H₂XO and (CH₃)₂XO Compounds (X = C, Si, Ge, Sn, Pb):  Double Bonds vs Carbene-Like StructuresCan the Metal Compounds Exist at All?

The relative stabilities of the R 2 XO, trans -RXOR, and cis -RXOR isomers (X = C, Si, Ge, Sn, Pb; R = H, CH 3 ) were investigated by ab initio and DFT pseudopotential calculations. Geometry optimizations and frequency computations were performed at the MP2 and Becke3LYP levels with DZ+P basis sets. MP4SDTQ/(DZ+P)//MP2/DZ+P and CCSD(T)/(DZ+P)//MP2/DZ+P single point calculations gave high level ab initio data. The CCSD(T)/(DZ+P)//MP2/(DZ+P) and the Becke3LYP/(DZ+P) relative energies both agree and show that the C 2 v H 2 XO isomers (with the exception of formaldehyde) are the least stable and that the cis - and trans -HXOH C s isomers are very close in energy. Acetone and dimethylsilanone are the most stable isomers in the Me 2 XO series. However, with the notable exception of HPbOH, the H 2 XO compounds are not stable thermodynamically toward cyclotrimerization or to hydration. The NPA/NLMO charges, bond orders, and orbital hybridizations support the polar description - O:::X + R 2 of all metal “double bond” species. The - O:::X + symbol implies the polarization of both π and σ bonds, which are stabilized electrostatically by methyl substitution. Only weak π interactions occur in the carbene-like RXOR metal isomers, and the σ bonds are mainly formed by p orbitals. In contrast, the X s orbitals participate strongly in the σ X-O bonds of the - O:::X + systems; the oxygen hybridization is far from sp 2 .