Linear Pt 2 M 2 Pt 2 hexanuclear clusters [Pt 4 M 2 (μ-H)(μdpmp) 4 (XylNC) 2 ](PF 6 ) 3 (M = Pt (2a), Pd (3a); dpmp = bis-(diphenylphosphinomethyl)phenylphosphine) were synthesized by site-selective reductive coupling of trinuclear building blocks, [Pt 2 M(μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 (M = Pt (1a), Pd (1b)), and were revealed as the first example of low-oxidationstate metal strings bridged by a hydride with M−H−M linear structure. The characteristic intense absorption bands around 583 nm (2a) and 674 nm (3a) were assigned to the HOMO−LUMO transition on the basis of a net three-center/twoelectron (3c/2e) bonding interaction within the central M 2 (μ-H) part. The terminal ligands of 2a were replaced by H − , I − , and CO to afford [Pt 6 (μ-H)(H) 2 (μ-dpmp) 4 ] + (4), [Pt 6 (μ-H)I 2 (μ-dpmp) 4 ](PF 6 ) (5), and [Pt 6 (μ-H)(μdpmp) 4 (CO) 2 ](PF 6 ) 3 ( 6). The electronic structures of these hexaplatinum cores, {Pt 6 (μ-H)(μ-dpmp) 4 } 3+ , are varied depending on the σ-donating ability of axial ligands; the characteristic HOMO−LUMO transition bands interestingly red-shifted in the order of CO < XylNC < I − < H − , which was in agreement with calculated HOMO−LUMO gaps derived from DFT optimizations of 2a, 4, 5, and 6. The nature of the axial ligands influences the redox activities of the hexanuclear complexes; 2a, 3a, and 5 were proven to be redox-active by the cyclic voltammograms and underwent two-electron oxidation by potentiostatic electrolysis to afford [Pt 4 M 2 (μdpmp) 4 (XylNC) 2 ](PF 6 ) 4 (M = Pt (7a), Pd (8a)). The present results are important in developing bottom-up synthetic methodology to create nanostructured metal strings by utilizing fine-tunable metallic building blocks.

Electron-deficient Pt2M2Pt2 hexanuclear clusters, [Pt4M2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt (7), Pd (8); dpmp = bis((diphenylphosphino)methyl)phenylphosphine), were synthesized by oxidation of hydride-bridged hexanuclear clusters [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2](PF6)3 (M = Pt (2), Pd (3)) and were revealed to involve a linearly ordered Pt2M2Pt2 array joined by delocalized bonding interactions with 84 cluster valence electrons, which are discussed on the basis of DFT calculations. The central M–M distances of 7 and 8 are significantly reduced upon the apparent loss of a hydride unit from the M–H–M central part of 2 and 3, indicating that the bonding electrons in the adjacent M–Pt bonds migrate into the central M–M bond to result in a dynamic structural change during two-electron oxidation of the hexanuclear metal strings. A similar Pt6 complex terminated by two iodide anions, [Pt6I2(μ-dpmp)4](PF6)2 (9), was synthesized from [Pt6(μ-H)I2(μ-dpmp)4](PF6) (5) by treatment with [Cp2Fe][PF6]. Complexes 7 and 8 were readily reacted with the neutral two-electron donors XylNC, CO, and phosphines to afford the trinuclear complexes [Pt2M(μ-dpmp)2(XylNC)L](PF6)2 (M = Pt, L = XylNC (1a), CO (10), PPh3 (11); M = Pd, L = XylNC (1b)) through cleavage of the electron-deficient central M–M bond. When complex 7 was reacted with the diphosphines (PP) trans-Ph2PCHCHPPh2 (dppen) and Ph2P(CH2)2PPh2 (dppe), the diphosphine was inserted into the central M–M bond to afford [(XylNC)Pt3(μ-dpmp)2(PP)Pt3(μ-dpmp)2(XylNC)](PF6)4 (12), which was transformed by treatment with another 1 equiv of diphosphine into the asymmetric trinuclear complexes [Pt3(μ-dpmp)2(XylNC)(PP)](PF6)2 (13). A further ligand exchange reaction of 13a (PP = trans-dppen) provided the diphosphine-terminated symmetrical Pt3 complex [Pt3(μ-dpmp)2(L)2](PF6)2 (L = trans-dppen (14a)). Complexes 7 and 8 were also reacted with [AuCl(PPh3)] to yield the Pt2MAu heterotetranuclear complexes [Pt2MAuCl(μ-dpmp)2(PPh3)(XylNC)](PF6)2 (M = Pt (15), Pd (16)), in which the Pt2M trinuclear fragment is inserted into the Au–Cl bond in a 1,1-fashion on the central M atoms of the Pt2M2Pt2 string.

