J. Am. Chem. SOC. zyxwvut 1993, 115, 1753-1759 1753 hydrogen atoms while maintaining the bond and dihedral angles, but shortening the bond lengths to 1.10 A. The coordinates of this tri- methylene were then submitted to Gaussian 90 for single-point calcula- tions using the D95V basis set (Dunning's basis set). The GVB(l/2) singlet energy was -1 16.955 zyxwvutsrqp 578 9 hartrees, and the ROHF triplet energy was -1 16.953 473 4 hartrees, indicating a singlet ground state and a singlet-triplet gap of 1.3 kcal/mol. Matched OD Photolysis of 4-(N2)2 and 31. MTHF solutions of 4- (N2)2 with A(342 nm) zyxwvutsrqpo = 1.4 and 31 with A(342 nm) = 1.3 were pre- pared. The solutions were pipetted into quartz EPR tubes fitted with vacuum stopcocks and submitted to five freeze-pumpthaw cycles. Samples were irradiated sequentially at 50 K using the same source and filter combination. Plots of signal intensity vs photolysis time are shown in Figure 7. Variable-Temperature EPR Study of 3. An EPR sample of 3-(N2)2 in 1.2-propanediol was photolyzed for 6 h at 50 K. The temperature was raised briefly zyxwvutsrqpo (-5 min) to 120 K in order to anneal the sample, and spectra were taken at decreasing temperatures down to 10 K. Simulation of EPR Spectra. We modified a published for the computation of EPR powder spectra. The EPR spin Hamiltonian used is shown in zyxwvutsrqp eq 6, shown again here for convenience. fi = g@B& + D(S2 - zyxwvu (S(S + 1)/3)) + E($: - 9;) (6) The expression for the first-derivative powder spectrum is26-36*37 ar -= aB (36) Aasa, R.; Vanngard, T. J. Mag. Res. 1975, 19, 308. (37) van Veen, G. J. Magn. Reson. 1978, 91, 30. Numerical integration of eq 13 was required in order to simulate spectra. Parameters 8 and 6 were varied in l o increments between Oo and 90°. At each orientation, the resonant fields for each of the Am, = 1 and hs = 2 transitions were computed by solving the resonance condition E, - Ej = g@B by interative diagonalization of the Hamiltonian matrix of eq 1. Once the resonance field was found for a given transition, the eigen- vectors li) and b) belonging to the two eFergy eigenvalues Ei and E, were computed as linear combinations of the S, basis functions. The transition probability could then be calculated according to eq 14.26-37J8 The derivative aB/av was approximated as h/gj3Am,.36*39 Powder spectra were obtained in final form by fitting a Gaussian line shape to the single transitions computed above and summing over all transitions. Line widths in powder EPR spectra are orientation-dependent; thus the line width was represented as a diagonal tensor u.26*38339 The width of the Gaussian applied to a given transition was determined from eq 15.26 U$ = (cos 8 cos u.,,)~ + (cos 9 sin zyxw 6 uyJ2 + (sin 0 uZJ2 (15) Acknowledgment. We thank Dr. Angelo J. Di Bilio for pro- viding us a copy of his program for EPR simulation, and Dr. Di Bilio and Dr. Sunney I. Chan for fruitful discussions of EPR results and spectral simulations. This work was funded by the National Science Foundation, to whom we are grateful. (38) Wasserman, E.; Snyder, L. C., Yager, W. A. J. Chem. Phys. 1964, (39) Kottis, P.; Lefebvre, R. J. Chem. Phys. 1964, 41, 379. 41, 1763. Molecular Solid-state Organometallic Chemistry of Tripodal (Po1yphosphine)metal Complexes. Catalytic Hydrogenation of Ethylene at Iridium Claudio Bianchini,**+ Erica Farnetti,*Mauro Graziani,*,t Jan Kaspar,r and Francesco Vizzat Contribution from the Istituto per zyxwvuts lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione, CNR, Via J. Nardi 39, 501 32 Firenze, Italy. and Dipartimento di Scienze Chimiche, Universitd di Trieste, Via Valerio 38, 34127 Trieste, Italy. Received March 27, 1992 Abstract: The solid-gas reactions of [(triphos)Ir(H)2(C2H4)]BPh4 (1) with CO, C2H4, and H2 are described [triphos = MeC(CH2PPh2),J. The gaseous reactants promote the elimination of ethane from 1 and the formation of [(triphos)Ir(CO),]BPh,, [(tripho~)Ir(C~H~)~]BPh~, and [(tripho~)Ir(H)~]BPh~, respectively. The latter 16-electron species is isolable in the solid state at temperatures <70 OC. At higher temperatures, [(triph~s)Ir(H)~]+ dimerizes in the solid state to give the tetrahydride [(tripho~)HIr(p-H)~HIr(triphos)]*+. Dimerization is avoided when the unsaturated fragment is incorporated into the lattice of a polyoxometalate cluster such as PW120a3-. The complex [ (triph~s)Ir(H)~(C~H~)] BPh4 is an effective catalyst for the hydrogenation of ethylene in the solid state at 60 OC. Comparisons are made with analogous fluid solution-phase systems. Molecular solid-state organometallic chemistry is experiencing rapid growth through the reactions of materials derived from either metal oxide clusters of the Keggin-ion type' or tripodal poly- phosphine ligands.2 We are developing the solid-state chemistry of Group VI11 metal complexes stabilized by tripodal polyphosphines such as P- (CH2CH2PPh2), (PP,), N(CH2CH2PPh2), (NP,), and MeC- (CH2PPh2), (triphos) (Scheme I). ' CNR, Florence. 'University of Trieste. 0002-7863/93/1515-1753$04.00/0 Scheme I pp3 NP3 triphos Earlier work has shown that (i) the Co(1) complex [(PP3)- Co(N2)]BPh4 (1) reacts in the solid state with a variety of gaseous 0 1993 American Chemical Society