3051
J . Phys. Chem. 1986, 90, 3051-3052
Bondlng of Water Ligands to Copper and Nickel Atoms: Crucial Role of Intermolecular Electron Correlation J. Sauer,* H. Haberlandt, Central Institute of Physical Chemistry, Academy of Sciences, DDR- I I99 Berlin, German Democratic Republic
and G. Pacchionit Institute of Physical Chemistry, Freie Universitat Berlin, D- 1000 Berlin- West 33, West Germany (Received: February 19, 1986)
For the HZMu(?3) and HzO-Ni(3D) complexes CI calculationsyield equilibriumdistances of 230 and 225 pm and stabilization energies of 34 and 45.5 kJ/mol, respectively. A substantial share of these energies is due to intersystem correlation effects (dispersion).
Introduction In recent theoretical studies'sz bonding of u ligands such as HzO and NH3 to transition-metal atoms has been considered. It has been concluded' that it is basically electrostatic in origin and due to penetration of the water dipole into the metal atom charge cloud. In this Letter we report calculations for the H20-Cu(?3) and H20-Ni(3D) systems that point to the crucial role of intermolecular electron correlation for bonding in these systems. We found it useful to look a t the problem of the transitionmetal-ligand bond from the point of view of intermolecular int e r a c t i o n ~ . At ~ ~ the SCF level of theory there will be an interplay of exchange-repulsion and attractive induction forces due to polarization of the metal atom by the water dipole. (For atoms with a permanent quadrupole moment, e.g. Ni(3D), some additional stabilization comes from the dipolequadruple term.) The result is a modest net stabilization predicted by SCF or CAS-SCF as shown, e.g., in Table I of ref 1. From studies of intermolecular interactions it is known that such results are sensitive4 (i) to basis set superposition error (BSSE)s and (ii) to the presence of polarization functions in the basis set of the ligand? Only with "double f plus polarization (DZP)" basis sets are reasonable values of the H 2 0dipole moment obtained.6 Indeed, using for the HzOmolecule a DZP basis set with proper exponents6 and making corrections for the BSSE by the function counterpoise method of Boys and Bernard? we did not obtain any net stabilization a t the SCF level. However, the results of a simple semiempirical estimate (London formula) of dispersion energy indicate that the latter may substantially contribute to the bonding? Hence, inclusion of electron correlation seems inevitable. Besides dispersion, which originates from intersystem electron correlation, there will be also a reduction of the HzO dipole moment due to intrasystem correlation. The latter will result in a small destabilizing effect, namely reduction of the induction energy (compared with SCF). Therefore, the configuration interaction (CI) method with the option to use a multideterminant reference state (MRD-CI method)' is employed. For nickel the 3D(d9s1)and for copper the 2S(d'os') states are considered. According to the criterion usually applied in the programs used' only ground-state configurations, Le. 3A1(HzO-Ni) - 3D d9s1 (Ni) and 2AI(Hz0-C~)-zS d10 SI (Cu)appeared in the reference set. Computational Details The C, approach of the metal atom toward the oxygen atom of H20is investigated. For HzOthe experimental geometry HOH bond angle = 104.52' and OH bond length = 95.72 pm is adopted. For this molecule, a standard double-f basis set (HuzinagaDunning) was used and augmented by flat polarization functions Present address: Dipartimento di Chimica Inorganica e Metallorganica, Universita di Milano, 1-20133 Milano, Italy.
TABLE I: Equilibrium Distance, r o in pm, and Stabilization Energy, -AIIO in kJ/mol, for Transition-MetalAtom-Water Complexes HzO-C~(zS) Hz0-Ni(3D)
method" LUFF
@CF
e
- eSCF
- eSCF + ED 1 -
