Metal-ligand cooperativity

'''Metal-ligand cooperativity (MLC)''' is a mode of reactivity in which a metal and ligand of a complex are both involved in the bond breaking or bond formation of a substrate during the course of a reaction. This ligand is an actor ligand rather than a [[Spectator ligand|spectator]], and the reaction is generally only deemed to contain MLC if the actor ligand is doing more than leaving to provide an open coordination site. MLC is also referred to as "metal-ligand bifunctional catalysis." Note that MLC is not to be confused with [[cooperative binding]].

'''Metal-ligand cooperativity (MLC)''' is a mode of reactivity in which a metal and ligand of a complex are both involved in the bond breaking or bond formation of a substrate during the course of a reaction. This ligand is an actor ligand rather than a [[Spectator ligand|spectator]], and the reaction is generally only deemed to contain MLC if the actor ligand is doing more than leaving to provide an open coordination site. MLC is also referred to as "metal-ligand bifunctional catalysis." Note that MLC is not to be confused with [[cooperative binding]].

The earliest reported metal-ligand cooperativity was from the Fujiwara group in the 1950s, in which they reported formation of stilbene from [[styrene]] and arenes using a palladium chloride catalyst.{{Cite journal|date=1967-01-01|title=Aromatic substitution of styrene-palladium chloride complex|url=https://www.sciencedirect.com/science/article/abs/pii/S0040403900906488|journal=Tetrahedron Letters|language=en|volume=8|issue=12|pages=1119–1122|doi=10.1016/S0040-4039(00)90648-8|issn=0040-4039|last1=Moritanl|first1=Ichiro|last2=Fujiwara|first2=Yuzo|url-access=subscription}} [[Shvo catalyst|Shvo's catalyst]] was developed for one of the earliest uses of ketone hydrogenation by an [[Outer sphere electron transfer|outer-sphere mechanism]].{{Cite book|last=Hartwig|first=John F.|title=Organotransition metal chemistry : from bonding to catalysis|date=2010|isbn=978-1-891389-53-5|location=Mill Valley, Calif.|oclc=310401036}} [[Ryōji Noyori|Noyori]] has developed many chiral catalysts for [[asymmetric hydrogenation]].{{Cite journal|last1=Ohkuma|first1=Takeshi|last2=Ooka|first2=Hirohito|last3=Hashiguchi|first3=Shohei|last4=Ikariya|first4=Takao|last5=Noyori|first5=Ryoji|date=1995-03-01|title=Practical Enantioselective Hydrogenation of Aromatic Ketones|url=https://doi.org/10.1021/ja00114a043|journal=Journal of the American Chemical Society|volume=117|issue=9|pages=2675–2676|doi=10.1021/ja00114a043|issn=0002-7863|url-access=subscription}} [[Transfer hydrogenation]], one of the most commonly used applications of MLC, is employed broadly in industry for large scale Noyori-type reductions.{{Cite book|last=Crabtree|first=Robert H.|title=The organometallic chemistry of the transition metals|date=2014|isbn=978-1-118-78824-0|edition=Sixth|location=Hoboken, New Jersey|oclc=863383849}}{{Cite journal|last1=Dub|first1=Pavel A.|last2=Gordon|first2=John C.|date=2017-10-06|title=Metal–Ligand Bifunctional Catalysis: The "Accepted" Mechanism, the Issue of Concertedness, and the Function of the Ligand in Catalytic Cycles Involving Hydrogen Atoms|url=https://doi.org/10.1021/acscatal.7b01791|journal=ACS Catalysis|volume=7|issue=10|pages=6635–6655|doi=10.1021/acscatal.7b01791|osti=1409775}}{{Citation|last1=Zhang|first1=Xumu|title=Industrial Applications of Asymmetric (Transfer) Hydrogenation|date=2021|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527822294.ch6|work=Asymmetric Hydrogenation and Transfer Hydrogenation|pages=175–219|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/9783527822294.ch6|isbn=978-3-527-82229-4|access-date=2021-05-17|last2=Shao|first2=Pan-Lin|s2cid=233588784 |url-access=subscription}}{{Cite journal|last1=Shimizu|first1=Hideo|last2=Nagasaki|first2=Izuru|last3=Matsumura|first3=Kazuhiko|last4=Sayo|first4=Noboru|last5=Saito|first5=Takao|date=2007-12-01|title=Developments in Asymmetric Hydrogenation from an Industrial Perspective|url=https://doi.org/10.1021/ar700101x|journal=Accounts of Chemical Research|volume=40|issue=12|pages=1385–1393|doi=10.1021/ar700101x|pmid=17685581|issn=0001-4842|url-access=subscription}}

