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Sir:Recent suggestions' that polynuclear metal complexes may e,xhibit heretofore unobserved modes of catalytic activity have stimulated interest in the behavior of such complexes and particularily those complexes (e.g., Os3H2(CO)la2 and ($-containing unsaturated metal-metal bonds. Here we report some addition reactions of two coordinatively unsaturated palladium( I) dimers. The most conspicuous feature of these reactions is the expansion of the palladium-palladium distance which occurs upon formation of an isocyanide or carbon monoxide bridge. In contrast, with most other complexes containing monohapto, doubly bridging carbon monoxide or isocyanide ligands, these ligands span two metal centers which are generally thought to be connected by a direct metal-metal bond.4When a dichloromethane solution of the phosphine bridged palladium(1) dimer Pd2(dpm)2Xzs (1, X = Cl, Br) (dpm, Ph2PCHzPPh2) is exposed to 1 atm of carbon monoxide, 1 equiv of carbon monoxide is absorbed and the dark red crystalline complex Pd2(dpm)z(CO)X2 (2) may be isolated. This product contains only a bridging carbonyl (UCO 1704 cm-I, X [Pd2(CNCH3)3[(C6H5)2PCHzP(C6H5)2]2][pF6]2 Scheme I Ph 2 I 1 + CNR I +zcm t 2 x -3 RNC, 2 +2 5 N C ' + -3CNR

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