Future Shock for the Web-apps era.... Compulsively readablefor nontechies, too. Fast Company Building on the success of his industry-shaking Does IT Matter? Nicholas Carr returns with The Big Switch, a sweeping look at how a new computer revolution is reshaping business, society, and culture. Just as companies stopped generating their own power and plugged into the newly built electric grid some hundred years ago, today it's computing that's turning into a utility. The effects of this transition will ultimately change society as profoundly as cheap electricity did. The Big Switch provides a panoramic view of the new world being conjured from the circuits of the World Wide Computer. New for the paperback edition, the book now includes an AZ guide to the companies leading this transformation.

"This book is important reading. It offers practical, real-world insight and pragmatic no-nonsense approaches for people who have a stake in corporate IT. " -Lynda Applegate, Henry R. Byers Professor of Business Administration, Harvard Business School"Information systems and processes are very important parts of our due diligence assessment of a company-yet the jargon is often more difficult to understand than many foreign languages. Baschab and Piot effectively translate IT into words and concepts that businesspeople can easily understand and act upon. This book is a helpful reference guide for corporate executives and private equity groups of all types." -Neal Aronson, Managing Partner, Roark Capital Group"Business success increasingly depends on effective use of IT. Effective use of IT depends on the kind of in-depth, practical insight in this book. Baschab and Piot provide a pragmatic approach to information systems investment that should be required reading for senior executives and CIOs alike." -Erik Brynjolfsson, Schussel Professor of Management, Director of the Center for Digital Business, MIT"This book should provide valuable guidance for management and technology consultants. The Executive's Guide to Information Technology provides field-proven insight on all important aspects of IT planning and execution, from governance to applications to operations and infrastructure." -Gary J. Fernandes, former vice chairman, EDS, member of the Board of Directors, Computer Associates"Baschab and Piot do a great job of laying out the fundamental issues and challenges that every IT organization faces. More often than not, the issues are not technical in nature, but are a reflection of how the IT and business teams work together to define, execute, and implement new business tools. The threshold issue is leadership. Often it is difficult for business leaders to feel that they have the skills and perspective to provide that leadership on technical projects. The Executive's Guide to Information Technology provides non-technical business leaders a solid framework for engaging with their IT peers." -Tom Nealon, Chief Information Officer, J.C. Penney

Whereas the cations [CpMo{P(OMe)(3)}(2){eta(2)(4e)-alkyne}](+) do not react with alkynes or P=CBut, the newly synthesized isostructural phosphaalkyne complex [CpMo{P(OMe)(3)}(2){eta(2)(4e)-P=CBut}][B(C6F5)(4)], which is unreactive towards PhC2Ph, readily reacts via an associative stepwise process with P=CBut to give [CpMo{P(OMe)(3)}(2){eta(4)-1,3-P2C2Bu2t}][B(C6F5)(4)]. A further interesting difference in alkyne and phosphaalkyne chemistry was observed when it was found that CpMoCl(CO){eta(2)(4e)-PhC2Ph} reacts with TlPF6 and P=CBut to give the unusual 16-electron cyclocotrimerization product [CpMo{=C(Bu-t)PC(Bu-t)=PC(Ph)=C(Ph)}(CO)][PF6], identified by single-crystal X-ray crystallography.

