Nuclear Spin-Spin Coupling Constants and Bonding Nature of the Carboxylato-bridged Dinuclear Rhodium(II) Phosphine Complex

Abstract

Nuclear spin-spin coupling constants (¹J(RhP), ²J(RhP), ¹J(RhRh), ³J(PP)) in a carboxylatobridged dinuclear rhodium(II) phosphine complex have been studied with the extended Hilckel molecular orbital method in the framework of Pople-Santry theory (Fig.1, T able 2). A characteristic large value of ³J(PP) was found to relate to the presence of Rh-Rh σ and σ^* orbitals which were mainly composed of Rh(4 d_z2) atomic orbitals (Fig.2). Despite relatively small valence s-orbital coefficients therein, these two oribitals are located near the highest-occupied and lowest-unoccupied orbitals due to the dinuclearity, and thus give the large ³J(PP) value. This feature is in contrast to the case of ²J(PP)_trans in mononuclear complexes, where both the valence s-orbital coefficients and the orbital-energy difference are large (Fig.2).
In a model ca lculation, the variation of the electron-donating ability in substituted phosphines was taken as the change of Coulomb integral of the P atom of PH₃, and the character of the Rh-P bonding was evaluated in relation to the corresponding mutual polarizability of valence s-orbitals ¹π(RhP). Linear relationship was found not only between the ¹π(RhP)and Rh(5 s)-P(3 s) overlap populations (Fig.3) but also between ¹π(RhP) and Rh-P bond populations (Fig.4). Since the substituent effect on the observed ¹J(RhP) (Table 3) showed a parallel trend with ¹π(RhP), ¹J(RhP) itself is considered to be a good measure of the Rh-P σa-bond strength for this complex.