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angle S2-P-O3 is opened up to 124.35 (8)O, the axial atoms are tipped away from these atoms toward 0 2 for an angle SI-P-Ol of 177.38 '. Conclusion. With regard to ring orientation, the study of new oxyphosphoranes possessing sulfur rings and pentafluorophenoxy ligands leads to axialwuatorial ring placement in TBP geometries similar to that found in other studies with cyclic pentaoxyphosphoranes."o*18 Thus far, use of electronegative ligands, steric and hydrogen bonding' effects, introduction of trans-fused rings,1° or varying ring size from five to eight-membered89 has not yielded a solid-state structure of a pentacoordinated phosphorus molecule displaying diequatorial ring occupancy. However, use of these variations has led to the formation of chair conformations4 in addition to the more prevalent boat form7-I0 for phosphorinane rings in axial-equatorial sites of TBP geometries. In the present study, the unique chair conformation found for the six-membered ring of 1 had a short P-O axial bond equal in length to the P-O equatorial bond within the accuracy of the measurement, the first observation of this kind in pentacoordinated phosphorus chemistry. Future work on structural preferences of cyclic oxyphosphoranes should prove valuable in assisting mechanistic studies of c-AMP action and more generally in interpreting mechanisms of reaction of cyclic phosphates.Acknowledgment. The X-ray crystal structure of Cr(~en)~+ (sen = 4,4',4''-cthylidynetris(3-azabutan-l-amine)) demonstrata that the coordination environments are very similar in this complex and Cr(en)33+. However, the photophysical behaviors of these complexes contrast markedly, with Cr(~en)~+ having a much shorter ' E excited-state lifetime (by a factor of at least lo') in ambient fluid solutions.The bond angles of the neopentyl cap ( N C C and C H 2 C C H 2 ) of Cr(~en)~+ are 5-10' larger than normal, and this is attributed to trigonal strain in the ligand induced by coordination to Cr(II1). MM2 calculations indicate that coordination geometry preferred by the ligand would be very nearly halfway between the ideal prismatic and antiprismatic microsymmetries. In contrast to the fluid solution behavior, (2E)Cr(sen)3+ doped into [Rh(~en)](ClO,)~ was found to have a 13-1s lifetime at 298 K. The facile, thermally activated fluid solution excited-state relaxation channel is attributed to a large amplitude ligand-promoted trigonal twist of the electronically excited complex. [Cr(sen)]Br3, CrN& lHmBr3, crystallizes in the monoclinic space group PZl/c with Z = 4 and u = 13.964 (4) A, b = 10.777 (2) A, c = 13.027 (3) A, and / 3 = 98.34 ( 2 ) O . The structure refined to R = 0.055 and R, = 0.050 with 1612 observed reflections.

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The emission spectrum and luminescent lifetime of (bpy) 2 Ru II (binicotinic acid) is affected by the presence of heavy metal ions in solution. As little as 1 µM Pb 2+ causes a red shift in emission, an increase in the emission quantum yield, and an increase in the room-temperature lifetime. A smaller red shift is observed in the 4,4′dicarboxy analogue in the presence of large quantities of lead; however, the emission lifetime and intensity are dimished. An X-ray determination of the ground-state geometry shows that the bipyridine rings of the binicotinic acid are twisted along the 2,2′ bond by 19.3°. The interaction between lead and the binicotinic acid complex was modeled by molecular mechanics and extended Hu ¨ckel calculations. The calculations show that interaction with lead flattens the bridged ring system of the binicotinic acid ligand, which affects the π* energy levels of the ligand, the d-orbital energies of the Ru(II), and the vibrational modes available to the substituted bipyridine ligand. The inverse energy gap law behavior observed in the binicotinic acid complex is explained in terms of an allosteric interaction between lead and the binicotinic acid complex.

The structures of [Cu(Diphen)(H20)] (C104)2 (1) and [Cu(Diphen)]2(C104)2 (2), where Diphen is 1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane, have been determined by single-crystal X-ray diffraction. Complex 1 crystallizes in the monoclinic space group P2\/c with cell dimensions a = 12.075 (2) k,b = 14.340 (2) A, c = 16.201 (2) A, ß = 103.27 (1)°, Z = 4, and Dcm = 1 692 g cm-3. Of the 3514 unique reflections (Mo Ka, 3.5°< 2 < 45°), 2431 reflections with F0 > 4a(F0) were used in full-matrix least-squares refinement. Final residuals were R{F) = 0.0434 and RV(F) = 0.0428. The copper cation is in a trigonal bipyramidal environment with four sites occupied