A detailed analysis of the changes in the electronic
structure of CO when a proton or a positive charge
approaches the carbon or the oxygen atom is reported using quantum
mechanical ab initio calculations and
several methods to analyze the theoretical data. The C−O bond is
shortened by nearly the same amount in
HCO+ and QCO+ compared to free CO, while
the nearly identical C−O bond lengths of COH+ and
COQ+
are longer than in CO. H+ and Q+ have
a strong electrostatic effect upon the atom to which they are
bonded,
which leads to an increased electronegativity of carbon and oxygen,
respectively. Inspection of the charge
distribution and the natural localized orbitals shows clearly that the
shorter C−O distances of HCO+ and
QCO+ and the longer C−O bond lengths of
COH+ and COQ+ are due to the changes in the
polarization of
the bonding orbitals which are caused by the positive charge of
H+ or Q+ that are bonded to the
molecule.
The bonding orbitals of CO are polarized toward the more
electronegative oxygen end. A proton or a positive
charge at carbon attracts electronic charge from the oxygen atom toward
the carbon end, which leads to less
polarized σ- and π-bonds and to a more covalent C−O bond. A
positive charge or a proton at the oxygen
atom has the opposite effect. The calculated curve of the C−O
bond length in MCO+ (M = Li, Cu, Ag, Au)
as a function of the M+−CO distance shows that the
C−O bond becomes shorter in the beginning when the
metal cation approaches the carbon atom. There is a turning point
at shorter M+−CO distances where the
C−O bond becomes longer again. The charge decomposition analysis
shows that the position of the turning
point is determined by the onset of the metal+ → CO
back-donation. A relatively small amount of M+
→
CO back-donation is sufficient to lengthen the CO bond. The
turning point for the curve of the C−O bond
length as a function of the M+−CO distance occurs at a
M+−CO value that is shorter than the
equilibrium
distance for M = Li and Ag, while it is longer for M = Cu and Au.
The trends of the bond strengths and
M+−CO interactions are explained with the radii and
orbital energies of the valence ns and (n − 1)d
orbitals
of the transition metals.
Quantum chemical investigations at the MP2 and CCSD(T) level with relativistic effective core potentials for the metals are reported for homoleptic carbonyl complexes of the Group 11 and Group 12 d 10 metal cations with up to six carbonyl ligands. Additional calculations for some compounds were carried out using density functional theory (DFT) methods (BP86 and B3LYP). There is good agreement between theoretical CCSD(T) and experimental bond dissociation energies (BDEs), which are known for eight of the 36 complexes studied. The bond energies predicted by DFT are too high. The complexes [Cu(CO) n ] and [Au(CO) n ] are predicted to be bound species for n 1 ± 5 only, whereas [Ag(CO) n ] and the Group 12 carbonyls [M(CO) n ] 2 are bound species for n 1 ± 6. The metal ± CO bonding has been [a] Prof.
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