A study of Hartree–Fock instabilities in Cr2(O2CH)4 and Mo2(O2CH)4

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Abstract
The Hartree–Fock instability of the SCF wave functions obtained for the quadruply bonded complexes Cr2(O2CH)4 and Mo2(O2CH)4 is studied at the ab initio level. Several cases of singlet, nonsinglet, and nonreal instabilities are found for each of these complexes at the experimental value of the metal–metal bond length. For Cr2(O2CH)4, these instabilities are still found at much shorter Cr–Cr distances. The ’’broken symmetry’’ wave functions corresponding to some of the singlet and nonsinglet instability roots of Cr2(O2CH)4 are computed and analyzed. Each of these broken‐symmetry wave functions is characterized by an important decrease of the bonding character of the metal–metal interaction with respect to the symmetry‐adapted wave function. Singlet instability can thus be considered as an attempt to reduce the strong bonding character imposed by the symmetry constraints to the whole class of D4h quadruply bonded complexes. The case of MoCr(O2CH)4 for which the symmetry of the system is broken from D4h to C4v, is analyzed. An examination of the singlet instability matrices As+Bs obtained for Cr2(O2CH)4 and Mo2(O2CH)4 suggests that the presence of a multiple metal–metal bond strongly favors instability. A single determinant Hartree–Fock wave function might lead in such a case to an erroneous description of metal–metal multiply bonded systems. For this class of molecules, singlet stability equations could provide a simple and accurate procedure to check the reliability of Hartree–Fock wave functions.