The azaboratantalacyclobutanes I (R/R' = Pr/CMe3, CHMe2/CMe3, CHMe2/CHMe2, CMe3CH2/CMe3CH2, Me3SiCH2/Me3SiCH2, CMe3/Me, CMe3/CHMe2, CMe3/SiMe3) are formed from the corresponding iminoboranes RB.tplbond.NR' and the neopentylidenetantalum complex [Me3CCH:TaCpCl2] by [2 + 2] cycloaddn. The elimination of HCl from these I and from the known I (R/R' = Et/CMe3, Bu/CMe3, Ph/CMe3, CH2CHMe2/CH2CHMe2) gives the azaboratantalacyclobutenes II. X-Ray structural analyses of II (R/R' = Ph/CMe3, Me2CH/Me2CH) show these cyclic carbenetantalum complexes to contain short Ta-C and Ta-N bonds and perhaps some transannular Ta-B interaction. The complex [Me3CCH:TaCl3(THF)2] (2b) adds iminoboranes RB.tplbond.NCMe3 with unbranched alkyl groups (R = Et, Pr, Bu) in a 1:2 molar ratio to yield the diazoniadiboratatantalacyclohexadienes III; III (R = Et) adopts a non-planar ring structure in the crystal. The 1:2 reaction of 2b with Me3CB.tplbond.NMes, instead, results in the cyclobutane IV (R' = mesityl).