MUTUAL INFLUENCE BETWEEN EQUATORIAL AND AXIAL LIGANDS IN TRIGONAL BIPYRAMIDAL METAL-COMPLEXES - A THEORETICAL RATIONALIZATION OF THE DIFFERENT M-N(AX) BOND STRETCHING IN 2 HIGH-SPIN D(5)-PSEUDO-ISOMERS

A comparison between the structures of the two pseudo-trigonal bipyram idal complexes [Mn(N(CH2CH2PPh2)3I]BPh4, 1 and [Mn(N(CH2CH2NMe2)3Br]BP h4. 2, has shown that, notwithstanding the unique Mn(II) high spin ele ctron configuration, the Mn-Na, bond is significantly stretched [2.19( 3) vs. 2.634(9) angstrom] when three equatorial amine donors are subst ituted by three phosphine analogs. A rationalization can be provided b y a qualitative MO analysis, based on EHMO calculations. The metal d-o rbitals (all half-filled) have no major role in the formation of any M n-L bond and the metal uses only its four s and p orbitals to ensure a s many as five bonds in 2. By contrast, such an hypervalency is strong ly reduced in 1, as almost 100 % of metal s orbital is engaged in bond ing-antibonding interactions with the equatorial phosphine lone pairs. Accordingly, the residual possibility for the metal to coordinate as many as two axial ligands is minimized.