Reaction of cis-[Mo(NCMe)(2)(CO)(2)(eta(5)-L)][BF4] (L = C5H5 or C5Me5) with 1-acetoxybuta-1,3-diene gives the cationic complexes [Mo(eta(4)-syn-s-cis-CH2CHCHCH(OAc)}(CO)(2)(eta(5)-L)][BF4], which, on reaction with aqueous NaHCO3/CH2Cl2, afford good yields of the anti-aldehyde substituted complexes [Mo(eta(3)-exo-anti-CH2CHCH(CHO)}(CO)(2)(eta(5)-L)]2(L = C5Me5), 4 (L = C5H5)]. The corresponding eta(5)-indenyl substituted complex 5 was prepared by protonation (HBF4 . OEt2) of [Mo(eta(3)-C3H5)(CO)(2)(eta(5)-C9H7)] followed by addition of CH2=CHCH=CH(OAc) and hydrolysis (aq. NaHCO3/CH2Cl2). An X-ray crystallographic study of complex 2 confirmed the structure and showed that there is a contribution from a zwitterionic form involving donation of electron density from the molybdenum to the aldehyde carbonyl group. Treatment of 2 and 4, in methanol solution, with NaBH4 afforded the alcohols [Mo(eta(3)-exo-anti-CH2CHCHCH2(OH)}(CO)(2)(eta(5)-L) [6 (L=C5H5), 8 (L=C5Me5)]; however, prolonged (30 h) reaction with NaBH4/MeOH surprisingly gave good yields of the methoxy substituted complexes [Mo(eta(3)-exo-anti-CH2CHCHCH2(OMe)}(CO)(2)(eta(5)- L)] [7 (L = C5H5), 9 (L = C5Me5)], the structure of 7 being confirmed by single crystal X-ray crystallography. This methoxylation reaction can be explained by coordination of the hydroxyl group present in 6 and 8 onto B2H6 to form the potential leaving group HOBH3-, which on ionisation affords [Mo(eta(4)-exo-buta-1-3-diene)(CO)(2)(eta(5)-L)](+) which is captured by reaction with OMe-. Complex 8 is also formed in good yield on reaction of 2 with HBF4 . OEt2 followed by treatment of the resulting cation [Mo(eta(4)-exo-s-cis-syn-CH2CHCHCH(OH)}(CO)(2)(eta(5)-C5Me5)][BF4] with Na[BH3CN]. Reaction of 4 with the Grignard reagents MeMgI, EtMgBr or PhMgCl afforded moderate yields of the alcohols [Mo(eta(3)-exo-anti-CH2CHCHCH(OH)R}(CO)(2)(eta(5)-C5H5)] [11 (R=Me), 12 (R=Et), 13 (R=Ph)]. Similarly, treatment of 2 with MeLi gave the corresponding alcohol 14. An attempt to carry out the Oppenauer oxidation [Al(OPr')(3)/Me2CO] of 11 resulted in an elimination reaction and the formation of the eta(3)-s-pentadienyl complex [Mo(eta(3)-exo-anti-CH2CHCH(CHCH2)}(CO)(2)(eta(5)-C5H5)], which was structurally identified by X-ray crystallography. Interestingly, oxidation of 6 with [(Bu4N)-N-n][RuO4]/morpholine-N-oxide affords the aldehyde complex, 4 in good yield.Finally, reaction of 11 with [NO][BF4] followed by addition of Na2CO3 affords the fur-3-ene complex [Mo(eta(2)-CH=CHCH2OC(H)Me)(CO)(NO)(eta(5)-C5H5)]. (C) 1998 Elsevier Science S.A.

