Approaches to quantifying the electronic and steric properties of metallodithiolates as ligands in coordination chemistry

• Metallodithiolate ligands are stronger donors than phosphines.
• Highly asymmetric metallodithiolates are characterized by wedge and hinge angles.
• Customizable donor and steric properties.
• Butterfly type structures expected useful in bimetallic catalysis.

The well-known steric and electronic factors developed for phosphines by Tolman have influenced the design of organometallic complexes for catalytic processes for nearly a half century. Metallodithiolates as in square planar MN2S2, MP2S2, and MS′2S2 complexes utilize cis-dithiolates as mono- or bidentate donors to exogeneous metals, such as low valent metal carbonyls and nitrosyls wherein their established donor abilities are similar to phosphines. The highly asymmetric heterobimetallic complexes have folded “butterfly” structures deriving from their hinged M(μ-SR)2M′ cores. Analysis of the electronic factors of representatives from a broad series of MN2S2 complexes is referenced to electrochemical redox events as well as the vibrational spectroscopy of diatomic ligand reporters. Steric properties are assessed from XRD studies that yield common metric parameters such as bite angle; they define as well the hinge angle of the bridging thiolate, and the wedge that encompasses the largely planar ligands in bidentate bonding. Adaptation of various approaches to defining ligand steric factors such as the solid angle and percent-buried-volume permits comparison with ligands such as phosphines or simple imines. According to the percent buried volume, spacial requirements of MN2S2 ligands are within a range of 33–36% and in-between the 1,1-bis(diphenylphosphino)methane, dppm, (38%) and bipyridine (30%). The asymmetry and tunability of such MN2S2 metalloligands for both electronic and steric control are prospects for applications in catalytic processes.

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