RELATIVISTIC EFFECTS ON THE HEAVY METAL-LIGAND NMR SPIN-SPIN COUPLINGS: A DENSITY FUNCTIONAL STUDY

Jana Khandogin and Tom Ziegler,

Department of Chemistry, University of Calgary, Calgary, AB T2N 1N4, Canada

    The one-bond nuclear spin-spin coupling is particularly sensitive to relativistic effects because contraction of the s orbitals can significantly alter the Fermi-contact contribution. The relativistic effects on the NMR coupling constant can be to the first order modeled by adding corrections on top of the nonrelativistic nuclear coupling formulation. Here, we present three different relativistic correction schemes based on the frozen-core approximation. The first two approaches involve the scalar relativistic Pauli¹ and Zero Order Regular approximation (ZORA)² Hamiltonians. In the third scheme, we replace s orbitals in self-consistent nonrelativistic molecular Kohn-Sham orbitals by the relativistic ones just for the evaluation of the Fermi-contact term, without changing orbital expansion coefficients.
    These schemes are applied to the calculation of metal-ligand coupling constants ¹J(M-L) in alkyl complexes [M(CH₃)_n]^m-, where M=W, Pt, Cd, Hg, Sn, Pb, and Tl, in metal-cyano complexes [M(CN)₄]^2- where M=Pt, Cd, and Hg, in Platinum-amine complexes Pt(NH₃)₄, and Pt(NH₃)₄Cl₂, as well as in some platinum-phosphine complexes Pt(PF₃)₄, PtCl₂(PMe₂)₂ and PtH₂(PMe₂)₂. It is shown that the latter method, similar to the hydrogen-like relativistic correction suggested by Pyykkö³, gives a surprisingly good agreement with experiment. This confirms that relativistic effects on Fermi-contact contribution can be to the largest extend attributed to the contraction of the metal valence and subvalence s-orbitals. However, more insight into how relativity affects the nuclear coupling interaction can be gained only by means of a rigorous relativistic coupling formulation. Finally, we present the molecular orbital-based studies on the connection between trans influence and ¹J(M-L) in some platinum-phospine and platinum-amine complexes.
 
 
 

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