Reaction of cis-[Mo(NCMe)2(CO)2(η5-L)][BF4] (L=C5H5 or C5Me5) with 1-acetoxybuta-1,3-diene gives the cationic complexes [Mo{η4-syn-s-cis-CH2CHCHCH(OAc)}(CO)2(η5-L)][BF4], which, on reaction with aqueous NaHCO3/CH2Cl2, afford good yields of the anti-aldehyde substituted complexes [Mo{η3-exo-anti-CH2CHCH(CHO)}(CO)2(η5-L)] 2 (L=C5Me5), 4 (L=C5H5)]. The corresponding η5-indenyl substituted complex 5 was prepared by protonation (HBF4·OEt2) of [Mo(η3-C3H5)(CO)2(η5-C9H7)] followed by addition of CH2CHCHCH(OAc) and hydrolysis (aq. NaHCO3/CH2Cl2). An X-ray crystallographic study of complex 2 confirmed the structure and showed that there is a contribution from a zwitterionic form involving donation of electron density from the molybdenum to the aldehyde carbonyl group. Treatment of 2 and 4, in methanol solution, with NaBH4 afforded the alcohols [Mo{η3-exo-anti-CH2CHCHCH2(OH)}(CO)2(η5-L)] [6 (L=C5H5), 8 (L=C5Me5)]; however, prolonged (30 h) reaction with NaBH4/MeOH surprisingly gave good yields of the methoxy-substituted complexes [Mo{η3-exo-anti-CH2CHCHCH2(OMe)}(CO)2(η5-L)] [7 (L=C5H5), 9 (L=C5Me5)], the structure of 7 being confirmed by single crystal X-ray crystallography. This methoxylation reaction can be explained by coordination of the hydroxyl group present in 6 and 8 onto B2H6 to form the potential leaving group HOBH3−, which on ionisation affords [Mo(η4-exo-buta-1-3-diene)(CO)2(η5-L)]+ which is captured by reaction with OMe−. Complex 8 is also formed in good yield on reaction of 2 with HBF4·OEt2 followed by treatment of the resulting cation [Mo{η4-exo-s-cis-syn-CH2CHCHCH(OH)}(CO)2(η5-C5Me5)][BF4] with Na[BH3CN]. Reaction of 4 with the Grignard reagents MeMgI, EtMgBr or PhMgCl afforded moderate yields of the alcohols [Mo{η3-exo-anti-CH2CHCHCH(OH)R}(CO)2(η5-C5H5)] [11 (R=Me), 12 (R=Et), 13 (R=Ph)]. Similarly, treatment of 2 with MeLi gave the corresponding alcohol 14. An attempt to carry out the Oppenauer oxidation [Al(OPr′)3/Me2CO] of 11 resulted in an elimination reaction and the formation of the η3-s-pentadienyl complex [Mo{η3-exo-anti-CH2CHCH(CHCH2)}(CO)2(η5-C5H5)], which was structurally identified by X-ray crystallography. Interestingly, oxidation of 6 with [Bu4nN][RuO4]/morpholine-N-oxide affords the aldehyde complex, 4 in good yield. Finally, reaction of 11 with [NO][BF4] followed by addition of Na2CO3 affords the fur-3-ene complex [Mo{η2-Download : Download full-size image(H)Me}(CO)(NO)(η5-C5H5)].

Reaction of cis-/trans-[ReBr2(CO)(2)(eta-C2H5)] with PhC(2)Ph and MeC(2)Ph in refluxing toluene afforded good yields of the eta(2)(4e)-donor alkyne complexes [ReBr2(eta(2)-PhC(2)Ph)(eta-C5H5)] 1 and [ReBr2(eta(2)-MeC(2)Ph)(eta-C5H5)] 2, respectively. Treatment of 1 and 2 with either AgBF4 or TIPF6 in the presence of PPh(3), PMePh(2) or P(OMe)(3) (L) gave monocations [ReBr{eta(2)(4e)-alkyne}L(eta-C5H5)](+), whereas a similar reaction with 2 equivalents of AgBF4 and 1 equivalent of Ph(2)PCH(2)CH(2)PPh(2) (dppe) afforded dications [Re(eta(2)-PhC(2)Ph)(dppe)(eta-C5H5)][BF4](2) and [Re(eta(2)-MeC(2)Ph)(dppe)(eta-C5H5)][BF4](2). The structural identity of [ReBr(eta(2)-PhC(2)Ph)(PMePh(2))(eta-C-5-H-5)][PF6] was confirmed by single-crystal X-ray crystallography. The alkyne C-C vector lies parallel to the Re-Br bond and the alkyne C-C bond length [C(1)-C(2) 1.26(4) Angstrom] is relatively short. Treatment of [ReBr(eta(2)-PhC(2)Ph)(PPh(3))(eta-C5H5)][BF4] and [ReBr(eta(2)-PhC(2)Ph)(PMePh(2))-(eta-C5H5)][PF6] with K[BHBu(3)(5)] in dichloromethane at -78 degrees C led to neutral eta(2)(3e)-vinyl complexes [Re{=C(Ph)CHPh}Br(PPh(3))(eta-C5H5)] and [Re{=C(Ph)CHPh}Br(PMePh(2))(eta-C5H5)]. The crystal structure of the latter showed that the C-C vector of the vinyl moiety lies almost parallel to the Re-Br bond. The stereochemistry of these reactions is discussed in the light of extended-Hiickel molecular orbital calculations. Reaction (-78 degrees C) of [Re(eta(2)-PhC(2)Ph)(dppe)(eta-C5H5)](2) with 1 equivalent of K[BHBu(3)(5)] in tetrahydrofuran afforded the X-ray crystallographically identified monocationic eta(2)(3e)-vinyl complex [Re{=C(Ph)CHPh}(dppe)(eta-C5H5)][BF4], which reacted at room temperature with a further equivalent of K[BHBu(3)(s)] to give the cis-stilbene-substituted complex [Re{eta(2)-Z-PhCH=CHPh}(dppe)(eta-C5H5)]. The crystal structure of the latter showed that the alkene phenyl substituents are orientated towards the cyclopentadienyl ring. In contrast, a similar reaction between K[BHBu(5)(s)] and [Re(eta(2)-MeC(2)Ph)(dppe)(eta-C5H5][BF4)](2) gave initially the eta(2)(3e)-vinyl complex [Re{=C(Me)CHPh}(dppe)(eta-C5H5)][BF4]; a further equivalent of [(eta-C5H5)] led to deprotonation and formation of the eta(2)-allene complex [Re{eta(2)-CH(Ph)=C=-CH2}(dppe)-(eta-C5H5)], in which the substituted allenic bond is co-ordinated to the rhenium. The dinuclear complex [Re2Br2(PPh(3))(2)(mu-O)(eta-C5H5)(2)][BF4](2) was also prepared and shown crystallographically to possess a single rhenium-rhenium bond [2.731(5) Angstrom].