The earliest reported metal-ligand cooperativity was from the Fujiwara group in the 1950s, in which they reported formation of stilbene from [[styrene]] and arenes using a palladium chloride catalyst.{{Cite journal|date=1967-01-01|title=Aromatic substitution of styrene-palladium chloride complex|url=https://www.sciencedirect.com/science/article/abs/pii/S0040403900906488|journal=Tetrahedron Letters|language=en|volume=8|issue=12|pages=1119–1122|doi=10.1016/S0040-4039(00)90648-8|issn=0040-4039|last1=Moritanl|first1=Ichiro|last2=Fujiwara|first2=Yuzo|url-access=subscription}} [[Shvo catalyst|Shvo's catalyst]] was developed for one of the earliest uses of [[ketone]] hydrogenation by an [[Outer sphere electron transfer|outer-sphere mechanism]].{{Cite book|last=Hartwig|first=John F.|title=Organotransition metal chemistry : from bonding to catalysis|date=2010|isbn=978-1-891389-53-5|location=Mill Valley, Calif.|oclc=310401036}} [[Ryōji Noyori|Noyori]] has developed many chiral catalysts for [[asymmetric hydrogenation]].{{Cite journal|last1=Ohkuma|first1=Takeshi|last2=Ooka|first2=Hirohito|last3=Hashiguchi|first3=Shohei|last4=Ikariya|first4=Takao|last5=Noyori|first5=Ryoji|date=1995-03-01|title=Practical Enantioselective Hydrogenation of Aromatic Ketones|url=https://doi.org/10.1021/ja00114a043|journal=Journal of the American Chemical Society|volume=117|issue=9|pages=2675–2676|doi=10.1021/ja00114a043|issn=0002-7863|url-access=subscription}} [[Transfer hydrogenation]], one of the most commonly used applications of MLC, is employed broadly in industry for large scale Noyori-type reductions.{{Cite book|last=Crabtree|first=Robert H.|title=The organometallic chemistry of the transition metals|date=2014|isbn=978-1-118-78824-0|edition=Sixth|location=Hoboken, New Jersey|oclc=863383849}}{{Cite journal|last1=Dub|first1=Pavel A.|last2=Gordon|first2=John C.|date=2017-10-06|title=Metal–Ligand Bifunctional Catalysis: The "Accepted" Mechanism, the Issue of Concertedness, and the Function of the Ligand in Catalytic Cycles Involving Hydrogen Atoms|url=https://doi.org/10.1021/acscatal.7b01791|journal=ACS Catalysis|volume=7|issue=10|pages=6635–6655|doi=10.1021/acscatal.7b01791|osti=1409775}}{{Citation|last1=Zhang|first1=Xumu|title=Industrial Applications of Asymmetric (Transfer) Hydrogenation|date=2021|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527822294.ch6|work=Asymmetric Hydrogenation and Transfer Hydrogenation|pages=175–219|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/9783527822294.ch6|isbn=978-3-527-82229-4|access-date=2021-05-17|last2=Shao|first2=Pan-Lin|s2cid=233588784 |url-access=subscription}}{{Cite journal|last1=Shimizu|first1=Hideo|last2=Nagasaki|first2=Izuru|last3=Matsumura|first3=Kazuhiko|last4=Sayo|first4=Noboru|last5=Saito|first5=Takao|date=2007-12-01|title=Developments in Asymmetric Hydrogenation from an Industrial Perspective|url=https://doi.org/10.1021/ar700101x|journal=Accounts of Chemical Research|volume=40|issue=12|pages=1385–1393|doi=10.1021/ar700101x|pmid=17685581|issn=0001-4842|url-access=subscription}}

== Modes of Metal-Ligand Cooperativity ==

== Modes of Metal-Ligand Cooperativity ==

The ligand can act as a Lewis acid and accept electrons from an incoming substrate as it binds to the metal, as in employed in dehydrogenation catalysis. Conversely, the ligand can be Lewis basic and bind the substrate; this Lewis basicity is most frequently seen in hydrogenation catalysis.

