Tertiary phosphine adducts of manganese(II) dialkyls. Part 2. Synthesis, properties, and structures of monomeric complexes

Abstract

Monomeric tetrahedral manganese dialkyl tertiary phosphine complexes of stoicheiometry MnR₂(PR′₃)₂ have been identified in solutions by electron spin resonance spectra and the alkyl complex Mn(CH₂CMe₂Ph)₂(PMe₃)₂ isolated and structurally characterised by X-ray crystallography. The molecule has a severely distorted tetrahedral geometry with P–Mn–P and C–Mn–C angles of 96.2 and 137.9° respectively, which reflect the relative sizes of the two kinds of ligand. The Mn–C and Mn–P distances are 2.149(6) and 2.633(4)Å respectively. Use of the chelating phosphine, 1,2-bis(dimethylphosphino)ethane (dmpe), has allowed the isolation of the tetrahedral monomers MnR₂(dmpe)(R = CH₂SiMe₃, CH₂CMe₃, and CH₂Ph). The chelate dialkyl o-xylylene, o-(CH₂)₂C₆H₄, gives an octahedral complex Mn[o-(CH₂)₂C₆H₄](dmpe)₂ whose structure has also been determined by X-ray diffraction. In this molecule, all metal-ligand bond lengths are shorter than the corresponding bonds in Mn(CH₂CMe₂Ph)₂(PMe₃)₂. This is consistent with a significant reduction in the Mn^II radius on adoption of the low-spin state observed. The Mn–C distances are 2.110(5) and 2.1 04(6)Å, while the Mn–P distances of 2.230(3)(trans to P) and 2.298(3)Å(trans to C) reflect the different trans-influence abilities of alkyls and phosphines. The X-band e.s.r. spectra of the monomeric complexes have been studied in detail and are discussed in terms of distorted tetrahedral high-spin Mn^II and octahedral low-spin Mn^II species.