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The synthesis, spectroscopic and structural characterization of the aryloxy and amino functionalized chloroboryl complexes (η5-C5R5)Fe(CO)2B(OMes)Cl (R = H, 2a; R = Me, 3a) and (η5-C5H5)Fe(CO)2B(NiPr2)Cl (7a) are reported. Compound 2ais shown to be a versatile substrate for further boron-centred substitutionchemistry leading to the asymmetric boryl complexes (η5-C5H5)Fe(CO)2B(OMes)ERn[ERn= OC6H4tBu-4, 2c; ERn= SPh, 2d] with retention of the metal–boron bond. The reactivities of 2a, 3aand 7atowards the halide abstractionagent Na[BArf4] have also been examined, in order to investigate the potential for the generation of cationic heteroatom-stabilized terminal borylene complexes. The application of this methodology to the mesityloxy derivatives 2aand 3agives rise to B–F containing products, presumably viafluoride abstraction from the [BArf4]−counter-ion. By contrast, amino-functionalized complex 7ais more amenable to this approach, and the thermally robust terminal aminoborylene complex [(η5-C5H5)Fe(CO)2B(NiPr2)][BArf4] (9) can be isolated in ca.50% yield. The reactivity of 9towards a range of nucleophilic and/or unsaturated reagents has been examined, with examples of addition, protonolysis and metathesis chemistries having been established.

The reactivities of the highly electrophilic boranes ClBC6F521 and HBC6F52n2 towards a range of organometallic reagents featuring metals from Groups 7–10 have been investigated. Salt elimination chemistry is observed between 1and the nucleophilic anions η5-C5R5FeCO2−R H or Me and MnCO5−, leading to the generation of the novel boryl complexes η5-C5R5FeCO2BC6F52R H 3 or Me 4 and OC5MnBC6F525. Such systems are designed to probe the extent to which the strongly σ-donor boryl ligand can also act as a π-acceptor; a variety of spectroscopic, structural and computational probes imply that even with such strongly electron withdrawing boryl substituents, the π component of the metal–boron linkage is a relatively minor one. Similar reactivity is observed towards the hydridomanganese anion η5-C5H4MeMnCO2H−, generating a thermally labile product identified spectroscopically as η5-C5H4MeMnCO2HBC6F526. Boranes 1and 2display different patterns of reactivity towards low-valent platinum and rhodium complexes than those demonstrated previously for less electrophilic reagents. Thus, reaction of 1with Ph3P2PtH2CCH2 ultimately generates EtBC6F5210 as the major boron-containing product, together with cis-Ph3P2PtCl2and trans-Ph3P2PtC6F5Cl 9. The cationic platinum hydride Ph3P3PtHis identified as an intermediate in the reaction pathway. Reaction of 2with Ph3P2Rhµ-Cl2, in toluene on the other hand, appears to proceed vialigand abstraction with both Ph3P·HBC6F5211 and the arene rhodiumi cation Ph3P2Rhη6-C6H5Me14 ultimately being formed.

A series of DFT calculations has been carried out with the aim of characterizing the metal - group 13 element interaction in the novel cationic borylene complex [(η5-C₅Me₅)Fe(CO)₂(BMes)]+ (1) and related species of the type [(η5-C₅R₅)M(L)₂(EX)]n+. In addition, comparisons have been made with charge neutral borylene complexes and with related group 14 based ligand systems (e.g. cationic metal carbonyls, carbenes and vinylidenes) for which models of bonding have previously been established. In this regard particular attention has been focused on the interpretation of (i) molecular orbital composition; (ii) bond dissociation energies (BDEs) and the ratio of ionic to covalent contributions (ΔE_elstat/ΔE_orb); and (iii) σ and π symmetry covalent contributions. The molecular orbital compositions for the prototype borylene complex 1 and for related cationic and neutral systems {e.g. [(η5-C₅H₅)Fe(PMe₃)₂(BMes)]+ and [(η5-C₅H₅)Mn(CO)₂(BMes)]} are consistent with the presence of bonding interactions between metal and borylene fragments of both σ and π symmetry. Furthermore, on the basis of BDEs, ΔE_orb values and σ/π covalent ratios, the bonding in cationic terminal borylene complexes such as 1 appears to have as much right to be termed a M&z.dbd;E double bond as does that in archetypal Fischer carbene and related complexes such as [(η5-C₅R₅)Fe(CO)₂(CCMe₂)]+ and [(η5-C₅R₅)Fe(CO)₂(CH₂)]+.

A series of DFT calculations has been carried out with the aim of characterizing the metal - group 13 element interaction in the novel cationic borylene complex η5-C5Me5FeCO2BMes1 and related species of the type η5-C5R5ML2EXn. In addition, comparisons have been made with charge neutral borylene complexes and with related group 14 based ligand systems e.g.cationic metal carbonyls, carbenes and vinylidenes for which models of bonding have previously been established. In this regard particular attention has been focused on the interpretation of i molecular orbital composition; ii bond dissociation energies BDEs and the ratio of ionic to covalent contributions ΔEelstatΔEorb; and iii σ and π symmetry covalent contributions. The molecular orbital compositions for the prototype borylene complex 1and for related cationic and neutral systems e.g.η5-C5H5FePMe32BMesand η5-C5H5MnCO2BMes are consistent with the presence of bonding interactions between metal and borylene fragments of both σ and π symmetry. Furthermore, on the basis of BDEs, ΔEorbvalues and σπ covalent ratios, the bonding in cationic terminal borylene complexes such as 1appears to have as much right to be termed a ME double bond as does that in archetypal Fischer carbene and related complexes such as η5-C5R5FeCO2CCMe2and η5-C5R5FeCO2CH2.