Organometallics zyxwvut 1989,8, 893-899 893 Nucleophilic Properties and Electrochemistry of Five-Coordinate Rhodium( zyxwvu I) a-Cyanide Complexes. Synthesis and Characterization of Stable Cis Hydride Cyanide Complexes of Rhodium( I I I) Claudio Bianchini, *st Franco Laxhi,$ M. Francesca Ottaviani,§ Maurizio Peruzzini,+ Piero Zanello,' and Fabrizio Zanobinit zyxw Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR, Via J. Nardi, 39, 50132 Firenze, Italy, Dipartimento di Chimica, Universiti di Siena, 53100 Siena, Italy, and Dipartimento di Chimica, Universits di Firenze, 50 100 Firenze, Italy Received June 17, 1988 The trigonal-bipyramidal (TBP) a-cyanide complexes of rhodium(I), [ (L)Rh(CN)] [L = N(CH2CH2PPh2),, 1; P(CH2CH2PPh2)3, 21, have been synthesized and spectroscopicallycharacterized. The redox properties of both compounds have been studied in detail and compared with those of the a-acetylide congeners [(L)Rh(C=CR)] (R = Ph, C02Et). One-electron oxidation of 1 and 2 yields the stable paramagnetic Rh(I1) derivatives [ (L)Rh(CN)]+ which exhibit square-pyramidal structures zyxw as determined by ESR spectroscopy. One-electron oxidation of the Rh(I1) compounds gives unstable Rh(II1) species of formula [(L)Rh(CN)I2+ which readily decompose to the dicyano complexes [(L)Rh(CN),]+. Electrophilic attack by Me+ on 1 and 2 occurs at the nitrogen atom of the a-bonded cyanide group to give the TBP methyl isocyanide complexes [(L)Rh(CNMe)]+. In contrast, protonation of 1 and 2 takes place at the metal, and the stable, octahedral cis hydride cyanide complexes [ (L)Rh(H)(CN)]+ form. Introduction As a part of our studies on electron-rich transition-metal complexes with tripodal polyphosphine ligands, we have recently focused much of our attention on a family of neutral, trigonal-bipyramidal (TBP) Rh(1) derivatives of formula [ (L)Rh(a-R)] [L = NP,, N(CH2CH2PPh2)3; PP,, P(CH2CH2PPh2),; R = halide, hydride, alkyl, aryl, acyl].' In all of these compounds, the metal center is highly basic and susceptible to attack by electrophiles such as H+ or alkyl+,thus forming octahedral (OCT) Rh(II1) derivatives in which the hydride (or alkyl) and R coligands are forced by the particular geometry of the tripodal ligand to lie in mutually cis positions. The electrons that populate the frontier a orbital responsible for the nucleophilic character of the metal in these d8-L,M systems2can also be removed, totally or partially, by chemical or electrochemical oxida- tion.lc In this way, Rh(I1) and Rh(II1) derivatives of formulas [(L)Rh(a-R)]+ and [(L)Rh(a-R)I2+, respectively, have been obtained. When the R ligand contains a nu- cleophilic center within its structure as in the case of a- acetylide,lc there is competition between the metal and the ligand in undergoing electrophilic attack. As an example, [ (NP3)Rh(CePh)], in which the @-carbon of the acetylide ligand is a nucleophilic site, reacts with H+ in THF or benzene forming the vinylidene complex [ (NP,)Rh(C= CHPh)]+.lC On the other hand, when the reaction of the PP3 analogue [ (PP3)Rh(C=CPh)] is carried out in benz- ene, the oxidation of the metal prevails over vinylidene formation and the Rh(I1) acetylide [(PP,)Rh(C=CPh)]+ is obtained., It is therefore apparent that a subtle balance of several factors, including redox potentials, solvent, and nature of the electrophilic reagent and of the eventual nucleophilic site on the R coligand decides which of the possible reaction pathways prevails over the others, i.e. electrophilic attack at the metal to form OCT Rh(II1) complexes, electrophilic attack at the a-ligand to give TBP Rh(1) complexes, or metal oxidation to yield Rh(I1) and Rh(II1) derivatives. ISSECC, CNR. University of Firenze. * University of Siena. In order to gain further insight into the factors that govern the reactions of multinucleophilic transition-metal complexes with electrophiles,we decided to investigate the chemistry and the electrochemistry of the u-cyanide com- plexes [(NP,)Rh(CN)] (1) and [(PP,)Rh(CN)] (2). Like their acetylide congeners, 1 and 2 possess two potentially nucleophic sites, namely, the metal and the nitrogen atom of the cyano group. Results and Discussion Synthesis and Characterization of 1 and 2. The TBP rhodium hydrides [(NP3)RhH] (3)ld and [(PP,)RhH] (4)ld react in THF at 40 "C with excess ethyl cyanoformate, NCC02Et, yielding ethyl formate and the yellow cyano complexes 1 and 2, respectively (eq 1). The use of NCC02Et as starting material to introduce cyanide ligands into complex frameworks has some pre~edent.~ It is generally agreed that the reactions proceed through oxi- dative cleavage of the C-C bond at the metal to give eth- oxycarbonyl cyanide complexes. When hydride ligands are present, the reductive elimination of ethyl formate may follow and cyanide complexes forme4 [(L)RhH] + NCCOZEt - [(L)Rh(CN)] + HCOZEt (1) An alternative two-step route by which 1 and 2 can be prepared is the one shown in (eq 2). This involves me- thylation by MeOSO2CF, of 3 and 4 in THF to give, via reductive elimination of methane from unstable cis hydride (1) (a) Bianchini, C.; Meli, A.; Peruzzini, M.; Zanobini, F. zy J. Chem. SOC., Chem. Commun. 1987,971. (b) Bianchini, C.; Meli, A.; Peruzzini, M.; Vacca, A.; Zanobini, F. Organometallics 1987,6,2453. (c) Bianchini, C.; Laschi, F.; Ottaviani, M. F.; Peruzzini, M.; Zanello, P. Ibid. 1988, 7, 1660. (d) Bianchini, C.; Masi, D.; Meli, A.; Peruzzini, M.; Zanobini, F. J. Am. Chem. SOC. 1988,110, 6411. (e) Bianchini, C.; Mealli, C.; Peruz- zini, M.; Zanobini, F. J. Am. Chem. SOC. 1987,109,5548. (f) Bianchini, C.; Mealli, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Zanobini, F. J. Organo- met. Chem. 1988, 346, C53. (9) Bianchini, C.; Peruzzini, M.; Vizza, F.; Zanobini, F. J. Organomet. Chem. 1988, 348, C9. (2) Bianchini, C.; Meli, A.; Dapporto, P.; Tofanari, A.; Zanello, P. Inorg. Chem. 1987,26, 3677. (3) Bianchini, C.; Peruzzini, M., unpublished results. (4) Bianchini, C.; Masi, D.; Meli, A.; Sabat, M. Organometallics 1986, 5, 1670. 0276-7333/89/2308-0893$01.50/0 0 1989 American Chemical Society