The ligand can act as a Lewis acid and accept electrons from an incoming substrate as it binds to the metal, as in employed in dehydrogenation catalysis. Conversely, the ligand can be Lewis basic and bind the substrate; this Lewis basicity is most frequently seen in hydrogenation catalysis.

The [[aromatization]] and dearomatization of a ligand can serve to facilitate a reaction. As shown in the figure, a ligand can be dearomatized by a base and thus activated toward cleaving a C-H or H-H bond and be subsequently rearomatized during substrate bond cleavage. [[N-heterocyclic carbene|NHC ligands]] and other [[Transition metal pincer complex|pincer]] ligands are frequently employed in this mode of MLC.{{Cite journal|last1=Khusnutdinova|first1=Julia R.|last2=Milstein|first2=David|date=2015|title=Metal–Ligand Cooperation|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201503873|journal=Angewandte Chemie International Edition|language=en|volume=54|issue=42|pages=12236–12273|doi=10.1002/anie.201503873|pmid=26436516|issn=1521-3773|url-access=subscription}} In some reports, with bidentate ligands, ligand dearomatization is not observed when the complex is treated with base but rather a complex with a formal metal-carbon bond is observed (that then acts as a Lewis basic ligand).{{Cite journal|last1=Vigneswaran|first1=Vipulan|last2=Abhyankar|first2=Preshit C.|last3=MacMillan|first3=Samantha N.|last4=Lacy|first4=David C.|date=2022-01-10|title=H 2 Activation across Manganese(I)–C Bonds: Atypical Metal–Ligand Cooperativity in the Aromatization/Dearomatization Paradigm|url=https://pubs.acs.org/doi/10.1021/acs.organomet.1c00606|journal=Organometallics|language=en|volume=41|issue=1|pages=67–75|doi=10.1021/acs.organomet.1c00606|s2cid=245571634 |issn=0276-7333|url-access=subscription}}{{Cite journal|last1=Fanara|first1=Paul M.|last2=Vigneswaran|first2=Vipulan|last3=Gunasekera|first3=Parami S.|last4=MacMillan|first4=Samantha N.|last5=Lacy|first5=David C.|date=2022-01-10|title=Reversible Photoisomerization in a Ru cis -Dihydride Catalyst Accessed through Atypical Metal–Ligand Cooperative H 2 Activation: Photoenhanced Acceptorless Alcohol Dehydrogenation|url=https://pubs.acs.org/doi/10.1021/acs.organomet.1c00648|journal=Organometallics|language=en|volume=41|issue=1|pages=93–98|doi=10.1021/acs.organomet.1c00648|s2cid=245315751 |issn=0276-7333|url-access=subscription}} In fact, the aromatization–dearomatization MLC pathway is governed by the acid–base properties of the reaction medium. Under basic conditions, the ligand preferentially undergoes dearomatization, whereas under acidic conditions, the aromatized form is thermodynamically favored.{{Cite journal|last1=Ajitha|first1=Manjaly|last2=Huang|first2=Kuo-Wei|date=2025-08-25|title=The Role of Substrate Acidity in PN3P–Ru Pincer Complex Catalyzed Formic Acid Dehydrogenation: Pseudo-Dearomatization vs Non-Dearomatization Pathways|url=https://pubs.acs.org/doi/full/10.1021/acs.organomet.5c00244|journal=Organometallics|language=en|volume=44|issue=18|pages=2099–2106|doi=10.1021/acs.organomet.5c00244|s2cid=245571634 |issn=0276-7333|url-access=subscription}}

