Metal-metal multiply bonded complexes with metal-metal vector having orientational disorder

Abstract

This dissertation presents detailed studies o f both trans and cis isom ers of W 2Cl4 (N H B ui)2 (PR 3 ) 2 (PR3 = PM e3, PM e2Ph, PEt3, PPr3). For trans PM e3 and P E t3 com pounds, low tem perature (less disordered) and room tem perature (more disordered) crystals were obtained. For trans PPr" and PM e2Ph com pounds, crystals obtained at low temperature and room temperature are the same. All trans com pounds undergo isomerization to cis isom ers in solution. Crystals of the cis isom er for PM e3 and PM e2Ph were obtained. The average W -P bond distances in the trans and the cis isom ers are 2.644(8) A and 2.52(1) A, respectively, and they are consistent with the ^ w -P coupling constants which are about 110 H z and 300 H z in the trans and cis isom ers, respectively. The short W -N distances in the m ore disordered crystals are not due to the presence o f a W -N triple bond, but are only apparent because the m etal-m etal orientational disorder is severe. The W -N bond distances in other trans and cis crystals are normal W -N double bond distances. The rate o f the trans to the cis isom erization is free phosphine dependent and a mechanism involving phosphine dissociation as the rate determining step is proposed. C ry stallizatio n at d ifferen t tem peratures affords d ifferen t cry stals for M 2Cl4 (PEt3)4 (M = Re, Mo, W ). Cubic crystals (space group I43m) were obtained at room tem perature for Re2Cl4 (PEt3)4 and M o2 Cl4 (PEt3)4 . W hen crystallization was carried out at -15 °C, tetragonal crystals (space group I42m ) were form ed. The d iso rd er ratios change accordingly from equal distribution along three m utually perpendicular directions to one principal orientation along the c axis (4 axis), and two equally distributed m inor orientations along the a and b axes. For W 2 C l4 (PEt3>4 , crystals obtained at room temperature and -15 °C are in the same space group I42m, but w ith different cell param eters and different disorder ratios. Once the crystals are formed, the disorder ratio will not change at room temperature.