Two specific aspects are studied within the relatively new and fascinating research area of „bioinorganic chemistry", i.e. the elucidation of the functions of inorganic elements in organisms : The ability of coenzymes to interact with metal centers  and the special role of copper in the biosphere .
Transition metal ions as well as redox active coenzymes (flavins, pterins, quinones) participate in the conversion of energy and matter in cells. However, only in a few cases is the interaction of these components chemically well documented; the best known example is the synergy of iron and the porphyrin macrocycle in the heme group. Yet there are other, only recently established such combinations in enzymes, namely between copper and quinones or between molybdenum and pterins [1,3].
Our work in this area aims at helping to understand the latest developments in protein structure analysis through research in coordination chemistry. The cooperation between metals and coenzymes with regard to electron transfer reactions and the (catalytic) activation of substrates is the central point of investigations in which electrochemical and spectroscopic methods  are used to probe novel, often structurally characterized compounds.
 “Cooperation of Metals with Electroactive Ligands of Biochemical Relevance: Beyond Metalloporphyrins”, W. Kaim and B. Schwederski, Pure Appl. Chem. 76 (2004) 351-364; “Non-innocent ligands in bioinorganic chemistry – an overview”, W. Kaim and B. Schwederski, Coord. Chem. Rev. 254 (2010) 1580-1588; "Coordination Compunds of Pteridine, Alloxazine and Flavine Ligands: Structures and Properties; W. Kaim, B. Schwederski, O. Heilmann and F. Hornung, Coord. Chem. Rev. 182 (1999) 323-342.
 "Kupfer - ein modernes Bioelement", W. Kaim and J. Rall, Angew. Chem. 108 (1996) 47; "Copper - a Modern Bioelement", W. Kaim and J. Rall,
Angew. Chem. Int. Ed. Engl. 35 (1996) 43-60; "The chemistry and biochemistry of the copper-radical interaction", W. Kaim, Dalton Trans. 2003, 761-768. (pdf)
 "Mixed-Valence Intermediates as Ideal Targets for Spectroelectrochemistry (SEC)", W. Kaim, B. Sarkar and G.K. Lahiri in , Eds. W. Kaim and A. Klein, Royal Society of Chemistry (Cambridge), 2008, p. 68-90, ISBN 9780854045501 ; "Spectroelectrochemistry: the best of two worlds" W. Kaim and J. Fiedler, Chem. Soc. Rev. 38 (2009) 3373-3382.
Transition metals are often essential for the catalysis of energy producing redox processes in biochemistry as well as in chemical technology. Elementary aspects of these reactions which are still little understood are being investigated. This includes thermally or optically induced intramolecular electron transfer and charge transfer processes between organic ligands and metal centers in coordination compounds, metal-metal communication in mixed valent polynuclear complexes  as well as magnetism, electrostatic interaction (electrochemistry) and magnetic resonance phenomena in structurally defined complexes. Based on model calculations the interesting, active states of such molecules are studied with regard to their reactivity.
A typical example  is the development of catalysts for hydride transfer reactions, including the formation of hydrogen from water. Coordination compounds of the transition metals rhodium or iridium with special ligands serve to reduce the activation energy, stabilize reactive intermediates and thus minimize electrical energy without leading to unwanted by-products.
Molecular catalysts and sensitizers for the use of light energy can be functionally optimized through a computer-aided design of functional ligands . The well defined structure of metal complexes and the combination of a variegated ligand structure with the number of suitable metal centers from the periodic table renders such systems especially attractive for performing molecular functions.
 "Exploration of Mixed-Valence Chemistry: Inventing New Analogues of the Creutz-Taube Ion"; W. Kaim, A. Klein and M. Glöckle, Acc. Chem. Res. 33 (2000) 755-763; "Mixed Valency in Ruthenium Complexes - Coordinative Aspects"; W. Kaim and B. Sarkar, Coord. Chem. Rev. 251 (2007) 584-594; "Unconventional Mixed-Valent Complexes of Ruthenium and Osmium"; W. Kaim and G.K. Lahiri, Angew. Chem. 119 (2007) 1808-1828; Angew. Chem. Int. Ed. 46 (2007) 1778-1796; “Concepts for metal complex chromophors absorbing in the near infrared”, W. Kaim, Coord. Chem. Rev. 255 (2011) 2503-2513.
 "Boron Atoms as Spin Carriers in Two- and Three-Dimensional Systems"; W. Kaim, N.S. Hosmane, S. Záliš, J.A. Maguire and W.N. Lipscomb, Angew. Chem. 121 (2009) 5184-5193; Angew. Chem. Int Ed. 48 (2009) 5082-5091.
