Structure, Bonding, and Activation of Dihydrogen Complexes

Transition metals can contain, within their coordination field, hydrogen in molecular (H2) and/or atomic (hydride) form. These types of metal-ligand complexes are unquestionably the most dynamic molecular systems known in terms of structural variability and atom motion/exchange processes. Until about 20 years ago, metals were known to contain only atomically bound hydrogen, that is, metal hydrides and metal hydride complexes (L MH,., where L is an ancillary ligand). However the discovery by Kubas and coworkers [1] in 1983 of side-on coordination of a nearly intact dihydrogen molecule (H2) to a metal complex has led to a new paradigm in chemistry that is the subject of many review articles [2-10]. 

Hydrogen-Transfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. Limbach, and R. L. Schowen Copyright 2007 WILEY-VCH Veriag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

Complexes with d H 1.6 A are considered to be hydrides, but even this is a subjective boundary. There is little H-H bonding interaction remaining when duH becomes 1.1 A, so intriguing questions arise such as at what point is the bond broken By one criterion, the H-H bond can be considered broken when 

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