Chapter 15
Transition Metal Diatomic and Monocarbonyl Molecules An Experimental Viewpoint W. Weltner, Jr., and R. J . Van Zee Department of Chemistry, University of Florida, Gainesville, F L 32611
Recent experimental data on the transition-metal diatomics i s used to test the "isoelectronic" p r i n c i p l e as applied to molecules with the same number of d + s valence electrons. Electron configurations and ground states i n the first-row transition-metal monocarbonyl molecules (MCO) are examined on the basis of their CO stretching frequencies, ESR evidence, and t h e o r e t i c a l calculations.
Our objective i s to discuss two of the simplest classes of t r a n s i tion-metal molecules of mutual interest to theorists and experi mentalists and basic to the understanding of c l u s t e r s . Our desire i s to f i l l i n the many gaps among their ground states by extrapo lating from the present data. Reviews of the status of information on the diatomics have been made by us {V), by Shim (2), by Morse (3), by Koutecky and Fantucci (4), and by Salahub (5)· Morse's review i s recommended here since it considers rather thoroughly both the heteronuclear and homonuclear diatomics. Apparently there i s no review of the MCO molecules, and we hope to provide a very brief one here. A more thorough discussion w i l l s t i l l be needed of the sophistications i n the many theoretical calculations that have been made. Gas-phase preparation and spectroscopic investigations of these molecules are d i f f i c u l t not only because of the need for reasonable concentrations and low temperature spectra, but also because of the analysis problems. High m u l t i p l i c i t i e s , large masses, large spino r b i t constants, and large nuclear moments contribute to produce low-lying electronic states, second-order spin orbit effects (zero f i e l d s p l i t t i n g s ) , small v i b r a t i o n a l frequencies, and large hyperfine interactions. Among the diatomic metals only C r lj>r2)' (10), Mo (6, V\), V 02_), F e (JLi) 2 ^lâ)' 2 ±L' 1Ê) ' ^ (17), CuAu ( 18) and Ag , Au ( 19) have been investigated i n the gas phase. A l l have been found to have the simpler ground state, except V , Fe , and N i . The ground states of F e and N i have not been d e f i n i t e l y established, but V demonstrates some of the compli cations mentioned above i n having Σ ~ ground state and an C
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0097-6156/89/0394-0213$06.00/0 o 1989 American Chemical Society
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THE CHALLENGE OF d AND f ELECTRONS
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