Over the past years the number of examples of compounds containing a planar-tetracoordinate carbon atom has increased. However, the presence of a carbon atom with a 360°s um of angles does not imply that the species is a derivative of planar methane; there must be an appropriate electronic stabilization. In the case of complexes 21a and 21b the central carbon atom is indeed stabilized by s-donors and pacceptors, as required for planar methane.

The syntheses of diboryl porphyrin complexes [(BX2)2(ttp)] (ttp: dianion of tetra-p-tolylporphyrin) and the B-B single-bond diboranyl complexes [(BX)2(ttp)] (X=F, Cl, Br, I) are given. The former are prepared from the reactions of BX3 (X=F, Cl) with [Li2(ttp)] and the latter from B(2)Cl(4) (X=Cl), the reaction of SbF3 with [(BCl)2(ttp)] (for X=F), and, in the cases of X=Br or I, in a remarkable reductive coupling reaction resulting directly from the reaction of BBr3 or BI3 with [Li2(ttp)]. Density functional theory (DFT) calculations on the thermochemical parameters for the reductive coupling reactions (and those calculated for related dipyrromethene complexes) indicate that a combination of the reducing ability of bromide and iodide ions combined with the constrained environment of the porphyrin ligand contribute to the driving force. The reductive coupling is also observed in the reaction of [(BCl2)2(ttp)] with nBuLi to give [(BnBu)2(ttp)], which was characterised crystallographically. The reaction of [(BCl)2(ttp)] with catechol gives a boron catecholato porphyrin complex, [B2(O(2)C(6)H(4))(ttp)]. Chloride abstraction from [(BCl)2(ttp)] gives the planar dication [B2(ttp)]2+, whereas chemical reduction of [(BCl)2(ttp)] by using magnesium anthracenide gives a neutral complex, [B(2)(ttp)], in which the TTP ligand has been reduced by two electrons to give an unusual example of an isophlorin complex. The cationic and neutral complexes [B2(ttp)]2+ and [B2(ttp)] were characterised through a combination of spectroscopic data that is supported by DFT calculations on the porphine analogues.

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Ferrocene reacts with BI3 and BBr3 to give 1,1′-bis(dihalogenboryl)ferrocenes; the cyclopentadienylring in cymantrene is twice borylated by BI3. By halogen exchange with AsF3 and AsCl3, by methylation with Sn(CH3)4, by cleavage of (C2H5)2O, by redox reaction with (CH3S)2 and by reaction with (C2H5)2NH the corresponding diborylferrocenes are obtained. The air-sensitive halogenborylferrocenes form donor-acceptor compounds with Lewis bases. 1H, 11B and 13C NMR data reflect increasing π-acceptor properties with increasing Lewis acidity of the boryl groups.

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Triple-decker complexes with a bridging diborolyl ligand CpCo(µ-1,3-C 3 B 2 Me 5 )M(ring) (M(ring) ) RuCp, 4; RuCp*, 5; Co(C 4 Me 4 ), 6) were synthesized by electrophilic stacking of the sandwich anion [CpCo(1,3-C 3 B 2 Me 5 )]with the [(ring)M(MeCN) 3 ] + cations. Structures of 4-6 were confirmed by X-ray diffraction. The electrochemical and spectroelectrochemical behavior of the complexes prepared was studied. DFT calculations of the redox potentials were also performed. Similar bonding properties of anions [CpCo(1,3-C 3 B 2 R 5 )]and [C 5 R 5 ] -(R ) H, Me) toward [M(ring)] + cations were established both experimentally (synthesis, electrochemistry, and X-ray diffraction) and theoretically (energy decomposition and Mulliken population analysis).

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