The revisit of Ni_(3)(dpa)_(4)Cl_(2) (dpa~(-) = dipyridylamido anion) by the groups of Hathaway, Cotton, and ours in the early 1990s extended the knowledge of metal-metal bonds beyond dinuclear metal complexes and started the journey of metal string complexes. The typical conformation of metal string complexes consists of a one-dimensional oligonuclear transition metal backbone supported by four organic ligands. Because of this novel geometry, metal string complexes can provide great insight into multiple metal-metal bonds, and may have potential applications as molecular wires. This field therefore has grown significantly in the past 20 years and continues to be an attractive and dynamic area of study. In this review, we will firstly describe the origin of this research field. Important properties associated with the multinuclear metal-metal multiple bonding such as the variety in types of metal cores and molecular lengths, electronic configurations, magnetic properties, and the efficiency of electron-transport through the metal strings will be introduced. Finally, two new generations of metal string complexes, mixed-valence nickel string complexes and heteronuclear metal string complexes, will be introduced. Mixed-valence nickel string complexes contain mixed-valent ~(3+) units, which show about 40% enhancement of the conductance in comparison with those of typical nickel string complexes. Moreover, the insertion of a heteronuclear metal ion in a homonuclear metal string complex provides a new way of tuning its physical properties, which is a potential approach to build bio-mimics and inorganic electronic devices. This review will aim to provide readers with general understanding of the past results and future challenges of this research field.
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