Theoretical study of cis-trans isomerization in simple transition-metal complexes

Abstract

PRDDO calculations on several simple octahedral transition-metal complexes are presented. Relative energies, optimized ge- ometries, and population analyses are discussed for a series of complexes of general formulas M(NH3)4(C0)2 and M(NH3)2(C0)4, M = Cro, Mn+, Fe2+, and Co. Back-bonding stabilizes the cis isomers of the chromium and manganese complexes, but not those of Fe2+ or Cos+. Similar calculations on Cr(CO)2(N2)4 and Cr(CO),(N,), also predict the cis isomers to be favored energetically and are consistent with available experimental evidence. The cis isomers of Cr(C0)2(PH3)4 and Cr(CO),(PH,), are both favored by less than 0.3 kcal/mol, even though the cis conformations of the corresponding amine complexes are strongly favored (6-13 kcal/mol). As reported previously, PH3 can act as a T acceptor even without d orbitals on phosphorus, the acceptor orbital being best described as P-H u*. The energetic manifestation of this effect is a reduction of the cis-trans isomerization energy. Finally, calculations on the conformational preferences of bis(pyridine)bis(acetylacetonato)metal complexes are discussed. The phenomenon of r bonding in organometallic compounds has been studied intensely from both experimental and theoretical perspectives. Some of the most obvious and important conse- quences of r-bonding effects are in the areas of site preferences, conformational stability, and relative bond lengths. A good ex- ample of how r bonding can influence site preference may be found in Hoffmann's work on transition-metal pentacoordination, where the site preferences of r-bonding ligands are detailed for trigonal-bipyramidal and square-pyramidal structures.' Con- formational preferences due to r bonding have been studied in a variety of systems.2 Experimental studies of the effects of r bonding on bond lengths abound in the literature, but some of the clearest results are found in chromium-carbonyl complexes with pho~phine,~ where a distinct trend of long metal-ligand bond lengths trans to strong .rr acceptors can be seen. Here, we present a theoretical study of conformational preferences and relative bond lengths in an isoelectronic and isostructural series of complexes of chromium, manganese, iron, and cobalt. Consider a (hypothetical) complex such as CT(NH~)~(CO)~. The pseudooctahedral coordination leads to the usual d-orbital splittings, with three occupied tp orbitals at low energy and two empty eg orbitals at higher energy (here and throughout this paper we employ octahedral symmetry notation for simplicity). Simple r-bonding arguments lead one to predict that this complex should exist in the cis form, since only that conformation allows r back-bonding to the carbonyls from all three tzg orbitals. Similar conclusions can be reached for systems such as Cr(NH3)4(C0)2 although here all three to orbitals are r bonding in both isomers. Nevertheless, the cis isomer reduces the number of mutually trans carbonyls and thus should be preferred. Indeed, in the absence of steric effects, mixed complexes of carbonyl and other non- or lesser-r-accepting ligands invariably assume the cis conformation. Examples are Cr(C0)2(PH3)4,3 Cr(C0)3(PH3)3,4 Cr(CO),(P- H3)2,' and MO(CO)~(PM~~)~,~ all of which exist in the cis form. AG for the cis-trans isomerization of the last compound is 0.32 kcal/mol, while more bulky phosphines are preferentially trans for steric reasom6 The effects of A bonding on the bond lengths of these and similar complexes are also clear. Thus, in ~is-cr(Co)~(PH~) ~, the Cr-P distance trans to CO is 2.34 A, while the corresponding distance trans to PH, is significantly shorter (2.28 A).3 In this paper, we study the geometric and energetic conse- quences of a bonding in a group of octahedral complexes of general