Structures and Bonding Character of Ge···CO Weakly Bonding Complexes

The structural properties and the bonding character of the subcarbonyl Ge, GeCO, and Ge(CO)<INF>2</INF> in their singlet and triplet states have been investigated using B3LYP, B3P86, B3PW91, and MP2 methods at 6-311+G* basis set level. Results indicate that GeCO species possesses a ³Σ ground state with a singlet ¹Σ excited state above by 17 kcal/mol at CCSD(T)/6-311+G* level. The ground-state GeCO may be classified as carbene-like, and its CO^δ- moiety possesses CO^- character. Formation of GeCO causes the weakening of CO bonding; the produced Ge−C bond is weakly bonding consisting of a weak σ-bond and two weak π-bonds. The calculated dissociation energy is 16.8 kcal/mol at CCSD(T)/6-311+G* level. The corresponding potential well for the bending mode is shallow; therefore, GeCO molecule is easily bent. For Ge(CO)<INF>2</INF>, there exist two v-type structures for singlet state and one for triplet state; no linear ones are found for either state. The singlet state (1 ¹A<INF>1</INF>) is more stable by 24.1 kcal/mol than the 2 ¹A<INF>1</INF> state and by 36.6 kcal/mol than the ³B<INF>1</INF> state at CCSD(T)/6-311+G* level; thus it may be considered as the ground state. The bond lengths in the 1 ¹A<INF>1</INF> ground state are very close to those in GeCO species, but the ∠GeCO angle is bent by ~10° and the ∠CGeC angle is only ~76°. The Ge−C bond lengths in the 2 ¹A<INF>1</INF> state are longer by 0.03−0.07 Å than those of the 1 ¹A<INF>1</INF> state, and the ∠GeCO and ∠CGeC angles are 140−144° and 119−125°, being significantly different from those of the ground state. The corresponding triplet state has ~50° of ∠CGeC angle and 162−166° of ∠GeCO angle, but its Ge−C and C−O bonds are longer than those in the 1 ¹A<INF>1</INF> state by ~0.07 and 0.03 Å, respectively. These Ge(CO)<INF>2</INF> species have essentially carbene-like character and should be referred to as bicarbonyl carbene-like. Comparison of the CO-dissociating energies between GeCO and Ge(CO)<INF>2</INF> in their ground states indicates that the first-CO-dissociating energy of Ge(CO)<INF>2</INF> is smaller by ~8.7 kcal/mol than that of GeCO; the averaged one over two COs is also smaller than that of GeCO. Detailed bonding analysis has implied that the possibility is small for the existence of the polycarbonyl Ge with more than two COs. This prediction may be also true for similar carbonyl complexes containing other nonmetal and nontransition metal atoms or clusters. Further study is very necessary.