Reaction of the linear trinuclear complex [Pt 2 M(μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 (M = Pd (1b)) with HgX 2 afforded the Pt 2 PdHg mixed-metal complexes [Pt 2 PdHgX 2 (μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 (X = Cl (2), Br (3), I (4)), which included an unprecedented Hg I −Pd I covalent bond formed via intramolecular metal−metal bond rearrangement. In contrast, reaction of 1a (M = Pt) with HgCl 2 afforded the novel pentagonal-shaped Pt 3 Hg 3 planar cluster [Pt 3 Hg 3 Cl 4 (μ-dpmp) 2 (XylNC) 2 ]Cl 2 (PF 6 ) 2 (5).M etal−metal-bonded small-size clusters are fine-tunable building blocks for nanostructured molecular devices and metal surface mimetic catalytic systems. 1,2 In particular, expanding heterometallic conjugates is a challenging synthetic subject, with the aim of exploring new heterometallic bonding systems and synergistic effects exerted within the heterometallic centers. 1,3 Taking this background into consideration, we have utilized linear tri-and tetraphosphines (bis[(diphenylphosphino)methyl]phenylphosphine (dpmp) and mesobis[((diphenylphosphino)methyl)phenylphosphino]methane (dpmppm)), in which each neighboring P atoms are connected by a methylene unit to support significant interactions between the metals and have synthesized a variety of linearly constrained multimetallic complexes. 4,5 Among them, the linear Pt 3 and Pt 2 Pd complexes [Pt 2 M(μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 2 (M = Pt (1a), Pd (1b); XylNC = 2,6-xylyl isocyanide), 4e recognized as good building blocks for molecular wires, have been investigated to expand by a direct metal−metal bonding interaction, leading to the hexanuclear metal wires of [ P t 2 M 2 P t 2 ( μ -H ) ( μ -d p m p ) 4 ( X y l N C ) 2 ] ( P F 6 ) 3 a n d [Pt 2 M 2 Pt 2 (μ-dpmp) 4 (XylNC) 2 ](PF 6 ) 4 (M = Pt, Pd), 4i and by bis-isocyanide linkers, forming the rigid-rod cluster polymers {[Pt 3 (μ-dpmp) 2 (bisNC)](PF 6 ) 2 } n , where bisNC is 2,3,5,6-tetramethylphenylene-1,4-bis-isocyanide. 4j Further, expanding and assembling the trimetallic units by additional heterometals would be a promising strategy, since d 10 metal ions with Lewis acidic properties, such as Au I , Ag I , and Hg II , are likely to form donor−acceptor interactions with low-valent metal centers. 3 In the present study, reactions of the Pt 2 Pd complex 1b with Hg II ions were examined to find a Hg I −Pd I covalent bond formed through intramolecular metal−metal bond rearrangement in a Pt 2 PdHg mixed-metal system. The reported structures including Hg−Pd fragments have really been limited only to donor−acceptor and metallophilic interactions (2.74− 3.28 Å), 6 and notably, the shortest Hg−Pd contact (2.6915(8) Å) reported thus far was a Hg II −Pd 0 (d 10 −d 10 ) interaction by Braunstein et al. in the Fe 2 PdHg heterometallic complex [Hg{Fe[Si(OMe) 3 ](CO) 3 (μ-dppm)} 2 Pd] (dppm = bis-(diphenylphosphino)methane). 7 The Hg−Pd bond length in the present report is unprecedentedly short (2.5830(5) Å) and is revealed to be the first example of a Hg I −Pd I covalent bond. The relevant reactions of the triplatinum complex...

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