The [[aromatization]] and dearomatization of a ligand can serve to facilitate a reaction. As shown in the figure, a ligand can be dearomatized by a base and thus activated toward cleaving a C-H or H-H bond and be subsequently rearomatized during substrate [[bond cleavage]]. [[N-heterocyclic carbene|NHC ligands]] and other [[Transition metal pincer complex|pincer]] ligands are frequently employed in this mode of MLC.{{Cite journal|last1=Khusnutdinova|first1=Julia R.|last2=Milstein|first2=David|date=2015|title=Metal–Ligand Cooperation|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201503873|journal=Angewandte Chemie International Edition|language=en|volume=54|issue=42|pages=12236–12273|doi=10.1002/anie.201503873|pmid=26436516|issn=1521-3773|url-access=subscription}} In some reports, with bidentate ligands, ligand dearomatization is not observed when the complex is treated with base but rather a complex with a formal metal-carbon bond is observed (that then acts as a Lewis basic ligand).{{Cite journal|last1=Vigneswaran|first1=Vipulan|last2=Abhyankar|first2=Preshit C.|last3=MacMillan|first3=Samantha N.|last4=Lacy|first4=David C.|date=2022-01-10|title=H 2 Activation across Manganese(I)–C Bonds: Atypical Metal–Ligand Cooperativity in the Aromatization/Dearomatization Paradigm|url=https://pubs.acs.org/doi/10.1021/acs.organomet.1c00606|journal=Organometallics|language=en|volume=41|issue=1|pages=67–75|doi=10.1021/acs.organomet.1c00606|s2cid=245571634 |issn=0276-7333|url-access=subscription}}{{Cite journal|last1=Fanara|first1=Paul M.|last2=Vigneswaran|first2=Vipulan|last3=Gunasekera|first3=Parami S.|last4=MacMillan|first4=Samantha N.|last5=Lacy|first5=David C.|date=2022-01-10|title=Reversible Photoisomerization in a Ru cis -Dihydride Catalyst Accessed through Atypical Metal–Ligand Cooperative H 2 Activation: Photoenhanced Acceptorless Alcohol Dehydrogenation|url=https://pubs.acs.org/doi/10.1021/acs.organomet.1c00648|journal=Organometallics|language=en|volume=41|issue=1|pages=93–98|doi=10.1021/acs.organomet.1c00648|s2cid=245315751 |issn=0276-7333|url-access=subscription}} In fact, the aromatization–dearomatization MLC pathway is governed by the acid–base properties of the reaction medium. Under basic conditions, the ligand preferentially undergoes dearomatization, whereas under acidic conditions, the aromatized form is thermodynamically favored.{{Cite journal|last1=Ajitha|first1=Manjaly|last2=Huang|first2=Kuo-Wei|date=2025-08-25|title=The Role of Substrate Acidity in PN3P–Ru Pincer Complex Catalyzed Formic Acid Dehydrogenation: Pseudo-Dearomatization vs Non-Dearomatization Pathways|url=https://pubs.acs.org/doi/full/10.1021/acs.organomet.5c00244|journal=Organometallics|language=en|volume=44|issue=18|pages=2099–2106|doi=10.1021/acs.organomet.5c00244|s2cid=245571634 |issn=0276-7333|url-access=subscription}}

The ligand can also be redox non-innocent to facilitate reactions that the metal would otherwise be unable to activate. The ligand can act as an electron reservoir, which is enabled when ligands contain [[frontier orbital]]s of suitable energy to participate in the redox event themselves, and can accept or donate electrons during the course of the reaction, allowing the metal to modulate its oxidation state. This allows metals which normally only participate in one electron regimes to be used in two electron regimes with a redox [[non-innocent ligand]] to store electrons during the reaction. Dithiolate ligands have been used extensively as one electron redox active ligands in metal complexes.{{Cite journal|last1=Eisenberg|first1=Richard|last2=Gray|first2=Harry B.|date=2011-10-17|title=Noninnocence in Metal Complexes: A Dithiolene Dawn|url=https://pubs.acs.org/doi/pdf/10.1021/ic2011748|journal=Inorganic Chemistry|volume=50|issue=20|pages=9741–9751|doi=10.1021/ic2011748|pmid=21913669|issn=0020-1669|url-access=subscription}} For example, dithiolates have been demonstrated to allow for the selective and reversible reduction of ethylene in the presence H₂, CO, and H₂S. This has applications in the purification of ethylene gas streams, in which ethylene can be reduced electrochemically by a dithiolate, selectively removed from the impurities in the stream, and then reversibly desaturated.{{Cite journal|last=Wang|first=K.|date=2001-01-05|title=Toward Separation and Purification of Olefins Using Dithiolene Complexes: An Electrochemical Approach|url=https://www.science.org/doi/10.1126/science.291.5501.106|journal=Science|volume=291|issue=5501|pages=106–109|doi=10.1126/science.291.5501.106|pmid=11141557|bibcode=2001Sci...291..106W|url-access=subscription}}

