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The IR-spectrum of (I) indicates that terminal hydrido hydrogen atoms are absent. In the 'H-NMR spectrum (80 MHz, [De]toluene) two sharp singlets, among other signals, appear at 6= -10.36 and -13.80 (2H each), which can be assigned to two groups of k2-hydrido bridges, which on the basis of the C,-symmetry of the cluster are chemically nonequivalent. Figure 2 shows the resulting structural scheme of (1). When the temperature is increased, the hydrido signals broaden, collapse together at CQ. 45 "C and eventually form a sharp signal at 6= -12.07; the other parts of the spectrum are essentially unchanged. At higher temperatures, therefore, a dynamic intramolecular exchange of hydrido hydrogen atoms, involving pairs of non-equivalent H-atoms, occurs.The protons of the propyne unit in (1) can be recognized in the NMR spectrum by two sharp singlets at 6= 3.06 (3 H) and 5.72 (1 H). These signals are absent in the spectrum of the product of the reaction of [Ir(COD)Cl]2 and i-C3D7MgBr-evidence that the propyne unit originates from the Grignard reagent.The complex (I), H4(COD)41r4C3H4, is one of the few organometallic clusters which does not contain CO-ligands and to our knowledge is, until now, the only closed hydridometal cluster with exclusively =-bonded olefinic ligands. In view of this, its thermal stability is surprizingly high; crystals of (1) do not decompose under N2-atmosphere at temperatures up to 230 "C and can be vaporized into the mass spectromer at CQ. 200 "C. It is therefore expected that further complexes of this type are synthesizable.

In the context of the development of enzyme electrodes for the determination of the substrates of dehydrogenases, different systems for the oxidation of NADH were tested. In a homogeneous system with the dehydrogenase, NAD+ and phenazine methosulphate, the oxygen consumption caused by oxidation of the substrate was determined with a Clark electrode. Satisfactory sensitivity and reproducibility were obtained in the range 0.04-1 mM for several substrates, but the enzyme was rapidly denatured by the mediator, The physical entrapment of dehydrogenases together with a water-forming NADH oxidase on a Clark electrode yielded enzyme electrodes for different substrates with linear responses in the range 0.1-10 mM and a response time of 2 min. Owing to the instability of the NADH oxidase, the half-life of the electrodes was less than 5 h. The most promising systems were obtained by the coupling of the dehydrogenases to 3-P-naphthoyl-Nile Blue-modified graphite electrodes leading to oxygen-independent catalysed electrochemical NADH oxidation. The systems were optimised with regard to the NAD+ concentration in the solution and the pore size of the membranes covering the electrodes. By immobilisation of the enzymes, mainly by chemical binding to Eupergit, a considerable increase in their stability could be achieved, and electrodes were obtained with a half-life of 8.5 d, a response time, q, of 3.5 min and linear responses over four orders of magnitude.

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