Scheme VI2efi factors.46 For the flavin-linked porphyrins the one-electron redox potentials are not correlated to the log k2nd, but we assume that geometrical factors play an important role. The methylene chain length considerably affects the chemical shifts in the 'H N M R spectra, which reflect the geometry of the flavin-linked porphyrin. In kinetic studies, we used the polar and protic solvent ethanol for practical reasons. In this solvent the flavin moiety is probably partly solvated, and the stability of the stacked conformation decreases with an increase of the flexible methylene chain length. The intermediate state clearly reflects the initial conformation. F1,xC,(TPP)MnlllC1, which has little freedom around the methylene spacer, is in a more folded conformation than l b and IC, and the reaction should proceed via a ternary complex [PyH,... F1,,-~(TPP)Mnllr+]. It has been well-known that the redox reaction between flavin and a 1,4-dihydropyridine proceeds via a preequilibrium charge-transfer-type complex [ PyH,.-FI,,] .46 In the proposed ternary complex, one electron can be rapidly transferred to the near manganese(II1) porphyrin. The reaction of lb, IC, Id, and l e proceeds only partly via this ternary complex and mostly via its open form. The proposed reaction mechanism is summarized in Scheme VI.Conclusion. Novel flavin-linked porphyrins have been synthesized. The key step, the condensation of the flavin carboxylic acid and the o-NH2TPPH2, which was carried out via the flavin acid chloride, proceeds in good yield. Spectrophotometric studies revealed that the flavin and the porphyrin moieties are in close proximity in the all-oxidized form. Electrochemical studies suggest an interaction of the chromophores in redox reactions. Especially, the potentials of the Flox/Fl'and Fl'-/F12couples are significantly shifted to more positive values. The flavin-catalyzed 2e/le electron-transfer reactions between NADH model compounds and (TPP)Mn"*Cl were investigated in intermolecular systems (FIox + PyH, + (TPP)Mn"'CI) as well as in intramolecular systems ( FIoxC,(TPP)MnlllC1 + PyH,). Reaction rates were accelerated by the intramolecular effect, and this acceleration was strongly affected by the methylene spacer length and the linking position. Abstract: Reaction of alkylidene complexes [(q5-C5H5)Re(NO)(PPh,)(=CHCH2R)]+PF6-(l-PF6-; a, R = H; b, R = CH,; C, R = CH2CH2CH3; d, R = CH2C6H5) with DBU or t-BuO-K+ gives vinyl complexes (E)-(q5-C5H5)Re(NO)(PPh3)(CH=CHR) ((E)-Za-d; 7246%). Complexes (E)-Zb-d equilibrate to 84-92:16-8

The structure of NMe, (SiMe,) has been studied in the crystalline phase at 1 1 6 K by X-ray crystallography and in the gas phase at 295 K by electron diffraction. The solid phase structure consists of essentially isolated molecules with a Si-N bond length of 1.71 9 4( 1 2) A. The geometry at silicon is nearly tetrahedral and that at nitrogen is slightly non-planar, with angles CNC

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cf. 1 and 2) for the planar form, whereas a B-P bond length of 1.91 Á is predicted for the most stable ground state, which has a pyramidal phosphorus, a 0°twist angle, and a dihedral angle of 124°. The HPH angle was calculated to be 103.4°. The corresponding angles and distances in the title compound show it to be somewhat nearer the planar form than the ground state of BH2PH2. However further variations of an R'2BPR2 molecule may show a range of these parameters. It is also interesting to note that when calculations were carried out on the orthogonal form (i.e. twist angle = 90°) of BH2PH2, a much longer BP distance of 1.98 Á was calculated.11In summary the structure of Mes2BPPh2 further supports the largely double B-P bond assignment in compounds 1 and 2. The pyramidal phosphorus should still be capable of ligand behavior. Complexes involving the title compound as a ligand and other variations involving its arsenic and antimony analogues are under investigation.Note Added in Proof. Professor Noth has informed us that the structure of [|B(PEt2)3|2], a dimer, has been solved. This also has pyramidal phosphorus centers in the terminal positions. Supplementary Material Available: A table summarizing the data collection and refinement and tables of atom coordinates, bond distances, angles, anisotropic thermal parameters, and hydrogen coordinates and thermal parameters (7 pages); a table of structure factors (19 pages). Ordering information is given on any masthead page.

The complexes cis-[Pt(C,F,),(RCrCR),] ( R = Et l a or Ph 1 b) have been prepared by treating cis-[Pt(C,F,),(thf ),] (thf = tetrahydrofuran) with the appropriate alkyne. They undergo facile ligand exchange with either cis-[Pt(C,F,),(thf ),I or [NBu",],[Pt(C,F,),] giving rise respectively to neutral cis-[Pt( C,F,),(thf ) ( RCECR)] 2 or anionic [NBu",] [Pt(C,F,),( PhCrCPh)] 7b mono(q2-alkyne) derivatives. A series of mono(q2-alkyne) complexes of formulae ciS-[Pt(C,F,),L( R C K R ) ] ( L = pyridine 3a, 3b; PPh, 4a; SbPh, 5b; or CO 6a, 6b) and [N(PPh,),][Pt(C,F,),(EtC=CEt)] 7a has also been prepared. None of these complexes except 7b shows v(C=C) absorptions in their IR spectra. The molecular structures of complexes 1 b and 7b have been established by X-ray diffraction studies: 1 b, monoclinic, space group C2/c, a = 19.1 77(4), b = 8.4971 (4), c = 19.790(3) A, p = 103.297(14)" and Z = 4 (C, symmetry imposed); 7b, monoclinic, space group P2,/c, a = 10.5179(9), b = 17.5834(20), c = 25.369(5) A, p = 99.376(11)" and Z = 4. Parameters within the acetylene moiety of 1 b suggest that the platinum-alkyne n-back bonding is minimal.Transition metal-acetylene complexes form a well defined area of study in organometallic chemistry, the interest in which can be ascribed to the wide variety of chemical processes undergone by alkynes when attached to mono-or poly-nuclear metal fragments' as well as to the richness of their co-ordination modes.,In contrast to the plethora of olefin complexes of platinum(i1) presently known, the number of simple platinum(ir) mono(acety1ene) derivatives is more limited.3 This is primarily due to the activity shown by some platinum(I1) complexes in effecting oligo-or poly-merization of acetylene^,^ including cyclization proce~ses,~ and to the ability of alkynes to insert into Pt-E bonds (E = halide,6 H7 or alkyl'). Since M-C,F, bonds are usually reluctant to undergo insertion reactions,' the choice of pentafluorophenyl derivatives as starting materials to prepare stable platinum(1r) acetylene complexes seemed appropriate. Accordingly, we here report the synthesis of the first bis(q'-alkyne)platinum(II) complexes and that of a number of mono(q2-alkyne)platinum(Ii) complexes. Some of the results have been briefly communicated. ' Results and DiscussionThe complex cis-[Pt(C,F,),(thf),] (thf = tetrahydrofuran) reacts either with hex-3-yne or with diphenylacetylene in 1:2 molar ratio to give in high yields the bis(q2-alkyne) complexes cis-[Pt(C,F,),(RC=CR),] (R = Et l a or Ph lb) as white, air-stable solids [equation (l)]. Analytical and relevant cis-[Pt(C,F,),(thf),] + 2RC&R ---+