Reaction of cis-/trans-[ReBr2(CO)2(η-C5H5)] with PhC2Ph and MeC2Ph in refluxing toluene afforded good yields of the η2(4e)-donor alkyne complexes [ReBr2(η2-PhC2Ph)(η-C5H5)]1 and [ReBr2(η2-MeC2Ph)(η-C5H5)]2, respectively. Treatment of 1 and 2 with either AgBF4 or TlPF6 in the presence of PPh3, PMePh2 or P(OMe)3(L) gave monocations [ReBr{η2(4e)-alkyne}L(η-C5H5)]+, whereas a similar reaction with 2 equivalents of AgBF4 and 1 equivalent of Ph2PCH2CH2PPh2(dppe) afforded dications [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2. The structural identity of [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] was confirmed by single-crystal X-ray crystallography. The alkyne C–C vector lies parallel to the Re–Br bond and the alkyne C–C bond length [C(1)–C(2) 1.26(4)Å] is relatively short. Treatment of [ReBr(η2-PhC2Ph)(PPh3)(η-C5H5)][BF4] and [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] with K[BHBus3] in dichloromethane at –78 °C led to neutral η2(3e)-vinyl complexes [[graphic omitted]HPh}Br(PPh3)(η-C5H5)] and [[graphic omitted]HPh}Br(PMePh2)(η-C5H5)]. The crystal structure of the latter showed that the C–C vector of the vinyl moiety lies almost parallel to the Re–Br bond. The stereochemistry of these reactions is discussed in the light of extended-Hückel molecular orbital calculations. Reaction (–78 °C) of [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 with 1 equivalent of K[BHBus3] in tetrahydrofuran afforded the X-ray crystallographically identified monocationic η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4], which reacted at room temperature with a further equivalent of K[BHBus3] to give the cis-stilbene-substituted complex [Re(η2-Z-PhCHCHPh)(dppe)(η-C5H5)]. The crystal structure of the latter showed that the alkene phenyl substituents are orientated towards the cyclopentadienyl ring. In contrast, a similar reaction between K[BHBus3] and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2 gave initially the η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4]; a further equivalent of K[BHBus3] led to deprotonation and formation of the η2-allene complex [Re{η2-CH(Ph)CCH2}(dppe)(η-C5H5)], in which the substituted allenic bond is co-ordinated to the rhenium. The dinuclear complex [Re2Br2(PPh3)2(µ-O)(η-C5H5)2][BF4]2 was also prepared and shown crystallographically to possess a single rhenium–rhenium bond [2.731(5)Å].

