Theoretical studies of inorganic compounds. 17 ¹⁾ Why are the homoleptic diyl complexes M(InR) ₄ with M = Ni and Pt stable compounds while only Ni(CO) ₄ but not Pt(CO) ₄ can become isolated? A theoretical study of M(EMe) ₄ and M(CO) ₄ (M = Ni, Pd, Pt; E = B, Al, Ga, In, Tl)

The equilibrium geometries and first bond dissociation energies of the homoleptic complexes M(EMe) ₄ and M(CO) ₄ with M = Ni, Pd, Pt and E = B, Al, Ga, In, Tl have been calculated at the gradient corrected DFT level using the BP86 functionals. The electronic structure of the metal-ligand bonds has been examined with the topologial analysis of the electron density distribution. The nature of the bonding is revealed by partitioning the metal-ligand interaction energies into contributions by electrostatic attraction, covalent bonding and Pauli repulsion. The calculated data show that the M-CO and M-EMe bonding is very similar. However, the M-EMe bonds of the lighter elements E are much stronger than the M-CO bonds. The bond energies of the latter are as low or even lower than the M-TlMe bonds. The main reason why Pd(CO) ₄ and Pt(CO) ₄ are unstable at room temperature in a condensed phase can be traced back to the already rather weak bond energy of the Ni-CO bond. The Pd-L bond energies of the complexes with L = CO and L = EMe are always 10 - 20 kcal/mol lower than the Ni-L bond energies. The calculated bond energy of Ni(CO) ₄ is only D _o = 27 kcal/mol. Thus, the bond energy of Pd(CO) ₄ is only D _o = 12 kcal/mol. The first bond dissociation energy of Pt(CO) ₄ is low because the relaxation energy of the Pt(CO) ₃ fragment is rather high. The low bond energies of the M-CO bonds are mainly caused by the relatively weak electrostatic attraction and by the comparatively large Pauli repulsion. The σ and π contributions to the covalent M-CO interactions have about the same strength. The π bonding in the M-EMe bonds is less than in the M-CO bonds but it remains an important part of the bond energy. The trends of the electrostatic and covalent contributions to the bond energies and the σ and π bonding in the metal-ligand bonds are discussed.