Vibrational Predissociation Spectroscopy of Metal Carbonyl Complexes 1

Vibrational Predissociation Spectroscopy of Metal Carbonyl Complexes 1: Co(CO)n+ (n=1 -10) Allen M. Ricks and Michael A. Duncan Department of Chemistry University of Georgia, Athens Ga. *(2 will be on Thursday RA 08)

Fe(CO)5, Co(CO)5+ and the 18 -electron rule • Iron pentacarbonyl, Fe(CO)5 is a known stable compound • Fe is d 6 so Fe(CO)5 satisfies the 18 -electron rule • Has a known D 3 h structure • So popular it even has its own wikipedia page • Co+ is isoelectronic to Fe • Presumably Co(CO)5+ would have the same structure as Fe(CO)5

full mass spectrum activate mass gate; select one cluster mass. excite at turning point. Production of cold cations and complexes with laser vaporization in a supersonic expansion. photofragments Mass selection of cations by time-of-flight. Tunable laser photodissociation with IR OPO; measure fragment intensity versus wavelength. parent ion depletion

Tunable IR Spectroscopy Laser. Vision Tunable Infrared Laser System designed by Dean Guyer Tuning range: 700 -4300 cm-1 Linewidth: ~1. 0 cm-1 2000 -4300 cm-1 Tunable mid-IR 2. 3 -5. 0 m OPA OPO 532 nm KTP oscillator 1 crystal angle tuned idler signal (not used) KTA diff. gen. + amp of idler beam 4 crystals angle tuned 1064 nm Pumped by pulsed, unseeded YAG e. g. , Spectra Physics PRO-230. 700 -2200 cm-1 Tunable 4. 5 -14. 3 m Ag. Ga. Se 2 diff. gen. 1 crystal angle tuned

Typical Mass spectrum observed in this experiment • Addition of water is necessary to facilitate ion formation • The peak corresponding to Co(CO)5+ is dominant over other cluster sizes • Indicates inherent stability of this complex • This is the 18 -electron species

Infrared photofragmentation breakdown spectra • Infrared photodissociation breakdown spectra show termination at n=5, again indicating the stability of this complex • We were able to fragment the n=5 complex, however it is inefficient and we believe it to due to multiphoton absorption

Infrared Spectra of Co(CO)n+ (n=5 -9) measured via elimination of CO • Dashed line shows νco stretch at 2143 cm-1 • The band at ~2166 cm-1 is due to “surface” carbonyls, those beyond the coordination sphere • This corresponds to a 23 cm-1 shift to the blue from the gas phase CO value • Is due to the electrostatic interaction of the carbonyls with the core ion • Bands at ~2140 and ~2150 assigned to Co(CO)5+ core ion • Spectrum of n=5 featureless, supporting our assumption that fragmentation of this complex is caused by multiphoton absorption

Infrared spectra of Co(CO)n+ complexes obtained via rare gas tagging • Spectrum of Co(CO)5+ obtained via rare gas tagging shows structure not observed in previous spectrum • Appears to be a 15 cm -1 frequency shift between Co(CO)4+ and Co(CO)5+ • Spectrum of Co(CO)3+ shows one CO stretch consistent with a D 3 h structure • Single peak in Co(CO)2+ spectrum indicates a D∞h structure • Doubly argon tagging required for Co(CO)1+ due to high binding energy

Comparison of experimental and theoretical spectra of larger argon tagged cobalt carbonyl complexes • Theoretical calculations help us elucidate the multiplicity of the complexes being studied • There is a spin change between n=4 and n=5 • Singlet Co(CO)5+ is isoelectronic to the 18 -electron Fe(CO)5 complex

Comparison of experimental and theoretical spectra of smaller argon tagged cobalt carbonyl complexes • Spectrum of Co(CO)5+ obtained via rare gas tagging shows structure not observed in previous spectrum • Appears to be a 15 cm -1 frequency shift between Co(CO)4+ and Co(CO)5+ • Spectrum of Co(CO)3+ shows one CO stretch consistent with a D 3 h structure • Single peak in Co(CO)2+ spectrum indicates a D∞h structure • Doubly argon tagging required for Co(CO)1+ due to high binding energy

Structures of small cobalt carbonyl and argon tagged cobalt carbonyl complexes • Structures of Co(CO)5+ complexes • Argon shows large structural perturbation? • Actually due to basis set superposition error • Usage of larger basis set on metal ion alleviates this problem

Current work in other systems • Current work on vanadium carbonyl complexes • Have achieved efficient Ne tagging! • Spectrum of the dimer shows two peaks when neon and argon tagging • Must be due to different spin states present

Acknowledgements • USAFRF and DOE for funding • Prof. Michael Duncan • Prof. Peter Armentrout