Closed-shell singlet ground states

In the case of carbenonitrenes, the nitrene subunit has an intrinsic preference for a triplet configuration, and, thus, for a A" ground state, a carbene with a very strong preference for a closed-shell singlet ground state is required. Such a case has not been realized yet. On the other hand, if both subunits are carbenes, then. 

Transition metal (TM) chemistry stands in contrast to this. Many compounds involve metal centres with partially filled d shells, and/or with one or several unpaired electrons. Therefore, it is not always straightforward to predict the orbital occupation pattern of a given stable compound. For intermediates on a reactive pathway, this is an even greater problem. This is also true for organometallic chemistry, despite the fact that many compounds obey the 18-electron rule and have closed-shell singlet ground states. Thus, there are many 16- or even 14-electron intermediates, odd-electron species [1], and polymetallic clusters and complexes for which the spin state is not readily predicted. 

The closed-shell singlet ground state ]g) of a ir system with n doubly occupied MO is given by the Slater determinant (65),... 

Duxbury (1985) has recently published an extensive review of laser Stark spectroscopy. Table 1 lists most of the molecules that have been studied in this way. The majority of these have closed-shell singlet ground states, and their Stark splittings follow eqns 2-5, at least for weak fields. However, one of the more reactive species, HCO, has a doublet ground state. The presence of the unpaired electron spin complicates the energy level pattern, but the same principles hold as for a singlet state. 

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