The ligand can also be redox non-innocent to facilitate reactions that the metal would otherwise be unable to activate. The ligand can act as an electron reservoir, which is enabled when ligands contain [[frontier orbital]]s of suitable energy to participate in the redox event themselves, and can accept or donate electrons during the course of the reaction, allowing the metal to modulate its oxidation state. This allows metals which normally only participate in one electron regimes to be used in two electron regimes with a redox [[non-innocent ligand]] to store electrons during the reaction. Dithiolate ligands have been used extensively as one electron redox active ligands in metal complexes.{{Cite journal|last1=Eisenberg|first1=Richard|last2=Gray|first2=Harry B.|date=2011-10-17|title=Noninnocence in Metal Complexes: A Dithiolene Dawn|url=https://pubs.acs.org/doi/pdf/10.1021/ic2011748|journal=Inorganic Chemistry|volume=50|issue=20|pages=9741–9751|doi=10.1021/ic2011748|pmid=21913669|issn=0020-1669|url-access=subscription}} For example, dithiolates have been demonstrated to allow for the selective and reversible reduction of ethylene in the presence H₂, CO, and H₂S. This has applications in the purification of ethylene gas streams, in which ethylene can be reduced electrochemically by a dithiolate, selectively removed from the impurities in the stream, and then reversibly desaturated.{{Cite journal|last=Wang|first=K.|date=2001-01-05|title=Toward Separation and Purification of Olefins Using Dithiolene Complexes: An Electrochemical Approach|url=https://www.science.org/doi/10.1126/science.291.5501.106|journal=Science|volume=291|issue=5501|pages=106–109|doi=10.1126/science.291.5501.106|pmid=11141557|bibcode=2001Sci...291..106W|url-access=subscription}}

== Mechanism of Hydrogenations with Metal-Ligand Cooperativity ==

== Mechanism of Hydrogenations with Metal-Ligand Cooperativity ==

MLC is most frequently used in hydrogenations, with many applications in asymmetric catalysis and in process scale production of chemicals. In a hydrogenation, there is a transfer of a hydride and a hydrogen to a substrate. Typical substrates include aldehydes, ketones, and imines. As this is a common use for MLC, it is instructive in understanding the mechanism of metal-ligand cooperativity. MLC occurs through an outer sphere mechanism. An outer sphere mechanism does not necessitate that the metal undergo oxidative addition or reductive elimination. Thus, H₂ is not added across the metal, but rather across the metal and a ligand; alternatively, the metal complexes are preformed to contain a hydride ligand as well as a ligand with a hydrogen alpha to the metal. Thus, the hydride and hydrogen are adjacent to one another, facilitating the transfer to the substrate; this transfer occurs without the substrate ever binding to the metal itself.{{Cite journal|last1=Eisenstein|first1=Odile|last2=Crabtree|first2=Robert H.|date=2013|title=Outer sphere hydrogenation catalysis|url=http://xlink.rsc.org/?DOI=C2NJ40659D|journal=New J. Chem.|language=en|volume=37|issue=1|pages=21–27|doi=10.1039/C2NJ40659D|issn=1144-0546|url-access=subscription}} Though amine is by far the most used ligand in cooperativity, other actor ligands include alkoxides and thiols.