 "From Electron Transfer to Chemistry: Electrochemical Analysis of Organometallic Reaction Centers and of their Interaction Across Ligand Bridges"; W. Kaim, in New Trends in Molecular Electrochemistry, Ed. A.J.L. Pombeiro, Fontis Media, Lausanne, p. 127-151.
 "Complexes with 2,2'-azobispyridine and related S-frame bridging ligands containing the azo function", W. Kaim, Coord. Chem. Rev. 291-221 (2001) 463488; "The coordination chemistry of 1,2,4,5-tetrazines", W. Kaim, Coord. Chem. Rev. 230 (2002) 127-139.
Metal complexes with M(NO) bonds are not only interesting due to their physiological relevance, but also due to their electronic structure which is often discussed controversially . Our own experiments using in situ-low temperature-techniques (ESR, IR, UV-VIS-NIR) are aimed at solving this problem (oxidation states !) in combination with quantum mechanical calculations. Several other complexes with non-innocent ligand behavior involving quinones or azo compounds are being investigated and reviewed .
 "Electronic structure alternatives in nitrosylruthenium complexes"; G.K. Lahiri and W. Kaim, Dalton Trans. 39(2010) 4471-4478.
 "Manifestation of Non-innocent Ligand Behavior"; W. Kaim, Inorg. Chem. 50 (2011) 9752-9765; "The Shrinking World of Innocent Ligands: Conventional and Non-conventional Redox-active Ligands"; W. Kaim, Eur. J. Inorg. Chem. 2012, 343-348.
(1) “The chemistry and biochemistry of the copper-radical interaction"
W. Kaim, Dalton Trans. 2003, 761-768.
(2) “Mixed Valency in Ruthenium Complexes - Coordinative Aspects”
W. Kaim and B. Sarkar, Coord. Chem. Rev. 251 (2007) 584-594.
(3) “Unconventional Mixed-Valent Complexes of Ruthenium and Osmium”
W. Kaim and G.K. Lahiri, Angew. Chem. 119 (2007) 1808-1828; Angew. Chem. Int. Ed. 46 (2007) 1778-1796.
(4) “Boron Atoms as Spin Carriers in Two- and Three-Dimensional Systems”
W. Kaim, N.S. Hosmane, S. Záliš, J.A. Maguire and W.N. Lipscomb, Angew. Chem. 121 (2009) 5184-5193; Angew. Chem. Int Ed. 48 (2009) 5082-5091.
(5) “Spectroelectrochemistry: the best of two worlds”
W. Kaim and J. Fiedler, Chem. Soc. Rev. 38 (2009) 3373-3382.
(6) “Electronic structure alternatives in nitrosylruthenium complexes”
G.K. Lahiri and W. Kaim, Dalton Trans. 39 (2010) 4471-4478.
(7) “Non-innocent ligands in bioinorganic chemistry – an overview”
W. Kaim and B. Schwederski, Coord. Chem. Rev. 254 (2010) 1580-1588.
(8) “Concepts for metal complex chromophors absorbing in the near infrared”
W. Kaim, Coord. Chem. Rev. 255 (2011) 2503-2513.
(9) “Manifestation of Non-innocent Ligand Behavior"
W. Kaim, Inorg. Chem. 50 (2011) 9752-9765.
(10) “The Shrinking World of Innocent Ligands: Conventional and Non-conventional Redox-active Ligands”
W. Kaim, Eur. J. Inorg. Chem. 2012, 343-348.
(11) “Electron Transfer Induced Coordination Changes in Organometallic Complexes with Non-innocent Hemilabile Ligands”
W. Kaim, M. Bubrin und R. Hübner, in Advances in Organometallic Chemistry and Catalysis, Hrsg. A.J.L. Pombeiro, Wiley, Hoboken, 2014, 667-675.
(12) “Complete and Partial Electron Transfer Involving Coordinated NOx”
W. Kaim in NOx Related Chemistry, Vol. 67, R. van Eldik, J. Olabe, Hrsg., ADIOCH, UK: Academic Press, 2015, S. 295-313.
(13) “Electron Transfer Reactivity of Organometallic Compounds Involving Radical-Forming Noninnocent Ligands”
W. Kaim, Proc. Natl. Acad. Sci., India, Sect. A Phys. Sci. 86 (2016) 445-457.
(14) “Interacting Metal and Ligand Based Open Shell Systems: Challenges for Experiment and Theory”
W. Kaim und A. Paretzki, Coord. Chem. Rev. 344 (2017) 345-354.