SYNTHESIS AND CHARACTERIZATION OF 7-COORDINATE TANTALUM(I) AND NIOBIUM(I) COMPLEXES WITH CIS CARBONYL AND ALKYL ISOCYANIDE LIGANDS

A series of seven-coordinate tantalum(I) and niobium(I) complexes of the form [M(CNR)(CO)(dmpe)2Cl] [M = Nb, R = Me, Cy, Bu(t); M = Ta, R = Me, Et, Bu(t); dmpe = 1,2-bis(dimethylphosphino)ethane] has been synthesized by photolysis of [M(CO)2(dmpe)2Cl] in the presence of CNR. The bis(isocyanide) complex [Ta(CNMe)2(dmpe)2Cl] was prepared by reduction of [TaCl4(dmpe)2] in the presence of CNMe. These extremely electron-rich compounds are fluxional in solution, displaying low-temperature NMR spectra consistent with a capped trigonal prismatic or pentagonal bipyramidal geometry, Single-crystal X-ray analyses of four of these compounds revealed extensively bent C-N-C isocyanide linkages. The isocyanide bend angles for [Ta(CNMe)2(dmpe)2Cl] are close to 120-degrees, suggesting that the ligands are bound to the metal as heteroallenes. The suitability of the niobium(I) and tantalum(I) compounds as precursors for reductive coupling of CO and CNR ligands is discussed. Crystal data: [Nb-(CNCy)(CO)(dmpe)2Cl], Monoclinic, P2(1)/c, a = 18.900 (3) angstrom, b = 9.180 (i) angstrom, c = 16.627 (2) angstrom, beta = 95.417 (6)-degrees, Z = 4, R(w) = 0.042; [Ta(CNEt)(CO)(dmpe)2Cl], monoclinic, P2(1)/n, a = 9.082 (4) angstrom, b = 30.453 (7) angstrom, c = 9.214 (4) angstrom, beta = 105.75 (2)-degrees, Z = 4, R = 0.031, R(w) = 0.042; [Ta(CNBut)(CO)(dmpe)2Cl], monoclinic, P2(1)/n, a = 9.399 (2) angstrom, b = 32.188 (6) angstrom, c = 9.406 (5) angstrom, beta = 103.03 (3)-degrees, Z = 4, R = 0.042, R(w) = 0.052; [Ta(CNMe)2(dmpe)2Cl], triclinic, P1BAR, a = 17.389 (1) angstrom, b = 17.467 (3) angstrom, c = 9.111 (2) angstrom, alpha = 99.99 (2)-degrees, beta = 94.75 (1)-degrees, gamma = 110.89 (1)-degrees, Z = 4, R = 0.034, R(w) = 0.047.