Bis(cyclopentadienyl) complexes (Cp 2 M) of the divalent first-row transition metals V-Ni have been known and used for over 50 years. For almost as long, an analogous series of compounds has been known with the indenyl ligand (i.e., Ind 2 M), with the conspicuous exception of M = Mn. Bis(indenyl) complexes of manganese(II), Ind 0 2 MnL n , have now been synthesized by halide metathesis from MnCl 2 and an appropriate potassium indenide. Depending on the indenyl ligand substituents and the presence of coordinated bases, a variety of structural motifs and bonding modes of the indenyl ligand are found in the resulting complexes. Single-crystal X-ray structures obtained for [2-(SiMe 3 )C 9 H 6 ] 2 Mn, [1,3-(SiMe 3 ) 2 C 9 H 5 ] 2 Mn, and [1,3-(i-Pr) 2 C 9 H 5 ] 2 Mn confirm that they possess classic η 5 -bound sandwich structures. In contrast, the unsubstituted parent complex recrystallizes from THF as a disolvate with two differently bonded indenyl ligands: i.e., (η 3 -C 9 H 7 )(η 1 -C 9 H 7 )Mn(thf ) 2 . Without the coordinated solvent, density functional theory calculations suggest that the complex would have two slipped η 5 -bound ligands. When methyl groups are present on the benzo portion of the indenyl ligand, specifically in the 4,7-positions, the corresponding manganese complex is isolated as a cyclic octomer, {(4,7-Me 2 C 9 H 5 ) 2 Mn} 8 , containing both bridging and terminal indenyl ligands. In the presence of 1,4-dioxane, however, attempted synthesis of (4,7-Me 2 C 9 H 5 ) 2 Mn results in the isolation of the [K(dioxane) 1.5 ][Mn(4,7-Me 2 C 9 H 5 ) 3 ] salt, in which each manganese atom is surrounded by a paddlewheel of three η 2 -bound 4,7-dimethylindenyl ligands. Cation-π bonding to the potassium and the presence of coordinated dioxane molecules generates a layered structure for the salt. Magnetic susceptibility measurements on the compounds indicate the presence of high-spin Mn(II) centers in all cases. These compounds demonstrate the high degree of conformational flexibility in the Mn(II)-indenyl bond.

Changes in the magnetic characteristics of transition metal complexes, particularly, their metal spin states, are most commonly achieved by varying the electron donor/acceptor properties of coordinated ligands. In some systems, it is possible to increase the steric bulk of ligands so that intramolecular crowding leads to changes in metal‐ligand bond distances; longer bonds produce weaker ligand fields and stabilize high‐spin or spin‐crossover complexes. Steric effects are also often associated, albeit indirectly, with a mechanism for influencing spin states that relies on modifying the symmetry‐based interactions of metal and ligand orbitals. Particular ligand orientations, as in bis(indenyl) compounds [M(R 9 C 7 ) 2 ], can be changed as the result of interligand steric pressure or crystal packing effects; such differences can alter metal d‐orbital energy levels and hence the stability of spin states. Both steric and symmetry effects add to the repertoire of techniques that can be used to control magnetic properties in transition metal complexes.

In the more than 50 years since the first manganese compound with MnC bonds was crystallographically characterized (Mn 2 (CO) 10 , in 1957), the number of structurally authenticated organomanganese compounds has grown to several thousands. The richness of structural features in organomanganese compounds now rivals that of any other first row transition metal. The range of structural features and bond types is large, and encompasses MnC, MnC, and MnC bonds, as well as complexes with MnMn, MnMn, and MnMn linkages. Crystallographically characterized examples are known with oxidation states from Mn 0 to Mn V , and the accessibility of two spin states of comparable energy in complexes of Mn II adds an additional variable to the fundamental structural types. The present review of structural organomanganese chemistry focuses largely on crystallographically characterized complexes, whose geometries are the most reliably established. Representative examples of major compound classes are considered, and statistical analysis of typical bond length ranges is provided for various Mn/C and Mn/Mn interactions. Bond lengths in organomanganese compounds are highly context‐sensitive, and even when only a specific bond order is under consideration, the spread of distances makes comparisons difficult unless closely related molecules are examined.

Changes in the magnetic characteristics of transition metal complexes, particularly, their metal spin states, are most commonly achieved by varying the electron donor/acceptor properties of coordinated ligands. In some systems, it is possible to increase the steric bulk of ligands so that intramolecular crowding leads to changes in metal‐ligand bond distances; longer bonds produce weaker ligand fields and stabilize high‐spin or spin‐crossover complexes. Steric effects are also often associated, albeit indirectly, with a mechanism for influencing spin states that relies on modifying the symmetry‐based interactions of metal and ligand orbitals. Particular ligand orientations, as in bis(indenyl) compounds [M(R 9 C 7 ) 2 ], can be changed as the result of interligand steric pressure or crystal packing effects; such differences can alter metal d‐orbital energy levels and hence the stability of spin states. Both steric and symmetry effects add to the repertoire of techniques that can be used to control magnetic properties in transition metal complexes.