ELSEVIER InorganicaChimicaActa240 (1995) 615--621 Carbonyl ligand substitution at rhodium in the heterometallic cluster [PPN] 2[Re7C(CO)21Rh(CO)2]. A reactivity comparison with mononuclear (r/5-CsHs) Rh (CO) 2 and related compounds Stephen F. Ralph, Scott W. Simerly, John R. Shapley * School of Chemical Sciences, Universityof lUinois, Urbana, IL 61801, USA Received 1 June 1995; revised 11 July 1995 Abstract The heterometallic cluster compound [PPN]2[ReTC(CO)21Rh(CO)2] reacts with tertiary phosphines and phosphites selectively at the rhodium center to form the mono-substituted products [PPN]2[ReTC(CO)EIRh(CO) (L) ]. Isolated compounds have been characterized by standard analytical and spectroscopic methods. Kinetics studies have shown that the reaction proceeds by an overall second-order process, first order in complex and first order in nucleophile. The second-order rate constants have been analyzed in terms of electronic and steric effects. The substitution rates at the rhodium center in the cluster are compared with those for the analogous mononuclear complexes ( ~/5_ C5H4X)Rh(CO) 2 previously studied by Basolo and co-workers. Keywords: Kineticsand mechanisms; Heterometallic clusters; Rhodiumclusters; Rheniumclusters;Carbonylclusters; Ligand substitutions 1. Introduction The analogy between [C5H5] - (-=Cp) and [ Re7C (CO) 2 t ] 3 -- ( ~ [ Re7 }) as six-electron donor ligands toward appropriate 12-electron [MLn] ÷ fragment s has been explored in several earlier papers describing the synthesis and characterization of the mixed-metal clusters [ReTC(CO)21MLn] 2- [ 1-6]. The present paper deals spe- cifically with the compound [ReTC(CO)2tRh(CO)2 ] 2- and its reactions with tertiary phosphines and phosphites to give the rhodium-substituted derivatives [ReTC(CO)2~Rh(CO)- (L)]2-: {Re7}Rh(CO)2 + L ~ {Re7}Rh(CO) (L) + CO (1) We were prompted to conduct this study in order to form a comparison with the classic work of Basolo and co-workers on the substitution reactions of CpRh(CO)2 and related com- pounds [7-10]. Despite its formally saturated character ( 18 valence electrons), CpRh (CO) 2 undergoes carbonyl ligand substitution by phosphorus donors in an overall second-order process [ 7 ]. The cyclopentadienyl ligand is held to be respon- sible for this facile expansion of the ligand shell because it In appreciationof Fred Basolo and his personal and professionalcon- tributions to inorganicchemistry. * Correspondingauthor. 0020-1693/95/$09.50 © 1995 ElsevierScience S.A. All rights reserved SSD10020-1693 ( 95 ) 04590-6 can remove a pair of electrons from the metal by 'slippage' from an ,/5 to an ,13 bonding mode. We have found that {ReT}Rh(CO)2 reacts with ligand dependent rates and acti- vation parameters that are qualitatively comparable to those of CpRh(CO)2. This study represents a unique case of a mononuclear/cluster reactivity comparison, as well as a rare example of a kinetics investigation with a higher nuclearity cluster [ 11,12]. 2. Experimental 2.1. General procedures All operations were carried out under an atmosphere of nitrogen employing standard Schlenk techniques, unless oth- erwise specified. Salts of [Re7C(CO)2tRh(CO)2] 2- were prepared as previously described [ 1 ]. Solvents were stored over appropriate drying agents and distilled prior to use. The phosphines and phosphites were obtained from a variety of sources; sample purity was determined by 3tp NMR. As needed, the solid phosphines were recrystallized from dis- tilled ethanol, and the liquid phosphines and phosphites were purified by drying with sodium, followed by distillation. IR spectra were obtained on acetone solutions with a Per- kin-Elmer model 1750 FT spectrometer, tH NMR spectra