Reaction of 2-(trimethylsilyloxy)furan with the compounds cis-[Mo(NCMe)2(CO)2L][BF4](L =η-C5H5, η-C5Me5 or η5-C9H7) afforded the η3-γ-lactonyl complexes [Mo{η3-[graphic omitted]H}(CO)2L]. The structure of one of these species, [Mo{η3-[graphic omitted]H}(CO)2(η-C5Me5)], has been established by a single-crystal X-ray diffraction study, which confirmed that the γ-lactonyl moiety is bound to the molybdenum via three carbon atoms as an η3-allyl. Treatment of these lactonyl complexes with nucleophilic reagents (amines, methoxide) resulted in lactone ring opening and overall addition of the nucleophile to the γ-carbon of the lactone ring, rather than at the lactonyl carbonyl carbon atom as might have been expected. The product of the reaction between [Mo{η3-[graphic omitted]H}(CO)2(η5-C9H7)] and PhCH2NH2 has been structurally characterised by an X-ray diffraction study as the zwitterionic, η2-alkene complex [Mo{η2-PhCH2NHCHCHCH(CO2H}(CO)2(η5-C9H7)]. Similarly, [Mo{η3-[graphic omitted]H}(CO)2(η-C5H5)] and methoxide anion gives, after acidification, [Mo{anti-η3-(MeO)CHCHCH(CO2H)}(CO)2(η-C5H5)], in which the lactone ring has been cleaved to give an η3-allyl moiety ligating the metal centre. An extended Huckel molecular orbital calculation on [Mo{η3-[graphic omitted]H}(CO)2(η-C5Me5)] suggests that these reactions proceed via initial attack at the metal centre, followed by a rearrangement which effectively transfers the nucleophilic moiety to the γ-carbon of the lactone ring. Reaction of the η3-lactonyl complex [Mo{η3-[graphic omitted]H}(CO)2(η-C5H5)] with HBF4·Et2O resulted in a remarkable ring-enlargement reaction, in which a co-ordinated carbon monoxide formally inserts into the lactone carbon–oxygen bond to form the crystallographically characterised complex [Mo{η3-[graphic omitted]O}(NCMe)(CO)(η-C5H5)].

A series of diplatinum dications [Pt-2(mu-H)(2)(L-L)(2)][BF4](2) [L-L = (C6H11)(2)P(CH2)(n)P(C6H11)(2), Bu(2)(t)P(CH2)(n)PBu(2)(t), n = 2 or 2] with two chelating diphosphine and two bridging hydride ligands has been n or wt prepared by the elimination of ethene from the agostic alkyl complexes [PtEt(L-L)](+) or alkene-hydride complexes [PtH(C2H4)(L-L)](+), or by the reaction of the dihydride complexes [PtH2(L-L)] with an excess of HBF4 . OMe(2). The complexes have been characterized by multinuclear (H-1, P-31 and Pt-195) NMR spectroscopy and for [Pt-2(mu-H)(2){Bu(2)(t)P(CH2)(3)PBu(2)(t)}(2)][BF4](2) and [Pt-2(mu-H)(2){(C6H11)(2)P(CH2)(3)P(C6H11)(2)}(2)][BF4](2) by single-crystal X-ray crystallography. The latter complex has a structure in which the platinum and phosphorus atoms are coplanar whereas in the former the co-ordination planes of the platinum atoms are twisted with respect tn Path other by 36.6 degrees The twisting in [Pt-2(mu-H)(2){Bu(2)(t)P(CH2)(3)PBu(2)(t)}(2)][BF4](2) pressure of the large diphosphine which destabilises the planar geometry. Significantly this complex is fluxional on the NMR time-scale at 290 K whereas the others are static and there is a shift in colour from yellow to red for the strained complex. The dinuclear species are useful synthetic precursors of the [PtH(L-L)](+) fragment, which has the most sterically particularly when L-L is large. Thus [Pt-2(mu-H)(2){Bu(2)(t)P(CH2)(3)PBu(2)(t)}(2)](2+), which has the most sterically demanding diphosphine, reacts with alkenes [e.g. ethene (reversibly) or norbornene] to form mononuclear alkyl complexes with a three-centre, two-electron (agostic) bond.

