Structure and Bonding 174 Series Editor: D.M.P. Mingos Stefanie Dehnen Editor Clusters – Contemporary Insight in Structure and Bonding 174 Structure and Bonding Series Editor: D.M.P. Mingos, Oxford, United Kingdom Editorial Board: X. Duan, Beijing, China L.H. Gade, Heidelberg, Germany Y. Lu, Urbana, IL, USA F. Neese, Mulheim€ an der Ruhr, Germany J.P. Pariente, Madrid, Spain S. Schneider, Gottingen,€ Germany D. Stalke, Go¨ttingen, Germany Aims and Scope Structure and Bonding is a publication which uniquely bridges the journal and book format. Organized into topical volumes, the series publishes in depth and critical reviews on all topics concerning structure and bonding. With over 50 years of history, the series has developed from covering theoretical methods for simple molecules to more complex systems. Topics addressed in the series now include the design and engineering of molecular solids such as molecular machines, surfaces, two dimensional materials, metal clusters and supramolecular species based either on complementary hydrogen bonding networks or metal coordination centers in metal-organic framework mate- rials (MOFs). Also of interest is the study of reaction coordinates of organometallic transformations and catalytic processes, and the electronic properties of metal ions involved in important biochemical enzymatic reactions. Volumes on physical and spectroscopic techniques used to provide insights into structural and bonding problems, as well as experimental studies associated with the development of bonding models, reactivity pathways and rates of chemical processes are also relevant for the series. Structure and Bonding is able to contribute to the challenges of communicating the enormous amount of data now produced in contemporary research by producing volumes which summarize important developments in selected areas of current interest and provide the conceptual framework necessary to use and interpret mega- databases. We welcome proposals for volumes in the series within the scope mentioned above. Structure and Bonding offers our authors and readers: • OnlineFirst publication. Each chapter is published online as it is finished, ahead of the print volume • Wide dissemination. The chapters and the volume will be available on our platform SpringerLink, one of the largest collections of scholarly content in the world. SpringerLink attracts more than 50 million users at 15.000 institutions worldwide. • Easy manuscript preparation. Authors do not have to spend their valuable time on the layout of their contribution. Springer will take care of all the layout related issues and will provide support throughout the complete process. More information about this series at http://www.springer.com/series/430 Stefanie Dehnen Editor Clusters – Contemporary Insight in Structure and Bonding With contributions by J.F. Corrigan Á S. Dehnen Á J.M. Goicoechea Á C. Heindl Á Y. Huang Á T.I. Levchenko Á M. Mehring Á G. Niedner-Schatteburg Á E. Peresypkina Á M. Scheer Á A. Schnepf Á A. Virovets Á F. Weigend Á B. Weinert Editor Stefanie Dehnen Fachbereich Chemie Philipps-Universita¨t Marburg and Wissenschaftliches Zentrum fur€ Materialwissenschaften Marburg, Germany ISSN 0081-5993 ISSN 1616-8550 (electronic) Structure and Bonding ISBN 978-3-319-52294-4 ISBN 978-3-319-52296-8 (eBook) DOI 10.1007/978-3-319-52296-8 Library of Congress Control Number: 2017935850 © Springer International Publishing AG 2017 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer International Publishing AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Preface What is a cluster? This question has been a topic of discussion over the years since the famous first definition by F. A. Cotton in the early 1970s, when he introduced the term to describe complexes that comprise at least three metal atoms that are connected by direct metal–metal bonds. The historic definition was rather narrow and restrictive regarding the choice of respective compounds, but at the same time was very clear. Later on, it became obvious, however, that it is also reasonable to call other molecules a cluster if they possess a certain size, even if the metal atoms in them were connected in part or exclusively via bridging non-metal ligands. Furthermore, the term was also used for large non-metal aggregates, including non-covalently bonded assemblies, like water clusters. During the last two decades, the increasing search for buzz words unfortunately led to application of the term also to small molecular assemblies, such as binuclear coordination compounds. The latter do apparently not fulfil any prerequisite of a cluster regarding the encyclo- pedic definition of the word as “a group of similar things or people positioned or occurring closely together”. This rather general definition is reflected by the use of the term not only in chemistry but also in other fields like Music, Arts, Linguistics, Urbanism, Astrophysics, Biology, Health Science, Engineering and Computer Sciences. So, where are we actually today? The IUPAC provides the chemical community with the following current recommendation: “A number of metal centres grouped close together which can have direct metal bonding interactions or interactions through a bridging ligand, but are not necessarily held together by these interac- tions”, which summarizes the aforesaid and excludes aggregates of uncountable units such as found in nanoparticles. Within this volume of Structure and Bonding, we try to provide the community with a collection of articles on different cluster compounds, all of which comply with the contemporary definition that relies on the literal meaning of the word. Hence, all of the examples called “clusters” within this volume exhibit a certain molecular size, with a critical minimum number of involved atoms or units equal to three. As outlined above, we do not recommend restricting the term on metal–metal bonded systems, as this would exclude a v vi Preface tremendous number of beautiful molecules that cannot be named other than a cluster in a reasonable way. It remains questionable whether or not to call poly- atomic non-metal main group molecules a cluster, as there are usually specific alternative terminologies for such systems, like fullerenes/-ides or boranes/-ides. However, we do not see a principal contradiction for it. The rather semantic question on the terminology is closely related to the nature of cluster molecules regarding their structures and bonding. With this book, we aim at providing a contemporary overview and insight into different families of clusters, their synthetic approaches and their specific properties, albeit representing a selection of examples, with no claim to completeness. Our selection includes the description of physical analysis methods and theoretical descriptions of the topol- ogies and the electronic situation. The synthetic approaches are as diverse as the products themselves; hence the book deals with compounds that were obtained by various techniques, which range from solid-state methods through solution chem- istry to gas phase techniques, and also includes theoretical treatment. However, as the chapters are reviews in nature, they do not provide too many details, which are subject to the more specified descriptions in the original and comprehensively cited literature. In all of the more experimental-based chapters, typical analytical tech- niques will be named and briefly described in the context of their application on the specific scientific questions. According to the great diversity of the cluster mole- cules, ranging from electron-deficient to electron-precise molecules, the bonding spectrum also covers a variety of different systems. Some cluster families feature electron-precise covalent bonds, while others exhibit semi-localized bonding or even possess highly delocalized electronic situations that are best described by the superatom concept for clusters in the jellium model of electron shells. A diversity of different bonding concepts have been developed during the past decades that serve – to some extent – to classify different families of clusters. Such concepts and their application to the discussed species are to be found within the different chapters of this book. The chapters are assigned