NEW MATERIAL:A polynuclear metal complex derived from a carboxylic acid of formula I: [R is formlas II, III, and IV (is CH2) or its derivative] and a basic metal salt. The metal atom has a coordination number ≥4, e.g. Cr, Fe or Co.

EXAMPLE: A polynuclear chromium complex of biscarboxybenzo-18-crown-6.

USE: Surface treatments for various substrates. The complex will be integrated with the treated surface, not easily removed, and can be utilized for protective films on semiconductor elements, solid catalysts in organic synthesis, and pollution prevention treatments by collecting heavy metals.

PROCESS: A metal salt, e.g. chromium chloride hexahydrate, of formula MXn.n'H2O (X is anion; M is metal; n and n' are positive numbers) is reacted with NaOH, etc. to form a basic metal salt, which is reacted with a compound of formula I to give the objective compound. The ratio of the number of metal in the metal salt to that of COOH in the compound of formula I is preferably 2:1.