Reaction of 2-(trimethylsilyloxy)furan with the compounds cis-[Mo(NCMe)(2)(CO)(2)L][BF4] (L = eta-C5H5, eta-C(5)Me(5) or eta(5)-C9H7) afforded the eta(3)-gamma-lactonyl complexes [Mo{eta(3)-OC(O)CHCHCH}(CO)(2)L]. The structure of one of these species, [Mo{eta(3)-OC(O)CHCHCH}(CO)(2)(eta-C(5)Me(5))], has been established by a single-crystal X-ray diffraction study, which confirmed that the gamma-lactonyl moiety is bound to be molybdenum via three carbon atoms as an eta(3)-allyl. Treatment of these lactonyl complexes with nucleophilic reagents (amines, methoxide) resulted in lactone ring opening and overall addition of the nucleophilic to the gamma-carbon of the lactone ring, rather than at the lactonyl carbonyl carbon atom as might have been expected. The product of the reaction between [Mo{eta(3)-OC(O)CHCHCH}(CO)(2)(eta(5)-C9H7)] and PhCH(2)NH(2) has been structurally characterised by an X-ray diffraction study as the zwitterionic, eta(2)-alkene complex [Mo{eta(2)-PhCH(2)NHCH-CH=CH(CO2H}(eta(5)C(9)H(7))]. Similarly, [Mo{eta(3)OC(O)CHCHCH}(CO)(2)(eta-C5H5)] and methoxide anion gives, after acidification, [Mo{anti-eta(3)-(MeO)CHCHCH(CO2H)}(CO)(2)(eta-C5H5)], in which the lactone ring has been cleaved to give an eta(3)-allyl moiety ligating the metal centre. An extended Huckel molecular orbital calculation on [Mo{eta(3)-OC(O)CHCHCH}(CO)(2)(eta-C(5)Me(5))] suggests that these reactions proceed via initial attack at the metal centre, following by a rearrangement which effectively transfers the nucleophilic moiety to the gamma-carbon of the lactone ring. Reaction of the eta(3)-lactonyl complex [Mo{eta(3)-OC(O)CHCHCH}(CO)(2)(eta-C5H5)] with HBF4 . Et(2)O resulted in a remarkable ring-enlargement reaction, in which a co-ordinated carbon monoxide formally inserts into the lactone carbon-oxygen bond to form the crystallographically complex [Mo{eta(3)-OH(O)CHCHCHCO}(NCMe)(CO)(eta-C5H5)].

Reaction of [W(eta(2)-PhC(2)Ph)(3)(NCMe)] with o-diphenyl-phosphino-styrene or -allylbenzene leads to either novel reorganisation reactions involving cleavage of carbon-carbon double and triple bonds, or unusual insertion reactions by PhC(2)Ph into carbon-hydrogen bonds.

A series of diplatinum dications [Pt2(µ-H)2(L–L)2][BF4]2[L–L =(C6H11)2P(CH2)nP(C6H11)2, But2P(CH2)nPBut2, n= 2 or 3] with two chelating diphosphine and two bridging hydride ligands has been prepared by the elimination of ethene from the agostic alkyl complexes [PtEt(L–L)]+ or alkene–hydride complexes [PtH(C2H4)(L–L)]+, or by the reaction of the dihydride complexes [PtH2(L–L)] with an excess of HBF4·OMe2. The complexes have been characterized by multinuclear (1H, 31P and 195Pt) NMR spectroscopy and for [Pt2(µ-H)2{But2P(CH2)3PBut2}2][BF4]2 and [Pt2(µ-H)2{(C6H11)2P(CH2)3P(C6H11)2}2][BF4]2 by single-crystal X-ray crystallography. The latter complex has a structure in which the platinum and phosphorus atoms are coplanar whereas in the former the co-ordination planes of the platinum atoms are twisted with respect to each other by 36.6°. The twisting in [Pt2(µ-H)2{But2P(CH2)3PBut2}2][BF4]2 is ascribed to the steric pressure of the large diphosphine which destabilises the planar geometry. Significantly this complex is fluxional on the NMR time-scale at 290 K whereas the others are static and there is a shift in colour from yellow to red for the strained complex. The dinuclear species are useful synthetic precursors of the [PtH(L–L)]+ fragment, particularly when L–L is large. Thus [Pt2(µ-H)2{But2P(CH2)3PBut2}2]2+, which has the most sterically demanding diphosphine, reacts with alkenes [e.g. ethene (reversibly) or norbornene] to form mononuclear alkyl complexes with a three-centre, two-electron (agostic) bond.