X-ray crystal structure of the trifluoroacetylcobalt complex CF 3 COCo(CO) 3 (PPh 3 ) – Implications for the relationship between structure and reactivity toward migratory insertion of carbon monoxide in cobalt alkyl complexes Nanda Gunawardhana, Stephen L. Gipson * , Andreas Franken Department of Chemistry, One Bear Place #97348, Baylor University, Waco, TX 76798, USA article info Article history: Received 4 February 2008 Accepted 6 March 2008 Available online 15 March 2008 Keywords: Trifluoroacetyl complexes Cobalt compounds Carbonyl insertion X-ray crystal structures abstract In order to gain insight into the bonding in perfluoroalkyl and perfluoroacyl complexes of transition met- als, the X-ray crystal structure of CF 3 COCo(CO) 3 (PPh 3 ) has been determined. Comparison of this structure with that previously reported by us for CF 3 Co(CO) 3 (PPh 3 ) and several other acyl/alkyl pairs reported by others highlights the cobalt–carbon bond contraction in the perfluoroalkyl complex and provides an explanation for why such complexes do not undergo migratory insertion of CO. Comparisons of cobalt–carbon bond lengths in hydrocarbon alkyl and acyl complexes show that the acyl complexes exhi- bit uniformly shorter bonds than the alkyl complexes, consistent with their ability to undergo CO inser- tion under mild conditions, and in contrast to the shorter Co–C bond length in the CF 3 complex relative to that in the COCF 3 species. Several other unique features of the bonding in the CF 3 complex become evi- dent upon comparison with the CF 3 CO complex and several hydrocarbon alkyl and acyl complexes. Other interesting comparisons include carbonyl stretching frequencies in the IR spectra of these complexes. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Cobalt alkyl and acyl complexes of the type RCo(CO) 4n L n have been extensively studied for almost 50 years now [1,2]. Interest in these compounds stems from the fundamental importance of the migratory insertion of carbon monoxide in transition metal orga- nometallic chemistry, as well as the involvement of these particu- lar compounds in the catalysis of industrially important reactions such as the hydroformylation of olefins. With respect to the equi- librium between the alkyl and acyl complexes, it has been reported that decreasing electron donation by the alkyl group favors the alkyl complex over the acyl, with hydrocarbon alkyls heavily favor- ing the acyl, fluorinated alkyls or other electron withdrawing groups favoring the alkyl, and benzyl derivatives displaying signif- icant concentrations of both forms at equilibrium [1]. It is generally accepted that transition metal perfluoroalkyl complexes do not undergo migratory insertion of CO [3]. The one reported example of this type of reaction (with iron [4]) has subse- quently been proposed to instead proceed through direct fluor- oalkylation at CO [5]. The reason typically given for the lack of CO insertion by perfluoroalkyl complexes is the greater strength of the metal–carbon bond in these compounds [3], and this strength is generally reflected in shorter metal–carbon bond lengths in perfluoroalkyl metal complexes compared to hydrocar- bon analogs [6–10]. The source of the added strength of perfluoro- alkyl–metal bonds has long been debated. Early on it was proposed to originate from d p  r * back-donation from the metal to the a-carbon of the perfluoroalkyl group [7]. However, careful study of the inductive effects of ligands in RMn(CO) 5 complexes, as man- ifested in carbonyl ligand force constants, showed that perfluoroal- kyl ligands, as exemplified by CF 3 , exhibit essentially no p-acceptor character [11]. In agreement with this experimental observation, molecular orbital calculations have shown that neither CH 3 nor CF 3 ligands in RMn(CO) 5 complexes act as p-electron acceptors [12]. Instead, the highly electronegative fluorine atoms of CF 3 with- draw enough electron density from the carbon atom to leave it substantially more positive than that in CH 3 . This relatively high positive charge on the carbon atom then significantly stabilizes the manganese energy levels in the CF 3 complex relative to the CH 3 complex, leading to stronger bonds between the metal and all its ligands, including the CF 3 group. The same effect is assumed to hold in all perfluoroalkyl complexes. X-ray crystallography has been used to demonstrate the bond contraction in perfluoroalkyl complexes [8–10]. In addition, several structural studies have compared the bonding in corresponding alkyl and acyl complexes of cobalt, RCo(CO) 3 L and RCOCo(CO) 3 L (L = PPh 3 , IMES) [13–15]. As part of a study of the reductively in- duced reactions of CF 3 COCo(CO) 3 (PPh 3 )(1) [16], some years ago we reported the X-ray crystal structure of the trifluoromethyl com- plex, CF 3 Co(CO) 3 (PPh 3 )(2) [17]. We have now succeeded in obtain- ing the X-ray crystal structure of complex 1 as well. This structure 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.03.030 * Corresponding author. Tel.: +1 254 710 4555; fax: +1 254 710 4272. E-mail address: stephen_gipson@baylor.edu (S.L. Gipson). Inorganica Chimica Acta 362 (2009) 113–116 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica