6.57 (t, 2 H)], underwent the e x p e~t e d~~.~] photoisomerization to cyclobutanedione 7 [UV (methylcyclohexane): /2,,,=496 nm (110); IR (methylcyclohexane :CH2C12 1 : 1) v,,, = 1770, 1795 cm-'1 upon irradiation at 404 or 436 nm, with isosbestic behavior in the early stages of the reaction (305, 374 and 482 nm).Irradiation of 7 in methylcyclohexane at room temperature (A = 496 nm) resulted in its conversion into benzene, presumably via the intermediacy of 5. The quantum efficiency for this reaction is greater than the maximum value observed for formation of 7 from 6 . When the irradiation was performed at -8O"C, the spectrum of a new compound was obtained with maxima in the ultraviolet at 294, 282, 271 and 260 nm (sh). The extinction coefficients were of the order of 3000. The new compound is stable indefinitely at -80°C (cf. Fig. 1).16-1 1 L . A .1 2 .1 0 -0 8 . 06.O L .

Abstract-Avariety of dinuclear MO(V), Mo(IV), Mo(II1) and Mo(1) complexes, and tri-and tetranuclear homo-and heterometallic complexes of molybdenum have been studied by "MO NMR. The MO

an Mo,Co-triangle bridged by two p3-sulfur ligands and a new structural unit, an q2-AsS ligand as bridge between two Mo atoms. The Ass-ligand, the two S-bridges and the Coatom lie in a plane bisecting the molecule. The carbonyl groups are located in the plane of the three-membered ring of metal atoms (Fig. 1). Hence, 3 constitutes the first example of a molecule containing complex-stabilized arsenic monosulfide, which, in the free state, is unstable. With the proviso that there is a 50% statistical distribution of arsenic and sulfur at the As and S! sites, the As-Sl distance in 3 can be said to be 2.237(3) A, a value which corresponds to the average length of the As-S single bond in a-As4SP1. We conclude, therefore, that the new ligand has a bond order of 1 and functions as a 5e-donor. Furthermore, if we consider the p3-S atoms as 4e-donors and the remaining ligands as 5e-donors (C,Me,-moieties) and 2e-donors (CO groups), then each metal atom acquires noble-gas configuration by formation of M-M single bonds. The experimentally determined metal-metal distances lend support to this conclusion.Remarkably, the Co(CO)z-fragment is incorporated exclusively in the q2-S2-group of 1 and 2, even though there are several sites open to attack in the ligand sphere of the starting complex. The S,-bridge over the Mo atoms is thereby enlarged to an SCoS unit, while elimination o f X

The reactivity of singly and doubly selenium-bridged complexes containing two Cp*Rh(CO) fragments (Cp" = q5-C,Me,) has been investigated under various conditions. The addition of SO, to Cp*,Rh,(CO),(p-Se)gives a novel p-SeSO, ligand, whose structure was determined by an X-ray diffraction study. The complexes Cp*,Rb,(CO),(p-E), (E = S, Se) undergo a disproportionation reaction in solution: the clusters Cp*,Rh,( p3-CO)(p3-E) were isolated and structurally characterized, whereas the remaining soluble, presumably mononuclear species was only detected by IR spectroscopy. In contrast, Cp*,Rh2(CO),(p-Se), dimerizes upon warming to give Cp*,Rh,(~3-Se), with the loss of its CO groups. The 72-e heterocubane clusters Cp*,CozRh,Se, and Cp*,Co,Se, have been synthesized from Cp*,CoM(CO),Se, (M = Co, Rh). The cluster Cp*,Rh,(CO), was isolated as a further byproduct from the synthesis of C~*,Rh,(co)~.The IR spectra and the X-ray crystallographic study show that the complex contains one p3-C0 and two p2-C0 ligands. Zusammenf assung Es wurde dieReal&&it von einfach und doppelt selenverbrtickten Komplexen mit Cp*Rh(CO)-Fragmenten (Cp" = $'-C,Me,) unter verschiedenen Gesichtspunkten untersucht. Die Addition von SO, an Cp*,Rh,(CO),(&3e) verlauft unter Bildung eines neuartigen p-SeSO,-Liganden, dessen Struktur durch Rijntgenstrukturanalyse bestimmt wurde. Die Komplexe Cp*2Rhz(CO),(~-E),(E = S, Se) unterliegen in LGsung einer Disproportionierungsreaktion: Die Cluster Cp*,Rh,( p3-CO)(p3-E) konnen isoliert und strukturell charakterisiert werden, w%hrend eine in

Reaction of Cp*,Ru 2(CO)4 (Cp" = $-CsMe,)