive in the arene-capped cluster II while basal cobalts are least positive in IV. Ionization potential calculations support qualitative Hückel studies that suggested (η-C₆H₆)M fragments bind more strongly to other metal centers than isolobal M(CO)₃fragments. Electronic absorption spectra of I, (η-MeC₆H₅)Co₄(CO)₉(V), Co₄(CO)₉(tripod) (VI) (tripod = (PPh₂)₃CH), (η-MeC₆H₅)Co₄(CO)₆(tripod) (VII), and (η-C₆Me₆)Co₄(CO)₆(tripod) (VIII) exhibit low-energy features between 1000 and 400 nm. Arene-capped clusters exhibit an extra low-energy absorption and calculations for the models II and IV assign this to a HOMO-LUMO transition that resembles intramolecular charge transfer from the apical to basal cobalt atoms. All substituted cobalt clusters V-IX (IX = (η-[2.2]-paracyclophane)Co₄(CO)₆(tripod)) exhibit reversible le reductions by cyclic voltammetry. Only clusters VI and VII display reversible le oxidations. Redox potentials correlate well with calculated tetracobalt core charges for the model systems I-IV but do not correlate well with HOMO or LUMO energies. Effects of apical and basal substitutions on redox potentials in the tetracobalt clusters are shown to be additive. This supports the notion of delocalized cluster bonding. For this reason cluster core charges better reflect gross properties of the cluster rather than HOMO or LUMO energies of localized two-electron orbitals.