1070-3284/05/3102- © 2005 Pleiades Publishing, Inc. 0121 Russian Journal of Coordination Chemistry, Vol. 31, No. 2, 2005, pp. 121–131. Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 2, 2005, pp. 132–142. Original Russian Text Copyright © 2005 by Varshavskii, Cherkasova, Podkorytov, Korlyukov, Khrustalev, Nikol’skii. Dirhodium(I) di(µ-carboxylato) tetracarbonyls [Rh(µ-RCOO)(CO) 2 ] 2 were obtained for the first time by reacting rhodium(I) carbonyl chloride dimer [Rh(µ- Cl)(CO) 2 ] 2 with silver carboxylates [1]. The authors [1] synthesized and characterized acetate, trifluoroacetate, p-fluorobenzoate, and phthalate complexes of this type and some bis(phosphine) derivatives, such as trans- Rh(RCOO)(PPh 3 ) 2 (CO). Later, some other methods were developed for synthesizing [Rh(µ-RCOO)(CO) 2 ] 2 complexes with R = CH 3 (II), CF 3 (III) [2–4] and their monocarbonyl triphenylphosphine derivatives trans- Rh(RCOO)(PPh 3 ) 2 (CO) [2, 5–12]. The authors of [4] reported the results of X-ray diffraction study of [Rh(µ- CF 3 COO)(CO) 2 ] 2 . The structure of trans- Rh(CF 3 COO)(PPh 3 ) 2 (CO) was previously determined in [13]. Rhodium(I) formate complexes became available considerably later. Thus, the carbonyl formate dimer [Rh(µ-HCOO)(CO) 2 ] 2 (I) was synthesized for the first time via a reaction of dirhodium(II) tetraacetate with carbon monoxide in formic acid [14]. Some other methods of synthesis of this compound were reported later on in [15, 16]. A series of bis(phosphine) com- plexes trans-Rh(HCOO)(PR 3 ) 2 (CO) was also synthe- sized [17–19]. The molecular structure of trans- Rh(HCOO)(PPh 3 ) 2 (CO) (IV) was determined by X-ray diffraction method [17]. It should be noted that in sub- sequent years, rhodium(I) formate complexes with car- bonyl and some other π-acceptor ligands (olefins, phos- phines) were studied more intensively than other car- boxylate derivatives. This is quite natural, since formate group is a C 1 ligand with composite structure and high reactivity. It is noteworthy that though separate IR and NMR parameters of Rh(I) carbonyl carboxylate complexes were reported in some of the above-mentioned papers, their comparison is difficult, since they were deter- mined under different conditions. In this work, simple reaction is suggested that can be used to synthesize [Rh(µ-HCOO)(CO) 2 ] 2 complex and its crystal structure is determined. The latter complex is shown to be suitable starting reagent in synthesis of Rh(I) carbonyl hydride HRh(PPh 3 ) 3 (CO). X-ray dif- fraction data available in the literature for bis(triphenyl- phosphine) complexes of the composition trans- Rh(RCOO)(PPh 3 ) 2 (CO) with R = H, CF 3 [13, 17] are supplemented with the data we obtained for analogous acetate complex. IR and NMR measurements of carbonyl formate complexes [Rh(µ-HCOO)(CO) 2 ] 2 and of trans-Rh(HCOO)(PPh 3 ) 2 (CO) together with their ace- tate and trifluoroacetate analogs made it possible to obtain for the first time a set of comparable spectral parameters that indicates a transfer of electronic effects of the ligands through a central atom. The results of this work were partially reported in the previous publica- tions [20–22]. Rh(I) Carbonyl Carboxylato Complexes: Spectral and Structural Characteristics. Some Reactions of Coordinated Formate Group Yu. S. Varshavskii*, T. G. Cherkasova*, I. S. Podkorytov**, A. A. Korlyukov***, V. N. Khrustalev***, and A. B. Nikol’skii* *Research Institute of Chemistry, St. Petersburg State University, Universitetskii pr. 26, Petrodvorets, St. Petersburg, 198504 Russia **Research Institute of Synthetic Rubber, Gapsal’skaya ul. 1, St. Petersburg, 198035 Russia ***Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia Received March 30, 2004 Abstract—Complexes [Rh(µ-RCOO)(CO) 2 ] 2 , where R = H, CH 3 , CF 3 (I, II, III, respectively) are synthesized by reacting anhydrous carboxylic acids with Rh(Acac)(CO) 2 crystals. In compounds I, II, III, and trans- Rh(RCOO)(PPh 3 ) 2 (CO), where R = H, CH 3 , CF 3 (IV, V, VI, respectively), ν(CO) and 1 J(CRh) increase and δ 13 C decreases with the increasing electronegativity of R (CH 3 < H < CF 3 ). In the case complexes IV, V, and VI, the values of δ 31 P and 1 J(PRh) decrease in the same order. Complexes I and V are studied by X-ray diffrac- tion analysis. Intramolecular (2.946 Å) and intermolecular (3.127 Å) Rh–Rh distances in a columnar structure I are close, i.e., the structure contains infinite chains of metal atoms. Interaction of IV with chlorinated solvents results in trans-RhCl(PPh 3 ) 2 (CO). When heated with an excess of PPh 3 in propanol-2, compound IV transforms to HRh(PPh 3 ) 3 (CO). The latter reaction was suggested as a basis of a new method that can be used to obtain HRh(PPh 3 ) 3 (CO).