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 Tdsymmetry 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 C3v→Tdon 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry