SYNTHESIS, STRUCTURES, H-1-NMR SPECTRA, AND REACTIVITY OF PARAMAGNETIC 46-ELECTRON TRINUCLEAR CLUSTERS OF THE FORM (CP-STAR-M)N(CPCO)3-N(MV-3-CO)2 (M = CO, RH, IR, N = 1, 2)

The preparation, structures, and spectroscopic characterization of two new series of paramagnetic 46-electron, heterotrimetallic clusters are reported, (Cp*M)(CpCo)2(CO)2 (*MCo2 series, M = Ir, 1; Co, 4; Rh 5) and (Cp*M)2(CpCo)(CO)2 (*M2Co series, M = Co 6; Rh 7; Ir 8). Their synthesis uses the bisethylene complex CpCo(C2H4)2 to act formally as a latent source of the CpCo fragment, which adds to either Cp*M(CO)2 (*MCo2 series) or the preformed, unsaturated dinuclear complexes [Cp*M(µ-CO)]2 (*M2Co series). Their structures reflect an important aspect of the bonding within the triangular M3(CO)2 cores. In each case, the trinuclear core may be divided into a dinuclear fragment that interacts with a mononuclear fragment determined by the positions of the carbonyl ligands in the solid state. In the *M2Co series, the two fragments are the original dinuclear precursor to which has been added a CpCo fragment. In the *MCo2 series the positions of the carbonyl ligands vary with the different metals involved. For M = Co and Rh the carbonyls bridge the CpCo–CoCp edge, while for M = Ir the carbonyl ligands bridge the Ir–Co bond. For both series, in every case where the carbonyl ligands remain primarily edge bridging (θ < 15°, θ = canting of the carbonyls off the perpendicular to the plane of the metal atoms, θ = 60° for μ3-carbonyls), a marked elongation of the bridged metal–metal bond is observed, as compared to the analogous bond in the parent dinuclear complex. In one case (*Rh2Co), where the carbonyl ligands are triply bridging in the solid state, the Rh–Rh bond shows no significant lengthening in the trinuclear complex as compared to the dinuclear parent, [Cp*Rh(µ-CO)]2. These data are consistent with the results of an extended Hückel MO calculation on the hypothetical complex (CpRh)3(CO)2 as reported by Pinhas et al. (Helv. Chim. Acta. 1980, 63, 29-49), where an elongation of the carbonyl-bridged bond is predicted as a CpM fragment is added to such a dinuclear species, as is observed in both of the above series. Furthermore, this analysis predicts that this in-plane M–M antibonding orbital should rise in energy and become unoccupied as the carbonyl ligands assume triply bridging coordination geometries. This prediction is consistent with the behavior of *Rh2Co, which is the only complex with μ3-carbonyls in the solid state. These calculations also indicate that the HOMO–LUMO separation should be small for these complexes. Consistent with this prediction is the observation of paramagnetically shifted 1H NMR spectra, for which a general model involving thermal equilibria between singlet (S = 0) and triplet (S = 1) isomers of each complex is proposed. Complexes 1,6, and 8 exhibit linear increasing shifts with increasing inverse temperature. Assuming a contact shift mechanism, the spin equilibria for complexes 4, 5, and 7 were modeled according to eq 1. The average enthalpy and entropy differences between the two isomers for these complexes are as follows: 4, ΔH = 10.56 (5) kJ/mol; ΔS = 35 (4) J/(mol·K); 5, 20 (2) kJ/mol, 45 (10) J/(mol·K); 7, 8.2 (1) kJ/mol, 34 (1) J/(mol·K). Members of the *M2Co series exhibit the interesting ability to exchange the dinuclear fragment [Cp*M(μ-CO)]2 (M = Co, Rh, Ir) from within their core for a new dinuclear complex, thus producing one of the other members of this series. This reaction is catalyzed by ethylene. The mechanism is proposed to involve initial coordination of ethylene to the CpCo fragment followed by loss of the dinuclear complex. The resulting CpCo mono- or bisethylene species is then captured by a second dinuclear complex. These reactions underscore the isolobal analogies between the title complexes and an unsaturated, 16-electron CpM(ethylene) species. The relationship between these reactions and fundamental cluster building and degradation reactions is also discussed. The crystal data were as follows: 4, monoclinic, C2/c, a = 15.254 (6) Å, b = 15.827 (5) Å, c = 8.341 (3) Å, β = 97.57 (3)° Z = 4, R(F) = 3.5%; 5, monoclinic, P21/n, a = 13.444 (7) Å, b = 11.342 (3) Å, c = 14.086 (7) Å, β = 107.11 (4)° Z = 4, R(F) = 3.19%; 6, orthorhombic, Ccmm, a = 10.197 (3) Å, b = 13.902 (6) Å, c = 17.851 (6) Å, Z = 4, R(F) = 6.74%; 7, orthorhombic, Ccmm, a = 10.292 (4) Å,b = 13.702 (4) Å, c = 18.544 (9) Å, Z = 4, R(F) = 6.15%; 8, orthorhombic, Pnma, a = 20.187 (6) Å, b = 15.779 (3) Å, c = 16.689 (4) Å, Z = 8, R(F) = 5.35%. © 1990, American Chemical Society. All rights reserved.