MLC is most frequently used in hydrogenations, with many applications in asymmetric catalysis and in process scale production of chemicals. In a hydrogenation, there is a transfer of a hydride and a hydrogen to a substrate. Typical substrates include aldehydes, ketones, and imines. As this is a common use for MLC, it is instructive in understanding the mechanism of metal-ligand cooperativity. MLC occurs through an outer sphere mechanism. An outer sphere mechanism does not necessitate that the metal undergo [[oxidative addition]] or reductive elimination. Thus, H₂ is not added across the metal, but rather across the metal and a ligand; alternatively, the metal complexes are preformed to contain a hydride ligand as well as a ligand with a hydrogen alpha to the metal. Thus, the hydride and hydrogen are adjacent to one another, facilitating the transfer to the substrate; this transfer occurs without the substrate ever binding to the metal itself.{{Cite journal|last1=Eisenstein|first1=Odile|last2=Crabtree|first2=Robert H.|date=2013|title=Outer sphere hydrogenation catalysis|url=http://xlink.rsc.org/?DOI=C2NJ40659D|journal=New J. Chem.|language=en|volume=37|issue=1|pages=21–27|doi=10.1039/C2NJ40659D|issn=1144-0546|url-access=subscription}} Though amine is by far the most used ligand in cooperativity, other actor ligands include alkoxides and thiols.

[[File:Inner Vs Outer Sphere Mechanisms.png|thumb|400px|The outer sphere mechanism for MLC compared to an inner sphere mechanism without ligand cooperativity.]]

[[File:Inner Vs Outer Sphere Mechanisms.png|thumb|400px|The outer sphere mechanism for MLC compared to an inner sphere mechanism without ligand cooperativity.]]

In contrast, in an inner sphere mechanism, the substrate will be inserted into the metal and reaction with hydrogen will then afford the hydrogenated product. This mechanism does not employ MLC. The differentiation between an outer sphere mechanism relying on MLC and an inner sphere mechanism is exemplified by cobalt hydrogenation with an amine pincer ligand. In the outer sphere mechanism, the hydrogen on the pincer ligand is added into the ketone along with a hydride ligand on the metal.{{Cite journal|last1=Zhang|first1=Guoqi|last2=Vasudevan|first2=Kalyan V.|last3=Scott|first3=Brian L.|last4=Hanson|first4=Susan K.|date=2013-06-12|title=Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions|url=https://doi.org/10.1021/ja402679a|journal=Journal of the American Chemical Society|volume=135|issue=23|pages=8668–8681|doi=10.1021/ja402679a|pmid=23713752|issn=0002-7863|url-access=subscription}} It is worth noting that there is debate over the concertedness of the [[transition state]] of this outer sphere hydrogenation step, and different reactions and catalysts may be either concerted or stepwise, and in some scenarios there may be multiple pathways at play. In comparison to the ketone hydrogenation, an olefin undergoes an inner sphere mechanism under the same reaction conditions, in which the olefin inserts directly into the metal. These mechanistic differences between the ketone and olefin are corroborated by the observation that the ketone hydrogenation will not occur with an N-Me pincer ligand, and the olefin hydrogenation will proceed with the N-Me ligand, suggesting the ketone requires the presence of the N-H bond while the olefin does not.

In contrast, in an inner sphere mechanism, the substrate will be inserted into the metal and reaction with hydrogen will then afford the hydrogenated product. This mechanism does not employ MLC. The differentiation between an outer sphere mechanism relying on MLC and an inner sphere mechanism is exemplified by cobalt hydrogenation with an amine pincer ligand. In the outer sphere mechanism, the hydrogen on the pincer ligand is added into the ketone along with a hydride ligand on the metal.{{Cite journal|last1=Zhang|first1=Guoqi|last2=Vasudevan|first2=Kalyan V.|last3=Scott|first3=Brian L.|last4=Hanson|first4=Susan K.|date=2013-06-12|title=Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions|url=https://doi.org/10.1021/ja402679a|journal=Journal of the American Chemical Society|volume=135|issue=23|pages=8668–8681|doi=10.1021/ja402679a|pmid=23713752|issn=0002-7863|url-access=subscription}} It is worth noting that there is debate over the concertedness of the [[transition state]] of this outer sphere hydrogenation step, and different reactions and catalysts may be either concerted or stepwise, and in some scenarios there may be multiple pathways at play. In comparison to the ketone hydrogenation, an olefin undergoes an inner sphere mechanism under the same reaction conditions, in which the olefin inserts directly into the metal. These mechanistic differences between the ketone and olefin are corroborated by the observation that the ketone hydrogenation will not occur with an N-Me pincer ligand, and the olefin hydrogenation will proceed with the N-Me ligand, suggesting the ketone requires the presence of the N-H bond while the olefin does not.